## Sunday, July 6, 2014

### Small loop antennas (magnetic loops)

Built well, these small antennas perform really well.

Built well, only the bandwidth suffers as a result of miniaturisation.

• Top left: 80m loop [3 x 1.5 x 0.8m], using 15mm copper tube.
• Top right (and detail): 80 and 40m loop [1.8m x 1m x 0.8m], using 1m wide 1mm thick aluminium sheet.
• Bottom: 160 and 80m loop [octagonal with 1.5m sides], using 28mm copper tube.

All these loops have integrated resonating capacitance. In the case of the sheet aluminium loop (top right) this is achieved by overlapping on one of the longer sides (see detail), with 8 or so nylon M6 bolts keeping the spacing resonably precise.

Coupling into the loop can either be by using a small loop within the main loop, or as shown, by tapping around the loop(via an isolating transformer, to allow the loop to float relative to ground). Using the tapping method gives a more obvious idea of loop impedance, and so has been used here. Calculating the capacitance value is easy enough: C(pF) = 0.0885A(square cm)/d(spacing in cm), giving 4425pF at 7MHz and 16 594pF at 3.7MHz. Ignoring effects due to the capacitors finite length, this corresponds to reactances of 5 ohms at 7MHz, and 2.72 ohms at 3.7MHz.

Assuming a constant current around the loop, the voltage across the capacitance is 5/1.45 x that across the feedpoint, which is 50 ohms. Thus for 100W drive, V = 70.7 x 5/1.45 = 244v. This gives the following loop current:

7MHz = 244/5 = 48.8A 3.7MHz = 244/1.7 = 100A

High currents are inevitable since the loop is not only much shorter than a quarter wave monopole, but being in phase, the two verticle section (say) currents are also in opposition (the radiatedsignal being a result of the small though finite difference between path lengths).

Return loss for loop tuned to 7MHz [-25dB at resonance]

Return loss for loop tuned to 3.7MHz [-32dB at resonance]

From these plots, you can see the 3dB bandwidth points (ie 6dB return loss) are 120KHz for the 7MHz antenna, and 30KHz for the 3.7MHz one. For solid state transceivers that have no output match tuning, this corresponds to an un-retuned operating range of about 60KHz at 7MHz (wide enough for the entire UK ssb segment) and 15KHz or so on 3.7MHz.

Using a remote tuner, these narrower figures could be trebled without incuring too much loss, but it would be sensible to use a coupling loop, not a ferrite transformer, if this is contemplated. A coupling loop circumference of 2.2m gave a good 50 ohm match when tried.

I tried to reduce the capacitor gap down to 2mm or so in order to resonate the loop on 1.94MHz, but even with a 1mm foam insulation, there was too much uneveness, and 2.45MHz was as low a frequency as could be managed,producing a 13KHz bandwidth as shown below:

I found 3mm foamed PVC ('foamalux') noticeably lossy - the bandwidth being 25KHzwhen tuned to to 2.45MHz (the 1mm foam used successfully was of unknown material).

This loop seems to work well enough on 3.7 and 7 MHz.

Notes on copper tube loops

With these, the trick is to implement the capacitance by reducing the tube diameter (from 28mm to 15mm, say) at one end of the loop, and putting a suitable length of this reduced diameter tube down the centre of the other (full diameter) end of the loop. So for example, the octagonal loop with 1.5m sides requires about 700mm of 15/28mm coaxial capacitance to resonate on 3.7MHz.For this particular antenna, I paralleled up six of the 1.5m lengths for the far side vertical (see pic) - all done using soldered copper fittings (except for the inners which used compression joints so as to be removable). With this arrangement, the loop easily tunes down to 1.9MHz.

That there are two closely spaced loops making up the octagonal antenna was done in attempt to reduce copper losses (prices for copper tubing greater than 28mm dia being very uncompetitive [in the uk, anyway])

The rectangular tube loop has four single loops in parallel, and uses 15mm tuning. The wide spacing was an attempt to reduce inductance and thus Q. Each of the four loops has a top horizontal that is actually coaxial (28/15mm), in order to achieve resonance. Individual capacitance also garrauntees equallity of loop currents. This antenna worked very well on 3.7MHz, but be warned, 15mm tubing is not very strong, and the loop will collapse under its own weight

### Make a magnetic loop antenna for 7-21 mhz

• Magnetic Loop Diagram

• Magnetic Loop antenna

This antenna has several advantages, not least being only 1 metre diameter! This loop relies on being horizontally polarised and receives only the magnetic wave, thus as most noise in the domestic environment is vertically polarised and electrical wave, it delivers low noise to your transeiver/receiver, which makes for nice clean listening. In addition any signal arriving in the direction of the loop end on will be nulled out, this can be useful to get rid of an interfering signal by simply rotating the loop leaving the desired signal in the clear. It can be used indoors with ease and works well at ground level which is not the case for long wire/dipole antennas at shortwave wavelengths.
So what are its disadvantages? Well its tuning is critical, such that for a small change in frequency the antenna will need to be retuned at the loop end. This is even more important for transmitting where a high reflected wave (swr) due to not being tuned correctly will damage the output stage of your transmitter! In addition due to the very high "Q" of the loop, very high voltages can build up on the loop tuning capacitor even with low amounts of power from your transmitter. It is for this reason I recommend this loop is used with a transmitter of no more than 8 watts, any more and the ordinary broadcast tuning cap will arc over with spectacular results. Of course should you wish, a higher spec/bigger air spaced tuning cap would allow higher power output transmitters to be used. Also I consider the use of remote tuning using a fairly high geared motor and insulated coupling on the tuning cap essential. For shortwave listeners manual tuning would suffice.
In setting up the tuning of the loop, connect to a receiver and tune to 14 mhz. Now tune the loop which as it nears peak tuning will cause a whooshing sound. Stop the tuning you should now hear good strength signals in your receiver. For tuning for a transmitter, 1st use receive method then apply low power and fine tune loop tuning and tweak gamma match for lowest swr.

Magnetic loop dimension details

• Diameter of loop 1000mm
• Diameter of tube 15mm
• Width of base 780mm
• Diameter of support pipe 42mm
• Loop end spacing (for tuner) 50mm
• Height of support 1590mm
• Nylon board 210x240mm
• Nylon board 240x70mm
• Gamma match width 310mm
• Gamma/loop spacing 110mm

Construction Tips

• Use a bicycle wheel with no tyre on to help form the curves of the soft annealed copper tube
• Clean the tube with wire wool before any soldering
• Use a 100 watt soldering gun for the joints, but use a small blow torch first to get the copper at temperature to take a joint
• Force some timber with the corners planed off down the plastic plumbing pipe this will stiffen the pipe as the loop is quite weighty
• Use inverted shelf brackets to support the mounting pipe and make a wooden frame wide enough to hold up the loop

### 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.