¼ wave Mag Mount as a Portable Antenna

 

If you need to operate portable and only have a mag mount antenna available, try placing any large piece of metal underneath it. This might be a refrigerator, stationary car, or a metal rain gutter. You might also try making your own portable ground plane by placing some aluminum foil over a large piece of cardboard
( 2 feet x 2 feet minimum).

300 ohm TV Twin-lead J-Pole Antenna (Approximately 3 dB Gain) This is an easy antenna to make from existing or inexpensive materials.  It rolls up for easy storage and can be deployed in seconds; just hang from the nearest ceiling or attach to the end of a fiberglass fishing pole or PVC pipe. 

¼ wave Mag Mount as a Portable Antenna

If you need to operate portable and only have a mag mount antenna available, try placing any large piece of metal underneath it. This might be a refrigerator, stationary car, or a metal rain gutter. You might also try making your own portable ground plane by placing some aluminum foil over a large piece of cardboard ( 2 feet x 2 feet minimum).

Simple Ground Plane Antenna For PMR446

 

Here is a design for a 446 Mhz band outside aerial with ground planes.
The aerial is built on a SO259 chassis socket, with small nuts and bolts to hold the ground planes on, after the ground planes are attached, and the vertical element is soldered in, it is worth putting Araldite over the ground plane bolts, so they cant work loose in high winds.

The whole assembly will fit into a piece of 1 inch/25mm plastic tube to allow it to be fitted to a mast, or other fixing. The plastic tube requires a few saw cuts down the top, so a jubilee clip can be tightened onto the coaxial PL259 plug. It is well worth using the better grades of low loss cable on the aerial. The aerial will be resonant about 446.2 MHz, but it would be worth checking with a VSWR meter before transmitting. I have used this design for years on 70cm band, and you will be surprised how well it works.

Brass brazing rod is really easy to solder, and easy to get.

162.5 MHz ground plane antenna

 

Materials Needed:

1 - 6" x 6" piece of aluminum

1 - 18 3/16" length of number 12 wire

4 - 18 3/16" lengths of 3/16" diameter aluminum rods

1 - SO-239 coaxial connector 

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[text instructions]

Matched long-wire antenna

by RU3AEP

Building an efficient antenna is a big problem for an radio amateur in many cases. HF antennas, especially for lower bands, have large sizes and have to be mounted on a big height. In many places, especially in the city, there is not enough room to erect the full-size dipole or 'inverted V' for lower (160 and 80 m) bands. Also, large antennas cause a long feeding cable to be used, and this is not good in terms of cost and construction: antenna with 'central' feeding should bear a weight of a cable, the feeder should be fixed somehow to protect all the system from strong winds, there is not always conditions to make whole antenna system suitable for the environment.

My situation.

When i started to think, what antenna to build for a top-band (160 m), i realized, that condition are too bad for it. I live in a 7-floor house, which has a roof with a high slope (about 35-40 degrees). Such roof is very dangerous to operate on it. Also, the house is almost completely surrounded by wide streets and electrical wires going along them. After long thinking, i concluded, that there is only one possibility to make an antenna - to hang up a long wire from my roof to the roof of another house. If i had built a dipole on that place, i would have had too many problems with the feeder. Any other places of antenna mounting were unacceptable from viewpoint of safety (antenna should never hang above electrical wires!!!, otherwise in case of fall down in may injury or kill you or somebody else and also cause severe electrical damage of the network and your equipment!) or from viewpoint of difficulty in mounting.

An old idea which is actual today for many city HAMs

Fortunately in that time i have read about one very old, but not frequently used antenna - so called Zeppelin-antenna with a matched feeding.  Classical design is presented below. As it can be seen, there is feeder with rather low impedance (~70-300 Ohm), and 1/4-wavelength matching line. From one end, this line is shortened, and here its impedance is just a zero (current is high, but voltage is low). Another end of this line is connected to the long wire, which has length exactly 1/2 wavelength. At this end, the impedance is very high (several kiloohms). That is why, a big voltage exists here during a transmission. Such impedance is quite suitable for a wire feeding, because a 1/2-wavelength wire has voltage maxima (high impedance) at the ends. The feeder from the transmitter with a specific impedance R(feeder) is connected to the matching line in the point, where impedance of the latter is equal to that of the feeder. Such point is usually located not so far from the closed end. If everything is done properly, feeder may have any length and SWR is closed to 1 in rather narrow band, central frequency of which is determined by the geometrical size of matching line and antenna.

This design can be used almost without change, but instead of symmetrical feeder coaxial cable can be used to connect the whole system to the unsymmetrical output of the transmitter. The using coaxial cable has also additional advantages- it is almost insensitive to the environment, can be placed everywhere and is very flexible.

Such antenna with feeding 'from the end' is much more easy to make, that a simple dipole. Here, antenna conductor bears only itself, and this reduces the mechanical loading and thickness of the wire to be used. Also,  you may use your window as one the point of antenna fixing. In this case, all the cable will be inside the room and could be tuned precisely in comfortable conditions. If the beginning of antenna is outside the apartment, most part of matching line can be used as the continuation of the feeding cable. On the next picture there is a design, that i implemented for using on 160 m.

All coaxial cables have 75 Ohm impedance, the antenna wire, as well as two bearing wires are made from very hard bimetallic insulated cable (outer diameter is about 3 mm). The most tricky part - the connector between cable and antenna - is shown on the picture. It should be noted, that voltage on it is quite high, so everything should be well insulated from each other. It is good to place this connector somewhere indoors (not far from window/hole), otherwise rains and snow may cause decreasing of insulation efficiency. This antenna uses a tuned line made from the coaxial cable, and for proper operation of the whole system the antenna should have the length equal to the l*0.95/2, and the coaxial line must resonate on the working frequency. It is a good idea, to connect the shortened end of the matching line to the ground to provide adequate safety and to reduce possible TV/RF interference while transmitting.

Tuning of the antenna

To achieve what was declared in the previous paragraph, first of all the precise length of the matching line should be determined. Theoretically, it should be equal to  l/(4*sqrt(d)) (sqrt - SQuare RooT, d - dielectric constant of the insulator used in the coaxial cable). SQRT(d) value  is typically about 1.52 for most cables, that is why, 'shortening coefficient' is about 0.66 (1/sqrt(d)). But the practical value will be a little different from that.

The lengths indicated on the picture are mine values, and they can be used as the approximate reference. To make your line resonate on the middle of the band (1890 kHz), you have to make the line about 1 m longer, that indicated on the picture (for example, 24 m). Then, connect the 1-2 kOhm resistor to the end of the line, and the transceiver trough SWR meter - to the feeder. Put some power into line and watch the SWR. If the line is completely out of resonance, SWR will be closed to infinity, and no power will be dissipated on the resistor. Then the frequency should be found, which gives the sharp minimum of the SWR. It should be somewhere 1750-1800 kHz. Here, the SWR should be no more than 1.5. After the resonance have been found, the end of the cable should be cut carefully in several steps, watching the resonance frequency each time. By the cutting, the resonance will be shifted up. After you achieve the desired frequency, your line is almost ready, and you can mount the antenna in the chosen place. The minimum of the SWR in mounted antenna is usually 10-15 kHz down, compared to the value achieved by the tuning. If the SWR in minimum is too great - the point of the feeder connection should be varied to achieve acceptable value (it means redistribution of the cable between short and long sections of the line - not easy task).

When i made the antenna by the way described, everything was OK, and i had a minimum of SWR at 1875 kHz (about 1.3), on the edges of the band SWR increased to 2.2-2.5, since this antenna is a narrow-band one. Compared to my previous dipole, which hanged on the low height along the building, this antenna exhibited much better transmission efficiency and higher signal/noise ratio while receiving. But unfortunately, nothing lasts forever, and this is not an exception. Having read the next section, you probably will understand, which problems are encountered by the ham operator in a big city like Moscow.

SIMPLE WIRE ANTENNAS - A FEW THOUGHTS AND IDEAS

by
By Ron - 6Y5/4S7RO

During these hi tech days of Amateur Radio, some of us may feel a sense of inferiority that we do not contribute enough technically, to safeguard our identity as true Radiomen. It is also no secret that in recent times our hobby has evolved more into a talking hobby than a technical hobby. Although in comparison, the older hams did a lot of home-brewing, standards those days were different. In the good old days one could air a homebrew TX with a little drift, chirp, squeak and spectral impurity, and still be tolerated. Those were also the days when HF transmitters were commonly built on breadboards, and were heard on every neighbors radio and television set! Over the years, we allowed our hobby to be commercialized, and now the commercial equipment manufacturers have set new and high standards. No longer is it possible for the average Ham to build and air a home made transmitter without drawing cynical remarks or even be reprimand by the authorities

The good news is that even if we are not technically versed to build transmitters, we can all have fun building antennas, and still avoid being called push button operators or mere talkers over the airwaves! It is a nice feeling to get a flattering signal report from a DX station even when we run a Japanese rig because the rig would be useless if the antenna was bad! This enables us to claim some of the credit for a good, strong signal and for the great distances our signals are reported to have reached.

Getting Started:

A dipole fed with 75 ohm coax is about the easiest and most efficient out of simple antennae to build. However, like the commercial rig and the commercial antenna, one could quickly feel monotonous using a dipole. This is partly because we quickly begin to realize the performance limitations of the "no gain" dipole. Thus, begins the quest for a better antenna. This often is within the many constraints involved, as well as a hesitancy to build rotatable antennas with intelligent aluminum! Some fear seemingly elaborate matching networks and the monstrous sizes of monoband antennas. There is also the all important worry about towers and heavy-duty masts. So, why not a simpler and a cheaper (but effective) approach to "gain antennas"?

Many are the antenna handbooks, articles and websites covering antenna theory and specific practical designs. So, I shall make this a very informal discussion about the first steps involving the basic of basics!

The Wire YAGI:

The best and the easiest upgrade is to add a reflector or a director to an existing dipole, to make it a simple 2 element wire Yagi. To do this you will need either suitably spaced trees or in the case of upper HF band antennas, spreaders out of bamboo, wood or similar non-conducting material (it is a good idea to weatherproof them with common varnish or the more fashionable "polyurethane" sold in handy spray cans). Even though it is only a start, to avoid going deep into antenna theory, we shall stick to parasitic elements which are typically 5% longer (reflector) and 5% shorter (director) than the driven element. Usually, all you need to do is to space a director around .1 wavelengths in front of the driven element and/or a reflector .15 wavelengths behind the driven element. Once again, to the rocket scientist, these figures vary according to the design objective. However, I shall keep such finer points for another write up at a higher level!

The Influence of the Parasitic Element:

We need to understand that placing a director or a reflector on the same boom as the driven element will always LOWER the typical 75 ohms impedance of our existing dipole. Even though this can typically be anywhere from 50-25 ohms - depending on the spacing and tuning of the elements, a 2 element array with either a director at .1 or a reflector at .15 wave spacing will usually yield a good enough match into a 50 ohm coax cable. With the above arrangement even in the crudest form, one should easily be able to achieve 3-4 db of gain over a dipole. This is not only as good as increasing your power from 100 to 200 watts, but also concentrates that power within a narrower beamwith. It helps reduce fading and also RECEIVE better. A popular saying in ham radio is "if you cant hear them you cant work them!" If you wish to optimize forward gain by tuning the parasitic element or varying the spacing between the two elements, you may do so, but this will lower the impedance at the feed point (It also will not permit direct feed with 50 ohm coaxial cable).

Matching: (if you decide to optimize)

Matching problems can be overcome by using many well known techniques, but a simple hairpin match is recommend (see any good antenna handbook or email me at sparkrohan@yahoo.com if you need more details on hairpin matches). Hairpin matching a wire beam, also makes the antenna less clumsy than if you were to try a gamma match (which will also add a lot of downward strain on a wire element). As tuning of the driven element will have little or no effect on the gain of the array, do not be afraid to trim the driven element a few inches plus or minus from the formulae, to facilitate a good match. However, always remember that tuning the parasitic element has everything to do with the gain, directivity and the feed point impedance of the array.

The influence of spacing and element lengths on gain, front to back, impedance etc., is a vast topic that I do not have space to cover here. However, briefly, spacing elements close to each other reduces the feed impedance and increases the Q (reduced bandwidth) of the beam. Wide spacing reduces front to back, but provides good gain and lowers the Q resulting in greater SWR and gain bandwidths. One other point to remember is that maximum front to back does NOT mean maximum gain and is not always a good way to evaluate a beam. Maximum front to back and maximum gain also does not occur on the same frequency. The normal practice is to strike a compromise. When using modest levels of power, what is most important is forward gain.

LOOPS: (Deltas & Quads)

Another easy upgrade from a dipole is a single element delta or a quad loop. For those using an inverted vee, this becomes an easier task as it only involves introducing an additional half wave of wire and a base wire to the existing vee (imagine a wire pyramid). The existing feedpoint at the apex is better shifted to the center of the base or to one of its corners. It is said that a single element loop has a gain of a little over a db over a dipole. Since loops are said to radiate at a low angle, they work better at low heights compared to a dipole (although for ANY antenna the higher the better). Loops are also quieter antennas in that they pick up less man made interference.

As 4S7RO, the author worked pileups of Ws on 40m, running 100w into a loop, which had the base only 6 off the ground. From Jamaica, a single element sloping loop (which was sloping from 20 at the apex to 5 at the base) and a small IC718, brought the author 20m DXCC in just 2 months!

The formula for calculating a resonant loop is 1005/F (in MHz). I.e. 1005/14.2 = 70.77 One interesting point to remember is if you are using insulated wire, to further multiply the answer by a factor of .95. The formula for calculating the length of a director is 975/f and a reflector is around 1030/f.

The 2el Loop:

As with the 2 el Yagi above, an easy way of achieving excellent directivity and gain is to make a 2 element delta loop array, by suspending the elements off a nylon cord strung between two trees or other anchor points. You could also have one support and use a cross boom at the top. The many Coconut trees in Kerala (I call them Organic towers!) should provide ideal supports for making fine loop arrays. The author is reminded of a very successful 2 element 40m delta loop, where the loops were suspended off a bamboo pole placed horizontally and tied just below the "crown' of a coconut tree back in 4S7!

If the bottom of the loop is reachable from ground level one could easily switch directions manually by adding or reducing wire from the parasitic element. To do this you will need to use a driven element and a director slightly shorter than normal. At the middle of this director is a stub of wire with a shorting bar, where by you could change the length of the parasitic element to make it either a director or a reflector the length of the stub determines the tuning of the element as a director or a reflector- by making the element shorter or longer than the driven element. The technically inclined ham can also incorporate remote relay switching to achieve the above

if you use pulleys to haul up the loops, you can easily make predetermined element length changes even if the bottom is not reachable from ground level. However, as propagation to different parts of the globe is seasonal, one could even keep things simple by having a well tuned fixed direction loop array aimed in the direction of interest. Take a walk out to the yard, have a good look at your wire antennas and the trees around them - you might be surprised at the many possibilities that suddenly dawn on you. With a bit of ingenuity the possibilities are numerous.

Matching Loops:

Direct matching a delta loop into a coax cable is not always as easily done as with a wire Yagi. Practical measurements indicate impedances between 90-120- ohms, depending on height above ground and enclosed angles. If you are lucky, you will get a decent match into a 75 ohm cable, or else have to live with an SWR of over 1.5, using our standard 50 ohm coax. With single and two element loop antennas, I prefer to use a simple wave coaxial stub/transformer to match the array for 50 ohm feed. This is easily done by measuring a wavelength of 75-ohm coaxial cable at the operating freq and multiplying it by .66 (a typical velocity factor for commonly available 50 and 75 ohm coax cable). Connect one end of the 75-ohm cable to the feedpoint and the other end to a 50-ohm cable (any convenient length) that feeds the TX. This method will always result in a very good SWR when using a single or a 2-element loop array

In conclusion, I also wish to make a brief mention of the wire Moxons rectangle. This is an interesting variation of the wire Yagi antenna with folded back elements. Practical tests and computer simulations have indicated that it has gain only a fraction less gain than that of a conventional full sized Yagi. It uses shorter horizontal element lengths, yields a high front to back ratio and provides a PERFECT direct match into a 50 ohm coaxial cable. To the active hams who have heard/worked me, it should suffice to mention that I use a Moxon on 40m! The Moxons rectangle was originally designed by Moxon/G6XN, but greatly improved, remodeled and popularized by L.B.Cebik/W4RNL.

I hope this has given you a simple insight into antennas in the real world. Much can be written, but this is a start. If sufficient interest is shown, I shall be glad to write a series on practical design and construction of HF and VHF antennas. In the past few years my focus has been on compressed antennas for the apartment dweller. However, my antenna interests range from 160m-VHF! I also enjoy computer design, simulation and optimizing of all forms of antennas. If any of you would like to carry this discussion further or add to this, please feel free to email me at sparkrohan@yahoo.com and share your real world antenna experiments and experiences!

In conclusion, I wish to remind the Gurus and the enlightened that the objective of this article was to provide the beginner with ideas for a few effective, but simple upgrades, whilst keeping language and theory as simple as possible. As there are many good antenna books out there, I figured that sharing my practical experience and knowledge would be a better way of getting the average Indian ham with a simple wire antenna, get started on the road to greater things!

HF RDF Antenna

 

The Adcock HF RDF Antenna

RDF Antenna - Combiner

The Adcock antenna has been used for a long time for RDF. It is basically an interferometer.

When both of the two dipoles see exactly the same signal strength and RF phase, no signal is delivered to the receiver. This condition happens only when the signal source is equal distance from the two dipoles.

As the antenna is rotated through 360 degrees, there will be two directions where the signal will drop to a very low strength. This can be a very deep and narrow null. The direction readout is then read, and the direction of the signal is then known, with an ambiguity; we don't know which of the two null directions is the correct direction.
The directional ambiguity will be resolved when another station takes a bearing from some significant distance away. The two correct headings will cross, while the two incorrect ones will not cross.

Since this antenna only responds to vertically polarized components of signals, poor performance can occur when the signal is horizontal or nearly horizontal. Since purely horizontal signals are rare, it should function on most signals.

Construction

Construction is not critical. The only really important factors are electrical balance, and a lot of clearance distance to other objects. These factors may require the antenna be placed out in the open, away from the radio operating position.

The simplest arrangement would be a free-standing support with the vertical mast pivoted on a compass rose on the ground. This can be wood. Metal can be used for the support and for the cross-boom, as long as the two dipoles both always see the same capacitance from the structure. Symmetry is the name of the game here.

A remote speaker and remote S meter can be mounted at the antenna in a weatherproof box. The operator can tune in the suspect signal, and then go out to rotate the antenna and take the heading.

Detail of the Combiner circuit

The combiner circuit mounted on the cross boom is not critical for the antenna to DF. If a tiny toroid is used, use small diameter wire. If a large toroid is available, larger wire can be used. #30 wire would probably work for a 1/2 to 1 inch diameter toroid.The number of turns is not critical, but slightly greater sensitivity for weak signal

s may be achieved through experimentation.The feed line lengths on each side must be identical.The length of the dipoles is not at all important, so long as they are not resonant at some frequency of major interest. Phase characteristics of a resonant antenna are strongly affected by very slight detunings. If the two dipoles are not exactly the same (they won't be!) then the antenna will exhibit maximum error at the resonant frequency.

Because this is a broadband low-cost antenna, it is not possible to have good impedance match between the antenna and the feed line at all frequencies. Experimentation with turns ratio (bifiler turns versus coax feed turns) may provide an optimum match at some desired frequency, such as 20 meters. Mismatch on a receiving antenna is usually not important. Even with a poor antenna one can hear most of the signals on the band, because receivers today are so very sensitive.

A Magnetic Loop Antenna

 

Various articles have appeared describing magnetic loops for HF use, some with complicated methods of remote tuning and the antenna can be quite a useful and interesting project.

The article in Sprat 61 my Tom, GM3MXN describes a loop useable from 7 - 21 mHz, using half inch copper tube 3 feet in circumference. As shown, there is a gap at the top of the loop, which may be about 3/4 inch, either side of which is connected to the tuning capacitor, which can be about 250 pF. The inner of the SO239 socket at the base of the loop is soldered onto the centre of the loop, with the inner forming a gamma match to the tube, about 9 inches from the earthy connection. This is adjusted for minimum SWR. As show, the loop can be mounted on a wooden support and is fed with 50 ohm coax.

Full Wave Slanted Delta Loop Antenna

 

The antenna is based on the basic principle of a full wavelength loop antenna. A single wavelength loop remains an efficient and effective antenna.
- Loop antennas provide additional gain over conventional wire type antennas (1.5 db over a dipole).
- A loop is most efficient when its circumference covers the largest surface area (delta, circular or quad shape).
- Loop antennas perform especially well over poor conducting earth like sand and lossy soils.
- Since a loop is a balanced antenna, it requires a balanced feed mechanism (using a 75 Ohm quarter wave coax).
- It has to be fed at a correct place to achieve respective polarisation. A loop is vertically polarised when fed from the side and horizontally polarised when fed from either the top or bottom.
- Loop antennas have less electrostatic noise in your receiver.

SPECIFICATIONS:
The delta loop can be coarsely cut using standard formula of wavelength (300 / Operating base frequency) but the exact formula for a full wavelength (m) delta loop is (299.8 / Operating base frequency) x 1.05 Quarter wave 75 Ohm matching transformer = 75 / Operating base frequency x velocity of the coax.

Calculation for 40m slanted delta loop :
Frequency : 7.020 MHz (for CW and Digimode)
Lambda = (299.8/7.020) x 1.05 = 44.84m of 2.5mm² copper wire
For equilateral triangle = 44.84m / 3 = 14.95m each side
Quarter wave 75 Ohm matching transformer (velocity of 75 Ohm TV coax = 0.85) = (75 / 7.020) x 0.85 = 9.08m of 75 Ohm coax
Operating bandwidth is 300-350 kHz 2:1 SWR with 2.5mm² copper wire

Calculation for 30m slanted delta loop :
Frequency : 10.120 MHz (for CW and Digimode)
Lambda = (299.8/10.120) x 1.05 = 31.10m of 2.5mm² copper wire
For equilateral triangle = 31.10m / 3 = 10.37m each side
Quarter wave 75 Ohm matching transformer (velocity of 75 Ohm TV coax = 0.85) = (75 / 10.120) x 0.85 = 6.30m of 75 Ohm coax
Operating bandwidth is 300-350 kHz 2:1 SWR with 2.5mm² copper wire

Calculation for 80m slanted delta loop :
Frequency : 3.540 MHz (for CW and Digimode)
Lambda = (299.8/3.540) x 1.05 = 84.69m of 1.5mm² copper wire
For equilateral triangle = 84.69m / 3 = 28.23m each side
Quarter wave 75 Ohm matching transformer (velocity of 75 Ohm TV coax = 0.85) = (75 / 3.540) x 0.85 = 18.01m of 75 Ohm coax
Operating bandwidth is 200-300 kHz 2:1 SWR with 1.5mm² copper wire

Installation :
I fed the antenna at upper corner which is fixed at the top of the tower.Direct feeding with 75 Ohm coax at one side and PL259 UHF male connector at the other side. Then UHF Female/Female and PL259 UHF male connector on 50 Ohm coax to the shack.

Opinion / Comments :
Incredible antenna. I am real pleased with its performance. Working DX on 40m and 30m is no more a problem for me ! I put in place a 80m delta loop antenna with same configuration.
Coaxial Cable used for the Quarter wave 75 Ohm matching transformer :
CAVEL 17PAtC/PH Class A (For digital TV connection and satellite system application)

1) Construction
Center Conductor: 1.13+0.01mm Solid Copper
Dielectric: 4.85+0.05mmFoam PE
Shield: Al/PES/Al
Braid Coverage: 35%X16X4X0.12TC+0.005mm
Jacket: 6.80+0.05mm PE
Generic Standard: EN50117-1
Sectional Standard: EN50117-6

2) Characteristics
Impedance: 75+3 Ohm
Capacitance: 50+3pF/km
Velocity of Propagation: 85%
Conductor resistance Max: Inner:16.8 ohms/km
Outer:26 ohms/km
Return Loss: 20dB min (5-2150MHz)

Super Loop Antenna

 

G5RV verses Superloop 80

Many operators with small lots, a G5RV is what can fit for the 80 and 40 meter bands. The G5RV is 102 feet long and has a 34 footsection of twinlead followed by coax into the shack, possibly with some sort of RF choke on the coax. The ends are typically supported by ropes up inthe trees. An 80 meter dipole would be about 134 feet long.

A tiny lot is limited in antenna potential and zoning laws prevent real towers.

RadioWorks “Superloop III” designed by Jim, W4FTU, and refined over the years, is a good alternative

PHYSICAL VARIATIONS

The standard arrangement is shown in Fig. 1. It looks like an inverted delta loop and is 112 feet across the top. It fit on the same ropes as my G5RV used and the coax even started at about the same point in space. The wire is heavy 14 gauge copper. If your space doesn’t quite allow this, the top corner insulators can be moved to shorten the 112 foot dimension; also additional insulators can be added to the diagonal wires to make a rectangular
shape and raise the bottom balun up in the air more. I also added 6 feet of wire to move the resonant freq closer to the band bottoms for digital work.

The loop can also be mounted upside down and slanted if you only have a single support available. As with all loops, the area enclosed is important and so is the average height; the standard inverted delta shape is a very good compromise.

ELECTRICAL CHARACTERISTICS

The “trick” to the Superloop is the 30′ length of ladder line hanging down from the center insulator. This length has been tuned so that appears to be a open-circuit stub on 40 meters; thus the antenna becomes two full-wave wires (at 40 meters) and is commonly referred to as the Bi-Square antenna. On 80 meters, it appears to be a short and the antenna becomes a single wave vertical loop. This happens automatically and no switching is involved.

A special balun is provided which gives a match between the 50 ohm coax lead-in and the higher resistance of the loop. For best matching, a 1/2 wavelength coax is recommended (e.g. 99′ of RG-8X); however mine is about 70 feet into my diff-T tuner and the SWR < 2 points are 3495 to 3787 but the short coax gives a minimum on 40 of 2.05 at 7090 KHz. If you need to run without a tuner, close attention to the coax length will help. The balun is the typical ferrite rod in a PVC pipe with foaming urethane inside. This has the effect of heat insulating; mine works fine on 500 RTTY watts contesting, but real high power may be a problem on RTTY; but those guys all have beams, right?

OPERATING RESULTS

The diagonal wires make it partially a vertical antenna with a nice reduction in polarization QSB. You can possibly double contacts on 80/40 over the G5RV. RITTY can help on the reception. The Superloop tunes up fine on the 20,15,10 bands Antenna, ropes, and coax will run you about $US 135. RadioWorks advertises in CQ and QST and have an interesting catalog.

Copyright and originally hosted at http://larc.hamgate.net/SuperLoop.htm

HOW TO MAKE YOUR OWN 11 METER ANTENNA

 

Easy 11meter horizontal loop antenna. DX only. Performance close to 1/2 wave vertical.
Materials needed:
50 ohm coax any length
75 ohm coax
37 feet of wire
To calculate the length of wire needed for a 11 meter loop antenna, we must use a formula. Divide 1005 by Frequency in Mhz will give you length in feet. For example, if we wanted our loop to receive on 27.555 Mhz we would divide 1005 / 27.555 = 36.47 feet or 36 feet, 6 inches.

Before we attach this loop to our 50 ohm coax we need to put a matching section in between the loop of wire and the 50 ohm coax. This matching section is a section of 75ohm coax.(cable tv coax) To figure the length of this 75 ohm coax This 75ohm section will be a quarter wavelength of the operating frequency. To get this we calculate 246 Divided by frequency. In this case it's 246 / 27.555 = 8.93 feet or approximate. 9ft. Now we need to figure the "Velocity Factor" before we cut this 75 ohm piece. The velocity factor of your coax is .66 or .80. Most RG-59u is .66 so we will use that. Now take that 8.93 feet from above and multiply that by .66. For this the answer is 5.89. Or 5 feet ten inches.

Putting it together.

Attach 36ft, 6 inch loop wire to one end of 5 foot ten inch piece of 75ohm coax. Put one end of the wire on the shield(braid) of the coax and the other end of the loop on the center conductor of the coax. Insulate connections so they do not touch each other. Now attach the other end of that 75ohm coax to the 50ohm coax that goes to your radio. Attach them together just as you would repair a cut coax.

Now hang the loop horizontally between some structures. Meaning flat and parallel with the earth. Put it as close to a circle you can get it, although it will work fine as a square, odd shaped rectangle or even a triangle. Just getting it up in the air 15 feet will produce significant results no matter what strange shape. Make sure the bare wire used for the loop is hanging in free air and insulted from everything accept the string it is hanging on.

Tee Antenna

 

Magnetic Loop Antenna

 

One solution for an indoor shortwave antenna is a magnetic loop antenna. I started this project because it is not very difficult to build this antenna type. Like most antennas it is cheap. The most critical point is to build - or find - a useful loop of copper tube.
I was lucky and found a 18 mm copper tube bound to a ring in a property market. It was circa 200 cm long and formed more or less a circle. I cutted the tube into 2 parts, polished and clear coated the surface. From plywood I made a stand. Two ends of the tubes were mounted with screws on a piece of wood, the both other ends hold with a piece of plastic tube together. I used a Hammarlund variable capacitor of 15 to 325 pF to close the circle at one side. For precise tuning I use an 6:1 slow motion drive, which allows better fine tuning into the targeted frequency range.
To close the other side of the copper ring and to couple out of the antenna ring I took a ferrite toroid T 80-2 and formed 2 windings of 0.5 mm copper wire. One winding with 20 turns is closing the circle of the copper tubes, the other winding of 5 turns connects to a TNC-plug. So we have a Balun with a 4:1 ratio.
This loop antenna works down to circa 6.000 kc and up to 23.000 kc.

Schematic of the loop antenna:

The balun at the top of the antenna:

Hammarlund variable air-cap with slow-motion drive

A Magnetic Loop Antenna for Shortwave Listening (SWL)

 

Now that we’re on the downward slope of sunspot cycle 23 (2004) you may have noticed that some of your favorite broadcast stations don’t come in as strong as they did a few years ago. This is especially apparent on weaker DX stations. The whip on your shortwave receiver used to be sufficient to pull in some good DX, but now you find yourself looking for something better.

Maybe you have been thinking, or even have already tried, putting up a wire antenna. This may be a great solution if you live in a reasonably quiet area, noise wise, and your shortwave receiver doesn’t easily overload in the presence of strong signals. Perhaps you live in an apartment or are situated where installing a wire antenna is simply not feasible. Or maybe you’re looking for something that offers a bit more mobility so you can take it into different rooms of your house. Consider the small single turn magnetic loop antenna if any of the above situations apply to you.

Small Single Turn Magnetic Loop

The small single turn magnetic loop (SSTML) antenna consists of a single winding inductor, about 3 feet (1 meter) in diameter, and a tuning capacitor. A second loop, which is one fifth of the diameter of the large loop, is connected to the feedline and this small loop is positioned in the large loop on the opposite side of the tuning capacitor.

The SSTML has some very interesting properties:

a)

It has a small footprint. The loop I describe here looks like a circle in the
vertical plane and is just a little over 3 feet (1 meter) in diameter.

b)

It is a rather quiet antenna. It doesn’t pick up as much man-made noise
from nearby sources as a wire antenna would in the same situation.

c)

This antenna is somewhat directional, which can benefit you in two ways. You
can either aim (rotate) the antenna for maximum signal strength, or for minimum noise pickup. I prefer to do the latter, and here’s why. This antenna has what is called a deep null on each side of the antenna, the broad sides, meaning that signals coming from that direction will be attenuated quite a bit (30 dB is an often-quoted figure). However, this is mostly true for signals we receive directly, like noise sources, and not so much for signals from broadcast stations coming to us through skywave propagation. I aim the antenna for minimum noise pickup, which results in the best signal to noise ratio. In some situations it is quite possible to fully tune out a noise source such as a TV or computer monitor.

d)

Since this antenna is really a tuned circuit, it also acts as a preselector. It only receives well in a narrow bandwidth of a few hundred kilohertz (kHz). The antenna requires retuning if you change the frequency on the radio by a hundred to two hundred kHz. This may sound like a disadvantage, but if you have ever tried a long wire antenna on a rather sensitive receiver, you probably have noticed that your receiver may get overloaded, resulting in hearing multiple stations at once or hearing broadcast stations on frequencies where there really aren’t any. This may make it impossible for you to pull in that DX station you’re really interested in or even make listening to a strong broadcast station rather unpleasant. This antenna will help prevent overloading your receiver.

When you search the World Wide Web for magnetic loops, you usually run into magnetic loops meant for transmission in the upper ham bands, perhaps 14 MHz and above. To be able to transmit on these loops you’ll have to follow stringent design rules in order to maximize the efficiency of the loop. If you’re interested in a receive-only loop, then the design rules become very relaxed. There’s no need for large diameter tubing and neither for a low loss capacitor to successfully build a loop antenna. On this page I will present a description of my SWL loop.

10dBi – 6 Element OWA Yagi Antenna for 2 Meter band plan

Here is a 6 element yagi my friend and I have built for experimenting with directional antenna. Theoritically the antenna has about 10dBi gain in its main lobe and a good Front-to-Back ratio, a perfect substitute if you don’t have means to get the popular V24 Silverthunder antenna.

Radiation Pattern Lobes

Dimensions (in inches)
Element, Length, Space from Reflector

Reflector – 40.52, —-
Driver – 39.96, 10.13
Director 1 – 37.38, 14.32
Director 2 – 36.31 25.93
Director 3 – 36.31, 37.28
Director 4 – 34.96 , 54.22

Calculated Center of gravity 23.87 inches

Dimensions (in cm)
Element, Length, Space from Reflector

Reflector – 102.92, —-
Driver – 101.50, 25.73
Director 1 – 94.95, 36.37
Director 2 – 92.23, 65.86
Director 3 – 92.23, 94.69
Director 4 – 88.80 , 137.72

Calculated Center of gravity :60.66 cm

The plan for the antenna has been taken from LB Cebik website. More article will follow that covers our construction of this Yagi antenna along with our reports of its improvement.

Practical Dipole Antennas

 

A classic dipole antenna is 1/2-l long and fed at the center. The feed-point impedance is low at the resonant frequency, f0, and odd harmonics thereof. The impedance is high near even harmonics. When fed with coax, a classic dipole provides a reasonably low SWR at f0 and its odd harmonics.

When fed with ladder line (see Fig 20.8A) and a Transmatch, the classic dipole should be usable near f0 and all harmonic frequencies. (With a wide-range Transmatch, it may work on all frequencies.) If there are problems (such as extremely high SWR or evidence of RF on objects at the operating position), change the feed-line length by adding or subtracting 1/8 l at the problem frequency. A few such adjustments should yield a workable solution. Such a system is sometimes called a "center-fed Zepp." A true "Zepp" antenna is an end-fed dipole that is matched by 1/4 l of open-wire feed line (see Fig 20.8B). The antenna was originally used on zeppelins, with the dipole trailing from the feeder, which hung from the airship cabin. It is intended for use on a single band, but should be usable near odd harmonics of f0.

Most dipoles require a little pruning to reach the desired resonant frequency. Here’s a technique to speed the adjustment.

How much to prune: When assembling the antenna, cut the wire 2 to 3% longer than the calculated length and record the length. When the antenna is complete, raise it to the working height and check the SWR at several frequencies. Multiply the frequency of the SWR minimum by the antenna length and divide the result by the desired f0. The result is the finished length; trim both ends equally to reach that length and you’re done.

Loose ends: Here’s another trick, if you use nonconductive end support lines. When assembling the antenna, mount the end insulators in about 5% from the ends. Raise the antenna and let the ends hang free. Figure how much to prune and cut it from the hanging ends. If the pruned ends are very long, wrap them around the insulated line for support.

Fig 20.8—Center-fed multiband "Zepp" antenna (A) and an end-fed Zepp at (B).

Dipole Orientation

Dipole antennas need not be installed in a horizontal straight line. They are generally tolerant of bending, sloping or drooping as required by the antenna site. Remember, however, that dipole antennas are RF conductors. For safety’s sake, mount all antennas away from conductors (especially power lines), combustibles and well beyond the reach of passersby.

A sloping dipole is shown in Fig 20.9. This antenna is often used to favor one direction (the "forward direction" in the figure). With a nonconducting support and poor earth, signals off the back are weaker than those off the front. With a nonconducting mast and good earth, the response is omnidirectional. There is no gain in any direction with a nonconducting mast.

A conductive support such as a tower acts as a parasitic element. (So does the coax shield, unless it is routed at 90° from the antenna.) The parasitic effects vary with earth quality, support height and other conductors on the support (such as a beam at the top). With such variables, performance is very difficult to predict.

Losses increase as the antenna ends approach the support or the ground. To prevent feed-line radiation, route the coax away from the feed-point at 90° from the antenna, and continue on that line as far as possible.

Fig 20.9—Example of a sloping 1/2-l dipole, or "full sloper." On the lower HF bands, maximum radiation over poor to average earth is off the sides and in the "forward direction" as indicated, if a nonconductive support is used. A metal support will alter this pattern by acting as a parasitic element. How it alters the pattern is a complex issue depending on the electrical height of the mast, what other antennas are located on the mast, and on the configuration of guy wires.

An Inverted V antenna appears in Fig 20.10. While "V" accurately describes the shape of this antenna, this antenna should not be confused with long-wire V antennas, which are highly directive. The radiation pattern and dipole impedance depend on the apex angle, and it is very important that the ends do not come too close to lossy ground.

Fig 20.10—At A, details for an inverted V fed with open-wire line for multiband HF operation. A Transmatch is shown at B, suitable for matching the antenna to the transmitter over a wide frequency range. The included angle between the two legs should be greater than 90° for best performance.

Bent dipoles may be used where antenna space is at a premium. Fig 20.11 shows several possibilities; there are many more. Bending distorts the radiation pattern somewhat and may affect the impedance as well, but compromises are acceptable when the situation demands them. When an antenna bends back on itself (as in Fig 20.11B) some of the signal is canceled; avoid this if possible.

Remember that current produces the radiated signal, and current is maximum at the dipole center. Therefore, performance is best when the central area of the antenna is straight, high and clear of nearby objects. Be safe! Keep any bends, sags or hanging ends well clear of conductors (especially power lines) and combustibles, and beyond the reach of persons.

Fig 20.11—When limited space is available for a dipole antenna, the ends can be bent downward as shown at A, or back on the radiator as shown at B. The inverted V at C can be erected with the ends bent parallel with the ground when the available supporting structure is not high enough.

Multiband Dipoles

There are several ways to construct coax-fed multiband dipole systems. These techniques apply to dipoles of all orientations. Each method requires a little more work than a single dipole, but the materials don’t cost much.

Parallel dipoles are a simple and convenient answer. See Fig 20.12. Center-fed dipoles present low-impedances near f0, or its odd harmonics, and high impedances elsewhere. This lets us construct simple multiband systems that automatically select the appropriate antenna. Consider a 50-W resistor connected in parallel with a 5-kW resistor. A generator connected across the two resistors will see 49.5 W, and 99% of the current will flow through the 50-W resistor. When resonant and nonresonant antennas are parallel connected, the nonresonant antenna takes little power and has little effect on the total feed-point impedance. Thus, we can connect several antennas together at the feedpoint, and power naturally flows to the resonant antenna.

There are some limits, however. Wires in close proximity tend to couple and produce mutual inductance. In parallel dipoles, this means that the resonant length of the shorter dipoles lengthens a few percent. Shorter antennas don’t affect longer ones much, so adjust for resonance in order from longest to shortest. Mutual inductance also reduces the bandwidth of shorter dipoles, so a Transmatch may be needed to achieve an acceptable SWR across all bands covered. These effects can be reduced by spreading the ends of the dipoles.

Also, the power-distribution mechanism requires that only one of the parallel dipoles is near resonance on any amateur band. Separate dipoles for 80 and 30 m should not be parallel connected because the higher band is near an odd harmonic of the lower band (80/3 » 30) and center-fed dipoles have low impedance near odd harmonics. (The 40 and 15-m bands have a similar relationship.) This means that you must either accept the lower performance of the low-band antenna operating on a harmonic or erect a separate antenna for those odd-harmonic bands. For example, four parallel-connected dipoles cut for 80, 40, 20 and 10 m (fed by a single Transmatch and coaxial cable) work reasonably on all HF bands from 80 through 10 m.

Fig 20.12—Multiband antenna using paralleled dipoles, all connected to a common 50 or 75-W coax line. The half-wave dimensions may be either for the centers of the various bands or selected for favorite frequencies in each band. The length of a half wave in feet is 468/frequency in MHz, but because of interaction among the various elements, some pruning for resonance may be needed on each band. See text.

Trap dipoles provide multiband operation from a coax-fed single-wire dipole. Fig 20.13 shows a two-band trap antenna. A trap is a parallel-resonant circuit that effectively disconnects wire beyond the trap at the resonant frequency. Traps may be constructed from coiled sections of coax or from discrete LC components.

Choose capacitors (Cl in the figure) that are rated for high current and voltage. Mica transmitting capacitors are good. Ceramic transmitting capacitors may work, but their values may change with temperature. Use large wire for the inductors to reduce loss. Any reactance (XL and XC) above 100 W (at f0) will work, but bandwidth increases with reactance (up to several thousand ohms).

Check trap resonance before installation. This can be done with a dip meter and a receiver. To construct a trap antenna, cut a dipole for the highest frequency and connect the pretuned traps to its ends. It is fairly complicated to calculate the additional wire needed for each band, so just add enough wire to make the antenna 1/2 l and prune it as necessary. Because the inductance in each trap reduces the physical length needed for resonance, the finished antenna will be shorter than a simple 1/2-l dipole.

Fig 20.13—Example of a trap dipole antenna. L1 and C1 can be tuned to the desired frequency by means of a dip meter before they are installed in the antenna.

Shortened Dipoles

Inductive loading increases the electrical length of a conductor without increasing its physical length. Therefore, we can build physically short dipole antennas by placing inductors in the antenna. These are called "loaded antennas," and The ARRL Antenna Book shows how to design them. There are some trade-offs involved: Inductively loaded antennas are less efficient and have narrower bandwidths than full-size antennas. Generally they should not be shortened more than 50%.

Simple 10 meter loop antenna

Here’s a diagram for a simple 10m loop antenna. Although 10 meter band is not very good at this time of the year, the operating condition is predicted to be improving in the next couple of years.

Simple Antenna for 10 meter operation

A/B Value for 28.2 MHz
A = 73″
B = 146″

A/B Value for 28.5 MHz
A = 72″
B = 145 3/4″

A/B Value for 27.5555 MHz (freebander)
A = 73 1/2″
B = 150 3/4″

Data from MMANA-GAL software

The antenna can be build from Copper wire or aluminum tube.

Hamradio Homebrew 2 Meter Square Dipole Plan

Here is a plan for homebrewing a 2 Meter Square Dipole plan. The advantage of this antenna is that it is unidirectional, and it takes less space than the regular 2 meter dipole. The calculation included on the diagram below is for building the antenna using copper tubing, you should use MMANA-GAL or other antenna simulation software to come up with new dimension for other materials (aluminium, wire, etc).

Click on the diagram to enlarge it. Hopefully this will help you in brewing new antennas! Original plan taken from KOFF website

2 Meter Slim Jim antenna from Ordinary Wires

 

Slim Jim (J Integrated Match J-Pole) is probably the most easiest and powerful 2 meter antenna to build provided you have the exact measurement and material to build it.

This how to will show you how to build a 2 meter slim jim antenna from ordinary insulated copper wire commonly used for carrying AC (alternate current) electricity in your household.

Slim Jim construction basic
I am not only going show you the measurement of slim jim antenna for specific frequency, but I’m going to show you how to calculate slim jim antenna by your own using the basic formula below.

The figure above shows that the longest side of slim jim is 3/4 wavelength long and the shorter side of the slim jim consist of 1/2 wavelength and 1/4 wavelength long seperated by a gap.

The feedline (coax cable) is normally connected 1/20 wavelength from the bottom of the slim jim antenna with the center conductor connected to the longest side and the shield/braid is connected to the shorter side.

Building the Slim Jim antenna
This guide assume you want to build a slim jim antenna that centered on 146MHz.

Calculation
The formula for calculating wavelength in metric system is 300/(freq MHz)

Using the formula from the figure, we have :

300/146 = 2.055 M
Wavelength = 205.5 cm

Wavelength x copper wire velocity factor = 205.5 cm x 0.94
= 193.17 cm

3/4 wavelength = 193.17 x 0.75
= 144.88 cm (57″)

1/2 wavelength = 193.17 x 0.5
= 96.585 cm (38″)

1/4 wavelength minus gap = 193.17 x 0.25 – 2.6 cm
= 45.69 cm (18″)

Coax tap = 193.17 x 1/20
= 9.6 cm (3 3/4″)

Building Materials

• 3/4″ diameter PVC (20mm) – 6 feet (180 cm)
• ordinary insulated copper wire for carrying altenate current (AC) – 11 feet (3.40 meter)
• Cable ties

Tools

• Soldering iron
• Glue gun
• Somthing to make a hole on PVC pipe

Wire Slim Jim Building Steps

• First take the PVC pile and measure it according to the 3/4 wavelength formula above (144.88 cm).
• Make two holes at the opposite side of the pipe. This hole is used for putting the copper wire through the pipe. Repeat this step 144.88 cm away from the top hole. Both of these holes will hold the copper wire.
• Insert the wire through the hole until both end reaches each other on one side of the PVC pipe. Then measure the length of the wire and cut the wire on that side so the setup resembles the figure above.
• Cut the wire insulation (but leave the wire uncut) 1/20 wavelength away (9.6 cm) from the bottom of the PVC pipe, again refer the figure above.
• Solder the center of the coax cable at the longest side of the slim jim (3/4 wavelength part) and the braid/shield at the shorted part of the antenna.
• Test the antenna using SWR meter to ensure that its SWR is at minimum or within acceptable level.
• There you go, you’ve build yourself your own 2 meter Omnidirectional Slim Jim antenna for less than USD2 (RM 6.00)

Slim Jim Antenna

 

Hello home builders, this is a vertically polarized omnidirectional free space antenna which offers approximately 1.8dB of gain. It has a radiation efficiency 50% better than a ground-plane antenna due to its low radiation angle. The slim jim antenna is a very old design from the 30's and plenty of information can be found about it on the net. This site however, will actually show you how to build your slim jim antenna from Home Depot items!

Antenna Description: Basically it is an end-fed folded dipole operated vertically. The matching stub provides a low impedance feed point (50 ohms) at the base and couples to the antenna section at high impedance at one end. As with all folded dipoles, the currents in each leg are in phase, whereas in the matching stub they in phase opposition, so little or no radiation occurs from this. Correctly matched, the VSWR (Voltage Standing Wave Ratio -Steve) will be much less than 1.5:1, and remains so across the band.

Slim Jim Antenna Feed

Here is a great diagram from Free Radio Berkeley of the antenna feed

The how to build information is derived from

an old FM-10 archive page titled "3 Meter Slim-Jim Antenna"

Construction: The Slim Jim should be constructed from 1/2" copper pipe. The bends are made with soldered 90 degree copper elbows. A slip sleave made from copper can be added to the element above the gap for tuning purposes, although the average length of the gap and spacing between the elements is 3" at 90MHz. No part of the antenna should be grounded to the tower or mast. The recommended mount is the use of PVC pipe and PVC pipe "T's." Make sure the space between the tower or mast and the antenna is one "freespace" 1/4 wavelength.

3 Meter Slim-Jim Antenna (JIM = J Intergrated Match)

1/2 wave section = 5610/MHz Example: 89.5MHz = 5610/89.5 = 62.68"

1/4 wave section = 2805/MHz Example: 89.5MHz = 2805/89.5 = 31.34"

The Slim Jim Antenna setup for portable use.

A Slim Jim for 4m

 

This antenna is made from a length of 300-ohm ribbon cable, which makes it easily portable, but you have to devise some method of suspending it!

The dimensions quoted in the diagram have been used successfully by some constructors, whilst others have found it to be off-frequency by a few megahertz. This may be due to a parasitic capacitance in the gap between the half-wave and quarter-wave sections, so be prepared to experiment a bit..

2 Metre band Slim Jim antenna

2 Metre band Slim Jim antenna using 300 Ohm ribbon cable

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Here are the dimensions for a 2 meter Slim Jim antenna, made from 300ohm ribbon cable.

50 ohm coax feeder length 3, 5, 7 m. etc.

The G5RV antenna

 

• 

  G5RV antenna

THE G5RV ANTENNA

The G5RV is a very popular antenna on the HF amateur band today. Despite it's widespread use on the bands, there are some myths and misconceptions concerning the G5RV that seem to have a life of their own. Working with text from the ARRL "Antenna Compendium", Volume 1, I would like to shed some light on this versatile antenna.

First, from Louis Varney, G5RV, of West Sussex, UK, here is some back- ground and insights into the G5RV. "The G5RV antenna, with its special feeder arrangement, is a multiband center-fed antenna capable of efficient operation on all HF bands from 3.5 to 28 MHz. Its dimensions are specifically designed so it can be installed in areas of limited space, but which can accommodate a resonably straight run of 102 ft for the flat-top."

Louis further states that, "In contradistinction to multiband antennas in general, the full-sized G5RV antenna was NOT designed as a half-wave dipole on the lowest frequency of operation, but as a 3/2-wave center-fed long-wire antenna on 14 MHz, where the 34 ft open-wire matching section functions as a 1:1 impedance transformer. This enables the 75-ohm twin-lead, or 50/80-ohm coaxial cable feeder, to see a close impedance match on that band with a consequently low SWR on the feeder. However, on all the other HF bands, the function of this section is to act as a "make-up" section to accommodate that part of the standing wave (current and voltage components) which, on certain operating frequencies, cannot be completely accommodated on the flat- top (or inverted-V) radiating portion.

The design center frequency of the full-size version is 14.150 MHz, and the dimension of 102 ft is derived from the formula for long-wire antennas which is:

LENGTH (ft) = 492(n-.05)/f(MHz)

= (492 x 2.95)/14.15

= 102.57 ft (31.27 m)

where n = the number of half wavelengths of the wire (flat-top)

"Because the whole system will be brought to resonance by the use of a matching network in practice, the antenna is cut to 102 ft."

As the antenna does not make use of traps or ferrite beads, the dipole portion becomes progressivily longer in electrical length with increasing frequency.

This effect confers certain advantages over a trap or ferrite-bead loaded dipole because, with increasing electrical length, the major lobes of the vertical component of the polar diagram tend to be lowered as the operating frequency is increased.

Thus, from 14 MHz up, most of the energy radiated in the vertical plane is at angles suitable for working DX.

Furtermore, the polar diagram changes with increasing frequency from a typical half-wave dipole pattern at 3.5 MHz and a two half-wave in-phase pattern at 7 and 10 MHz to that of a long-wire pattern at 14, 18, 21, 24 and 28 MHz.

Although the impedance match for 75-ohm twin-lead or 80-ohm coaxial cable at the base of the matching section is good on 14 MHz, and even the use of 50-ohm coaxial cable results in only about a 1.8:1 SWR on this band, the use of a suitable matching network is nessessary on all the other HF bands. This is because the antenna plus the matching section will present a REACTIVE load to the feeder on those bands.

Page 2 Thus, the use of the correct type of matching network is essential in order to ensure the maximum transfer of power to the antenna from a typical transceiver having a 50-ohm coaxial (unbalanced) output. this means unbalanced input to balanced output if twin-lead feed is used, or unbalanced to unbalanced if coaxial feeder is used.

A matching network is also employed to satisfy the stringent load conditions demanded by such modern equipment that has an automatic level control system. The system senses the SWR condition present at the solid state transmitter output stage to protect it from damage, which could be caused by a reactive load having an SWR of more than 2:1."

2m 4-element Quad you can build

 

A four element quad for 2 metres which is supposedly equal to a Cushcraft 11 element beam antenna.

The antenna boom is made out of pvc pipe, 1"size, which is 1.5" in diameter, actual size. The boom is approximately 42" long. The cross pieces are 1/4 inch bicycle flag poles made out of fiberglass, or you could use fiberglass fence posts.

TOP VIEW LOOKING DOWN

YOU FEED A QUAD FROM THE HORIZONTAL SPREADER END.

A short dipole for 80 meters

 

• 

The antenna above has been described by Nadisha, 4S7NR and may be of interest to anyone wishing to get on 80M (3.5MHz) that have limited space available.

L1 is 12 feet. L2 also is 12 feet and the overall length is 48 feet.

The two loading coils are described as 67.83uH and can consist of 104 turns of insulated wire, wound over 3.5 inches. The coil diameter is not stated however. Maybe it will be a case for experimentation here.

A Stealthy 75 meter Antenna

 

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  A Stealthy 75 meter Antenna

The 75 meter dipole at K7ZB was put up to meet the need for making contacts with friends in the Southwest and Southern California. As usual, it had to be put up so no one can see it from the surrounding neighborhood.

The objective was accomplished as shown below, which is the site drawing for the installation. It follows the contour of the home, yard and landscaping as much as possible - sacrificing gain and pattern for merely pumping a signal into the clouds and surrounding area.

K7ZB painted the wire forest green to match the background of the foliage or desert sand to match the usual house paint used in the majority of residences in the Valley of the Sun.
It is very hard to see even when you know it's there!

A good technical explanation of dipoles that are tortured like this one is available at L.B. Cebik's (W4RNL) antenna webpage shown below.

Fold, Bend, and Mutilate or Making a Dipole Fit the Space Available L. B. Cebik, W4RNL

70cm vertical antenna

9 element YAGI antenna for 2m band

magnetic loop antenna for HF

 

by Peter Parker VK3YE – first appeared in Amateur Radio, December 1997

October 1997′s Novice Notes looked at compact antennas that amateurs use to operate from confined locations. The smallest antenna described for 80 metres was a magnetic loop. This article provides all the details needed to build your own.

Description

Able to cover all frequencies between 3.5 and about 10 MHz, the loop described here is directional, does not require a radial system, and stands just 1.8 metres tall. Most parts needed can be purchased at a hardware shop. The antenna can be put together in an afternoon and requires only hand tools to assemble. It should cost less than sixty dollars to build.

Shown below is the schematic diagram for the loop. Note that the element is continuous except for a gap at the top across which the variable capacitor is wired. The feedline is connected to the bottom of the loop. Also shown is the physical construction of the antenna. The loop element is 1.5 metres square and is supported on a wooden cross. To minimise losses, thick aluminium strip is used for the element. At the top of the loop is a high-voltage variable capacitor. This is used for adjusting the antenna to the operating frequency. Because of its narrow bandwidth, the tuning is very sharp and a vernier drive has been added to make tuning easier. Dimensions are not particularly critical, provided it is possible to bring the loop to resonance on all operating frequencies with the variable capacitor used.

Parts needed

The following materials are required to build the antenna:

 

• 3 2m lengths of 3x20mm aluminium strip

• 1 1.8m length of 20x44mm pine

• 1 1.5m length of square (12x12mm) wood

• 1 polyethylene chopping board (medium or large size)

• 1 150 x 80×4 mm piece of stiff high-voltage insulating material (eg bakelite)

• 2 right angle metal brackets

• 1 20-400pF high voltage variable capacitor

• 1 6:1 vernier reduction drive (Dick Smith No P-7170)

• small length of coaxial cable braid

• RG58 coaxial cable (any length) and PL259 plug

• screws, nuts and miscellaneous hardware

  Many of the above items can be bought at hardware shops. The main exception is the wide-spaced variable capacitor.

These are almost unobtainable commercially, though you could try Daycom in Melbourne. Other possible sources include old high power transmitting equipment, hamfests and deceased estates. The exact value of the variable capacitor is not particularly important, provided it is at least about 400pF. The capacitor used in the prototype was a two gang 200pF unit with 2mm spacing between the plates. The gangs were connected together to provide the needed maximum capacitance.

If your attempts to obtain a suitable capacitor fail, there is always the possibility of making one. Full construction details appear in DK1NB’s magnetic loop design program (details later).

Construction

The first step in assembling the loop is to make the wooden cross that supports the aluminium element. This is done by bolting a 1.5m horizontal cross piece to the 1.8m vertical section. A white polyethylene chopping board is used for the antenna’s base. The two right-angled brackets are used to attach this to the vertical section. The next step is to bend the three lengths of aluminium so that they form a 1.5 metre square loop able to fit on the frame when bolted together. As is visible in Figure Two, two pieces are “L” shaped, while the other is bent into a shallow “U”. Note that the two L-shaped pieces are about 10cm apart at the top of the loop. These are physically joined by the bakelite insulation block that is attached to the top of the length of pine. The upper L-shaped pieces meet with the lower U-shaped piece at points ‘v’ and ‘w’. The overlap is about 40-50 millimetres. Make the electrical connection at these points as good as possible. To achieve this, sand the aluminium at the point of contact and use two or more small bolts to hold the pieces together. Use special conductive paste if available.The variable capacitor is mounted on a home made metal bracket so that its shaft faces downwards. To the shaft is attached a vernier reduction drive. Use either small brackets, fishing line or glue to fasten the frame of the reduction drive to the 1.8 metre vertical section. Note the thick, low-resistance conductors between the end of the loop and the tuning capacitors. Braid from a length of coaxial cable was used in the prototype. Make these connections short to minimise losses.

The loop is fed at the bottom. The braid of the feedline connects to the centre of the lower horizontal element (see diagram, point ‘x’). The inner conductor connects to the loop at point ‘y’ via a 900mm length of coaxial cable (inner and braid soldered together). At both ‘x’ and ‘y’, a small bolt, nut and eye terminal connector is used to make connections to the aluminium element. The distance between ‘x’ and ‘y’ and the length of the coaxial cable may both have to be varied for proper matching – this is discussed later.

Adjustment

The object of the adjustment process is to adjust the section between ‘x’ and ‘y’ until the antenna’s feedpoint impedance can be made to equal 50 ohms on the bands of interest. The first step is to connect the antenna to an HF receiver tuned to 7 MHz. Set the receiver’s RF and AF gain controls to near maximum and the antenna’s capacitor to minimum capacitance (plates fully unmeshed). Then gradually increase the capacitance. Not much will happen at first, but the noise from the receiver should gradually start to increase. Further adjustment of the capacitor will result in the received noise falling. Turn the capacitor back to the position where the noise peaks. Depending on the value of your capacitor, the plates should be around a quarter meshed at this point. This test confirms that the antenna can be tuned to 7 MHz.

Repeat the process for 80 metres. This time, the noise should peak when the capacitor is near maximum capacity. If it is not possible to obtain a peak, try setting the receiver to a higher frequency (4 or 5 MHz) and tune for a peak. If a peak is obtained there, but not on 3.5 MHz, it is likely that the variable capacitor’s maximum capacitance is too low for eighty metres. Possible remedies include substituting a larger capacitor, connecting high voltage fixed capacitors in parallel with the variable capacitor or making the loop larger.Having confirmed that noise peaks can be obtained on all frequencies of interest, it is now time to ensure that the antenna’s impedance is 50 ohms at these frequencies. This entails making adjustment to the antenna’s feed pont.The use of a resistive antenna bridge is recommended so that you can make antenna measurements without radiating a signal. If all you have is a conventional SWR bridge, make adjustments during the day to minimise the risk of interference to other stations. Position the antenna near its final operating position (which should be out of other people’s reach). Set your transceiver to about 3.580 MHz. Adjust the variable capacitor for maximum received noise. Transmit a steady carrier and note the reflected power or SWR. Adjust the transmitter up and down 40 or 50 kilohertz to find the precise frequency where the SWR is lowest. Note the reading at this frequency. If you are lucky, the reflected power should be nearly zero. Otherwise, adjust the length and position of the 900mm lead joining the feedline to point ‘y’ and/or the spacing between points ‘x’ and ‘y’. You will find that there is some interaction between these adjustments and the setting of the variable capacitor.

Every time a change has been made, adjust either the transmitting frequency or the antenna’s variable capacitor for the point where reflected power is lowest. Repeat these procedures until reflected power is either zero or close to it.

When making these adjustments, there is a temptation to leave the transmitter keyed while making changes to the antenna or adjusting the variable capacitor. This should not be done for two reasons. The first is that the voltages at the top of the antenna element can be quite high (hundreds or even thousands of volts) even with quite low transmitting powers. The second is that the loop is detuned when people are near it. Thus any adjustment made when you are near the loop will not be optimum when you move away. This effect is particularly pronounced on higher frequencies, and applies to metal objects as well as humans.

Once a length and position for the 900mm coaxial cable has been found, along with an appropriate spacing between ‘x’ and ‘y’, all further adjustments can be done with the antenna’s variable capacitor. Operating the antenna is described in the next section.

Operation

The Q of this antenna is very high. This means that it can only operate efficiently over a narrow frequency range (5-10 kHz typical). Almost every time you change frequency, you will have to change the setting of the variable capacitor.

As mentioned before, this is done by peaking the capacitor for maximum received noise at the desired operating frequency. If the reflected power is high, make further adjustments until it is acceptable. Again the use of a resistive-type bridge (rather than a conventional SWR meter) is preferred because of the ability to tune up without causing interference.

Note that the loop is directional, with a sharp null when the element is facing the direction of the incoming signal. This makes its behaviour different to that of full-sized quad elements, where the null is off the sides of the loop. This directivity can be useful when nulling out interference. It is also useful to remember when other stations report difficulty in hearing you – turning the loop may improve your signal.

Results

This loop has been used extensively on eighty metres. Most contacts have been made with the antenna indoors. Though performance is well down on a dipole, contacts into Western Australia and New Zealand have been made with it. The power used was twenty watts. Lower powers have been tried, but results have not been good. Contests are always good events to test the effectiveness of new antennas. During July 1997′s hour-long 3.5 MHz Australasian CW Sprint, twelve contacts were made with the loop. This was despite the added handicap of having to retune the antenna with every significant frequency shift.

As would be expected, the loop’s disadvantage when compared to full-sized antennas falls with increasing frequency. On 7 MHz for instance, the theoretical difference between the loop and a half-wave dipole is barely one s-point. Tests have confirmed the effectiveness of the loop on 40 metres, though all contacts have so far been within VK/ZL.

Improving the loop’s efficiency

The antenna described is capable of good results on 80, 40 and probably 30 metres. However, it is a compromise, designed for low cost and easy construction with basic tools. Doing any of the following will increase its efficiency and/or usefulness.

1. Use copper rather than aluminium. Copper is more conductive (but more expensive) than aluminium. This means that a version of this antenna using copper rather than the specified aluminium is likely to be more efficient than the prototype. Copper water pipe (the thicker the better) should be suitable.

2. Soldering the loop element directly to the variable capacitor will also improve performance and long-term reliability, especially if the antenna is used outdoors. The reason why this wasn’t done in the prototype was due to the difficulty in soldering to aluminium.

3. Use a single piece of metal for the conductor to reduce resistive losses. Where this is not possible, either solder/weld pieces together, or use conductive paste to minimise losses.

4. Make the loop a circle or octagon instead of a square. Square loops are the easiest to make, but cover less area for a given perimeter than other shapes. This lowers efficiency.

5. Make the antenna rotatable. The loop’s deep nulls can be used to advantage in nulling out interference from power lines, TV sets and other stations.

6. Use a larger loop. Efficiency increases rapidly with loop size. Even a 2 or 2.5 metre square loop should be noticeably more efficient than the 1.5 metre antenna presented here. The use of magnetic loop simulation software (see elsewhere) allows one to estimate the improvement possible by making this and other changes suggested above.

7. Use more reduction on the variable capacitor to make adjustment easier. The first prototype had only one vernier drive on the capacitor’s shaft. With this arrangement, getting the antenna tuned to the desired frequency was tedious because the tuning is sharp. If you routinely change frequency, a second drive is well worth the cost, particularly if 40 and 30 metres are the main bands of interest.

To perform this modification, install the two vernier drives in tandem, as shown in Figure Two. If the front drive contains a 0-100 dial, you may find that the knob is limited to three turns and the back part restricted to 180 degree rotation. To overcome this, remove the knob, unscrew the 0-100 dial, and remove the c-shaped bracket that is restricting movement.

Information about magnetic loops

All information used in the construction of the prototype came from the following Internet sites:-

http://ourworld.compuserve.com/homepages/csl/magloop.htm

http://www.gqrpclub.demon.co.uk/ants.htm

http://www.cdrom.com/simtel.net/msdos/hamradio.html Hans Joachim Kramer, DK1NB has developed a DOS computer program useful for those who design magnetic loops. Able to calculate efficiencies and bandwidths, this freeware program also contains much useful constructional advice (including pictures) to assist those who experiment with magnetic loops. This excellent program (mloop31.zip) is available from the last mentioned site on the list above

2M + 70cm Open Sleeve Vertical Dipole

Johnny Pedersen (LA3AKA)

2m/70cm Open Sleeve Vertical dipole

I was playing around with the MMANA Antenna analysis software and wanted to design a 2m/70cm vertical antenna. I tried different antenna models, J-pole, half wave dipoles, Ground Planes …. I then remembered a chapter in the 18. edition of the ARRL Antenna Handbook covering Open Sleeve antennas to make Broad band antennas. I thought that this might be useful for making a dual band VHF/UHF antenna.

The Antenna is planned built using 6mm aluminium rods. According to MMANA this will with a distance of 3.2cm between driven element and sleeve elements give an Feedpoint Impdance of 75 ohms on 2 meter and 50 ohms on 70cm. One nice thing with this antenna is that you get a good gain on 70 cm (approximately 3dB over a g Gain:

Band Gain Takeoff Angle

2 m 6.7 dB 3.9^o

70 cm 11.0 dB 1.4^o

The Centre Frequencies for the antenna shown is 145 MHz and 435 MHz

WB5CXC 2 Meter Vertical Moxon

 

Here is some data on a 2M vertical polarized antenna I built this week. I'm not sure about the gain but the front to back ratio is very good and conforms to the model. Using the local repeater as a signal source I measured the F/B ratio using an ICOM IC-746PRO. With the attenuator active the reading is S5+ and with the antenna rotated around the reading is not registering on the S meter. I made the antenna out of 1/2" PVC and #6 copper ground wire. I also made a 10M Moxon out of PVC and wire. Haven't got it installed yet. When I install it this week and have some data, I'll send you pictures etc.

Antenna pattern - Blue trace is the design @ 146 Mhz and Red trace is at the upper limit of 148 Mhz.

Gain and SWR plots.

Diagram of antenna.