2007-03-11
End-Fed Half-Wave Antenna & Tuner
For the on-going 2007 ARNSW Homebrew Group Challenge work I needed a simple 80 metre antenna. As a home-unit resident the size of a full half-wave is pretty impractical, but I decided to try it so I had a base-line to compare future shortened verticals to.
While centre-fed would be the simplest to get going and probably the easiest to install in a semi-permanent manner, I went with end-feeding. I would like to have the option of using the antenna portable, where a transmission line is just something else to carry and end-feeding is probably the easiest to set-up.
I read everything I could on the subject, the ARRL antenna book is good, but I found Steve Yates AA5TB's website especially useful. Steve explains the physics of the counterpoise well, and gave me confidence that it would indeed work as I had planned without stray-RF problems.
The Tuner
A matching box was constructed, containing both a matching circuit and a resistive 50 Ohm VSWR bridge that borrows heavily from Dan Tayloe N7VE (et al). The matching inductor value was picked to resonate the available variable capacitor gang just below 80 metres at maximum capacitance. The Hi-Z side floats to make it more versatile, a simple clip lead can be used to return the counterpoise side to the coax braid, if so desired.
The final device tunes a resistive load of 3-7 kilo-Ohms from 3.2 MHz to 12.6 MHz. 40 metres is covered (and 30 metres too, but I lack a transceiver for it currently), it is unfortunate that 20 metres could not be covered as well, without switching out some of the windings. W0CH switches his inductor taps, but this also changes the impedance match too much for my taste (Note that you can compensate somewhat for a few kOhms of resistive mismatch and an undetermined amount of reactive mismatch with the tuning). His circuit is otherwise very similar to mine (and just about everyone else's who has an internal bridge).
The unit was built in a small plastic box, and it was quite a squeeze to fit the tuner and the bridge inside. The resulting unit is very small and should work fine in portable operations. The tuner can be seen running about 1 Watt CW from the challenge transmitter into a 4k7 dummy load and a neon bulb which is getting stinking hot!
This gross abuse of a Neon bulb shows the voltage step-up of the tuner and proves it can feed a high impedance load fairly efficiently.
The Antenna
The radiator itself is simply a half-wave long piece of wire. I used an unzipped length of "zip cord", a full free-space half wave length (about 41.6 metres) which makes the antenna a bit reactive, but this is easily tuned out. Managing the wire while it was measured and unzipped was actually one of the more challenging parts of the project, but with the help of the XYL and the parents it took only 20 minutes or so. Black plastic egg insulators from the WICEN stand at Wyong were strung on the wire, two fixed at each end and two floating to allow various geometries for deployment. Banana plugs were added to each free end of the wire, so either might be used as a feed point.
Testing
After some trial and error it was found possible to just fit the antenna's enormous length on my property. The centre was strung up at my 3rd story bedroom window and the ends sloped down to the extreme boundaries, one being the car park railing at the edge of a cliff, the other being the adjoining property's fence. The car park end was the most practical to feed from, so I took some gel-cells, a few radios and a fold-up chair down to the cliff edge and set-up there.
The antenna works much better than any I've used before. The inverted-V configuration may have something to do with this. I used the railing around the car park as a counterpoise, but it was possible to run using just the radio and its coax as a counterpoise too, by clipping the counterpoise connector to the coax socket outer with an alligator clip lead. It was very easy to tune in either way, on both 80 and 40 metres. The ARNSW morse practice beacon was full-scale on my VR-500, were normally is is barely S1 on my north-south horizontal 40 metre dipole. With a homebrew 80 metre transceiver I could hear lots of DX and local QSOs, and I listened to the WIA broadcast using my 80 metre VXO receiver for the first time at a good signal level. As the sun went down the mosquitoes got too bad to stand any longer, so packed up and went back indoors.
I had to take down the antenna, it is simply too big to leave up all the time, if I want to keep the neighbours on side anyway. The car park end is also a bit of pedestrian hazard, so this antenna will only be going up for special occasions. The experiment was otherwise a complete success, and I am very impressed with the antennas performance. I also now have good figures to shoot for with a shortened vertical.
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