For my handheld QRP transceiver I have developed a rod antenna for outdoor use when cycling or hiking. In this article I’ll first give the reader some basic considerations on shortened antennas and afterwards the pracital consequences of these.
A full-sized vertical antenna usually has got a length of a quarter wavelength. Some special constructions of a 1/2, 5/8 wavelength and others are also commonly known. But for a rod antenna the basic construction is a 1/4 wavelength based circuit. Full-size 1/4 wavelength antennas that have the correct length (1/4 multiplied by 0.95 shortening factor) have got a feed point impedance of between 40 to 50 ohms. So they can be directly connected to a 50 ohm coaxial cable. The problem: For 14 MHz this antenna would have an overall length of about 5 meters. Not very appropriate for a handheld transceiver except you were Arnold Schwarzenegger.. 😉
When the mechanical length of an antenna is shorter than 1/4 of the wavelength that it is desired for, a mismatch can be obeserved. The antenna not longer is resonant to the operating frequency. An additional capacitive reactance appears. This must be compensated by an inductive reactance. Therefore shortened antennas have an integrated coil either at the bottom (bottom loading coil), anywhere in the middle (center loading coil) or top (top loading coil).
Another problem must be faced: Feed point impedance is usually much lower than for a full size antenna. Sometimes only between 10 and 15 ohms. Thus some sort of impedance matching circuit must be integrated, too,
Last, degree of effiency will also decrease. So, don’t expect too much from a shortened antenna!
To sum up the things that have been mentioned before: We have to face 2 general electrical problems with short antennas:
- A correct loading coil must be found, and
- proper impedance matching must be performed.
Into my antenna I integrated 2 loading coils. One at the bottom to serve as an impedance matcher and one slightly below the center to compensate capacitive reactance.
The antenna is mounted to a standard male BNC-connector.
Mechanical deimensions and coil data:
Bottom coil (L2):
L2, the bottom coil, is 50 turns of 1 mm diameter enameled wire wound on a 8mm diameter plastc tubing from the local hardware store. From the bottom end of L2 a 120 pF capacitor is lead to the ground potential of the BNC plug. This, together with L2 sets up low pass filter serving as an impedance matcher.
Edit: Another impedance matching circuit I’ve described here.
The plastic tubing used for L1 and L2 is hollow. Into this tubing a 6mm diameter aluminium rod fits in exactly. The length of the rod between the two coils is 45 cm. Following is another piece of the plastic tubing carriyng L1.
Center coil (L1): L1 is 45 windings of 0.6 mm diameter enameled wire.
The top rod of the antenna is a telescopic antenna with an overall length of 120 cm that can be bought on the internet. It’s outer diameter is also 6 mm, so it also fits into the plastic rod.
Here are the pictures to make clearer how the antenna is constructed:
To increase the usually low degree of efficiency of such an antenna I have manufactured a sort of “counterpoise” that represents the 2nd part of a dipole : A 5 meter long insulated cable with a large crocodile clip that is clipped to the ground potential of the metal BNC connector.
Edit: In the meanwhile I have used this rod antenna for 14 MHz / 20 meters several times during outdoor activities. What I’d never have believed: I could make lots of QSOs with it! Particularly if you are in a high place (e. g. on a look-out) you can work distances of about 1000 to 3000 kilometers right with 4 watts out of your hands. Provided the station you’re answering is strong. Then there is a realistic chance that he might hear you with reasonable signal strengh.