A portable, light-weight antenna that is easily switched between 10-12-15-17-20m using wire-jumpers and plug-in capacitors.

End fed half wave antenna matching unit (G4IRN)

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.

 

Construction Notes

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.

Tuning

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.

Final Measurements

 

Band Antenna wire length (m) Capacitor length (cm)
20 10.05 56.0
17 7.87 28.5
15 6.72 17.0
12 5.72 7.5
10 5.00 4.8
Transformer Turns Ratio 2:12

Next Steps

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.

Gallery

 

 John Warburton G4IRN 17 May 2014