Portable Half Wave Wire Antenna
One of the challenges of working portable QRP from field locations is getting an effective antenna. A half wavelength end-fed wire is a lightweight trail friendly antenna system that is both easy to erect and works reasonably well in the field.
I take no credit for the design of this antenna or the tuner /swr indicator devices included in this antenna system. Instead, I have taken information published to the web on the Adventure Radio Society and other amateur web sites to build a simple, easy to duplicate antenna that I have used many times from portable locations with good results. Credits and links for the authors are listed below.
This antenna is nothing more than a half wavelength of wire for the desired band that is installed vertically using whatever support is available. In the field, I use a slingshot to shoot a string over a convenient tree and pull up the wire. A quarter wavelength wire is placed on the ground or run through the bushes to act as the counterpoise. Only one support is needed for this antenna and it can be erected by one person in just a few minutes.
The feed point impedance of a half wave end fed wire is several thousand ohms, so it can't be connected directly to the 50 Ohm transceiver. However a simple LC tuned circuit can be used to transform the high impedance at the end of the antenna to a low impedance that the rig will be happy with.
I built a compact LC tuner and SWR indicator in a Radio Shack 270-1801 project box that makes the rig connection and tuning easy. Since I like to work both 40 and 20 meters, I constructed the LC tuner with a tapped inductor and a band switch to permit operation on both bands.
Inside this tiny box is both the dual band LC tuner and a SWR bridge. The tuner uses a toroid inductor and a little trimmer tuning capacitor. The trimmer cap is adjusted with a tuning tool through the access hole below the red binding post. The SWR circuit is a resistive bridge and uses a LED for the indicator.
I also included a toggle switch and an auxilary BNC jack that permits the SWR bridge portion of the circuit to be used to feed other antennas without the LC tuner.
The resistive SWR bridge circuit was copied from Richard Fisher KI6SN's article entitled A Bright Idea for Wrestling With SWR in the Field published in the September 1998 issue of the ARS Sojourner. It is based on work by Dan Tayloe, N7VE, and Charles Lofgren, W6JJZ and is a very simple way to determine when SWR is reduced to a minimum. It uses a bridge circuit with three legs composed of 50 Ohm resistors and the antenna load as the fourth leg.
When the antenna load impedance is less than or greater than 50 Ohms, the bridge is unbalanced and a voltage appears across the center of the bridge. This voltage is stepped up by the transformer and lights the LED. When the antenna impedance equals 50 Ohms, there is zero voltage across the center of the bridge and the LED is extinguished.
Each of the three 50 Ohm resistors are actually two 100 Ohm half watt resistors in parallel to increase the power handling ability of the bridge. The transformer is 5 turns and 20 turns of #24 wire on a T50-2 toroid core. The LED is Radio Shack stock number 276-309, a clear lens low voltage LED. The diode should be a 1N34 but I used a 1N914 in my version.
The LC tuning circuit and half wave antenna system was inspired by Bill Jones, KD7S's article Keeping in Perfect Tune: A Film-Can Transmatch in the January, 2000 issue of The ARS Sojourner. Bill's tuner was designed for 20 meters. I added some additional turns of wire on a T50-2 toroid and a switch to make a two band unit for 40 and 20 meters. I used 4 turns on the primary and a total of 45 turns of #26 wire on the secondary with a tap at 27 turns above ground. The tuning cap that I used was a small trimmer cap with an unmarked value which I am estimating at approximately 90 to 100 pF maximum capacity.
Operation is simple. Connect a half wave wire to the red antenna connection and a quarter wavelength wire to the black counterpoise post. Place the Tune-Operate SWR bridge switch to the Tune position and apply RF to the Rig connection. Rotate the trimmer cap for minimum illumination of the LED. Flip the Switch back to Operate and you are ready to go on the air.
Note that even though the LC tuner is a dual band device, it can not be used on both bands with the same length of antenna wires. The length for the antenna and counterpoise wires are as follows:
The formulas for calculating the lengths are:
1/2 wave wire = 468/Frequency in MHz
The inside view of the homebrew bridge/tuner shows the point-to-point wiring. The trimmer capacitor is soldered to a scrap of dual side PC board which has been fastened to the Red antenna post. The back side of this PC board has been scored to divide it into two areas and the trimmer cap has been soldered between those two lands.
Another small scrap of PC board material is glued to the center of the aluminum plate to act as a ground soldering point.
Enamel wire was used for the LC tuner toroid secondary winding and plastic covered wire was used on the primary and for both windings of the SWR bridge transformer.
My thanks to Richard Fisher, Bill Jones and the Adventure Radio Society for publishing the ideas which led to the development of this trail friendly antenna system.
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Updated July 29, 2011