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The Portable HF Antenna. What Makes A Physically Small Antenna Work Right For Two Way Radio?

A portable hf antenna is usually a compromise antenna, because resonant transmitting antennas for the HF frequencies can be pretty large if they are made full-sized.

For example, a full-sized half-wave wire antenna for the ham radio 80 meter band would be a full 40 meters long, or about 135 feet in length. And for maximum efficiency, the ideal height for such an antenna would be around 60 feet high in the air, if you plan on having two-way communications with long distance radio stations (DX signals).

A vertical antenna for 80 meters would be a quarter-wavelength high, which works out at 20 meters or around 65 feet high. And that doesn't include a set of radials (or counterpoise wires) around a quarter-wavelength long, radiating out from the base of the vertical antenna like the spokes on a giant bicycle wheel.

That's an awful lot of real estate needed to erect those full-sized HF antennas. Even a lowly 27 MHz CB radio antenna is going to be an eyesore. A quarter-wave CB ground plane antenna is almost 9 feet tall, while a half-wave vertical CB antenna will be close to 20feet in length. If you put one of those on your house the neighbors are pretty sure to notice your new interest in two way radios!

So how can people work their radio equipment as a portable station, or installed in a car or truck as a mobile station? The answer is a portable hf antenna that is physically smaller than a full-sized aerial, but which can still radiate enough of a radio signal for you to be heard out there.

The military use portable hf antennas all the time, so do organizations like the United Nations and the International Red Cross who need two-way radio communications in places where there are no cellphone towers. The same goes for many non-governmental aid organizations around the world, and for licensed Amateur Radio operators in most countries. Satellite phones may or may not be available but they are very expensive. That's one reason why HF radio is often called the poor-man's satellite phone!

The portable hf antenna is kept smaller than full-sized by having a loading coil or tuned circuit that makes the transmitter think it has the right antenna attached, so it can pump out its full rated power into the aerial. Loading-coils can be at the base of an antenna, in the center or at the top. These are called base-loaded, center-loaded or top-loaded antennas, and on vehicles are usually known as whip antennas, because they are bendy and whip about as you drive. There are also helically-wound antennas, where a wire is wound round and round a fiberglass whip or sometimes a big plastic tube.

Many military or four-wheel drive vehicles with HF radios get by with a 15 foot whip antenna fixed on top of an aerial tuning unit. While on fighting ships, you are likely to see the same kind of arrangement where a 30 foot whip sits on a one foot tall insulator and a large automatic tuning box.

These antenna tuners are placed at the base of the compromise HF antenna to make it resonate at the frequency the radio will transmit on. That too is a portable hf antenna, even though it is a big one.

Portable HF radios used by soldiers in the field have their own antenna tuning unit built-in, and these can make even a modest whip antenna seem right to the radio. While radio hams working portable ops often throw a simple wire dipole into the trees or a use a wire doublet antenna with a small tuner.

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