12 June 2009

EFHWA tuner quirks

For the upcoming Summer holidays I decided to try something new: an End Fed Half Wave Antenna (EFHWA). Since all operating conditions will be known (same place of last year), I should be able to compare two different antennas.

I chose the EFHWA because it is reported to have a decent radiation angle even when low above the ground. This antenna has a high impedance between 2500 and 5000 ohm depending on the way it is installed, so it requires a matching device.

Given I will be using QRP, I built a transformer on T50-2 code with 3:27 turns ratio. This 1:81 step-down impedance transformer matches 50 ohm to ca 4000 ohm. With a polyvaricon capacitor it tuned between 6.2 and 22 MHz.

Then I reduced the turns ratio to 3:25, so that it would match a 3300 ohm antenna, in between the EFHWA expected Z ranges. This increased the resonance to 9-26 MHz since the secondary inductance had reduced. Good 30 to 12m coverage, I wouldn't be able to throw a 20m of wire for a 40m EFHWA anyway.

Ready for boxing the tuner... surprise! The tuning range has now shrinked to 11-26 MHz! I have lost 30m when fitting it into a plastic box. This means that the maximum capacitance or inductance available has reduced (I did shorten wires of 2-3 cm if that matters), or both. Need to get back to it and try something different.

For those interested: I found out the tuning range using an antenna analyzer (MFJ259B) and connecting a 3k3 resistor in place of the antenna. Gave full capacitance and looked for the SWR dip with the 259. Same at minimum capacitance.