Thursday, May 29, 2014

HF Antennas

At HF, antennas can be fairly large. An effective antenna is usually at least 1⁄4-wavelength in some dimension. On 40-meters, for example, a 1⁄4-wavelength vertical antenna is a metal tube or wire 33 feet high. At the higher HF frequencies, antenna sizes drop to 8–16 feet, but are still larger than even a big TV antenna. Your physical circumstances have a great effect on what antenna you can put up. Rest assured that a large variety of designs are available to get you on the air.

Wires, verticals, and beams are the three basic HF antennas used by ham radio operators all over the world. You can build all these antennas with common tools or purchase them from the many ham radio equipment vendors.

Wire antennas

The simplest wire antenna is a dipole, which is a piece of wire cut in the
middle and attached to a feed line,  The dipole gives much better performance than you may expect from such a simple antenna. To construct a dipole, use 10- to 18-gauge copper wire. It can be stranded or solid, bare or insulated. When completed, its length should be:

Length in feet = 468 / frequency of use in MHz

This formula accounts for a slight shortening effect that makes a
1⁄2-wavelength of wire slightly shorter than a 1⁄2-wavelength in air. For example, a dipole for 21.1 MHz is 468 / 21.1 = 22.2 feet long. Allow an extra 18 inches on each end for attaching to the end insulators and tuning and another foot (6 inches × 2) for attaching to the center insulator. The total length of wire you need is 22.2' + 18" + 18" + 12" = 26.2'.

click to enlarge
To assemble a dipole, follow these steps:

1. Cut the wire exactly in the middle and attach one piece to each end
insulator, just twisting it back on itself for the initial check.
2. Attach the other end to the center insulator in the same way.
3. Attach the feed line at the center insulator and solder each connection.
4. Attach some ropes and hoist it up in the air.
5. Check the dipole.
6. If the SWR is low enough at too high a frequency or is lowest at the
high end of the band, loosen the connections at the end insulators and
lengthen the antenna by a few inches on each end.
7. When you adjust the antenna length so that the SWR is satisfactory,
make a secure wrap of the wire at the end insulators and trim the

If the frequency of lower SWR is too low, shorten the antenna by the
same amount.

Make some short, low power transmissions to measure the SWR
(standing wave ratio) as explained in your radio’s operating manual.
The SWR should be somewhere less than 1.5 to 1 on the frequencies
you wish to use.

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