A portable, light-weight antenna that is easily switched between 10-12-15-17-20m using wire-jumpers and plug-in capacitors.
Thinking about antennas for travelling, I was wanting to build something very portable and versatile that I could pop in a suitcase or in the back of the car whenever I’m on the move. Inspired by articles written by Steve Nichols, G0KYA, I embarked on building a half-wave end-fed matching box that can be easily switched from band to band. The result is a light-weight, ‘fast QSY’ solution with low SWR on each band.
Steve’s article provides all the instructions and measurements for the matching box and antenna: http://www.infotechcomms.net/downloads/Endfed_halfwave_dipoles.pdf
The difference with my matching unit is that the coaxial capacitors are ‘plug-in’ – basically I have put an extra SO239 socket on the matching box so I can plug in the correct bit of coax for the band I’m on. The wire antenna has jumpers to select the correct length and is tie-wrapped to a fibreglass pole which can be dropped in a few seconds to change the jumpers.
An antenna analyser is essential for this exercise – having built the transformer with the suggested turns ration of 2:13, I initially tried to match an end-fed half wave on 10m (wire length = 5m). I plugged in a 15cm long piece of coaxial-capacitor and started snipping away to bring the antenna to resonance. Unfortunately I just couldn’t get the resonant frequency high enough before I had no coax left, thus I decided to take a turn off the secondary transformer winding. Although the length of the resulting capacitor at resonance is much shorter that Steve’s values, it works fine. So – I ended up with a transformer turns ration of 2:12.
Unlike Steve, I used some insulated, stranded copper wire for the transformer windings – I’m not sure what the pros and cons are, but it seems to work OK. The ground socket is from Clas Ohlson: http://www.clasohlson.com/uk/Ground-Screw/31-4569.
For all-band selection between 10m and 20m, four jumpers are needed to extend the antenna and five plug-in coaxial-capacitors are required. Note that I have followed Steve’s advice and connected two 5m long pieces of wire to the antenna ground, to act as a counterpoise.
Having built the antenna for 10m initially, I then put the coax capacitor to one side and started with another one to tune on 12m. I used crimp-connectors on the antenna wire to extend it, making it a half-wave on 12m. The coaxial-capacitor was then trimmed for resonance and thereafter, the exercise was repeated for each band – each band having it’s own plug-in piece of coax and additional piece of wire to connect to the antenna.
|Band||Antenna wire length (m)||Capacitor length (cm)|
|Transformer Turns Ratio||2:12|
This set-up is great as a portable take-anywhere solution. My next experiment will be try the antenna without the fibregalss pole – end-fed from the house, out of an upstairs window. I’ll post result here when I get round to it.
John Warburton G4IRN 17 May 2014