18 March 2014

Matching efficiency tests for a W3EDP antenna

Today I conducted some interesting experiments on my 84 ft end fed antenna. Also known as a W3EDP antenna. Based on recent signal reports I have felt that this antenna does not perform as well as an end fed halfwave on 40m.

I replaced the homebrew coupler (see schematic below) with my Emtech Z Match. I was able to easily load the antenna against a 17ft or a 33ft counterpoise respectively, laid out on the lawn. These are insulated wires.

I received good reports from ZS4SF in Welkom on several tests throughout the day. All 599+ reports.
On swapping between the two tuners I consistently received better reports on the Z match with marginally better reports using the 17ft counterpoise.

I then set up my power meter at a distance of about 6 ft from the antenna. I inserted a 3ft piece of wire into the RF socket of the Power Meter to act as an antenna. In effect this is a very sensitive field strength meter. The meter certainly registered 'background' RF that was approximately -60dBm. A preselector tuned to 7020KHz would be an interesting experiment.

I then connected the MFJ259B to each coupler in turn and switching between the two different counterpoises.  The MFJ generates a signal at 7020KHz of +6dBm. This registered above the noise on the Power meter by a good margin (not measured) and enough to conduct the planned tests. By adjusting the tuner, I was clearly able to see the power meter detect increased  power as the tuners were matched and the SWR was 1:1. It is quite interesting to see how the radiated power drops off significantly as the SWR exceeds 3:1. Thus I was able to peak the radiated power in line with a 1:1 SWR on the Power (Field Strength) meter.

I determined that the Z Match with the 17ft counterpoise was in fact generating a measured 7 dB greater power into the antenna when compared to the home brew coupler connected to the same counterpoise.

I then spent time adjusting the turns on the home brew coupler coil. It appeared that more efficiency could be obtained by increasing the number of turns on the coil from 7Turns to 13Turns. Resonance was then obtained with the capacitor at almost it's minimum setting. Link turns were adjusted to achieve a 1:1 SWR. However I was unable to achieve the radiated power output level seen when the Z Match was used..

Further experimentation needed.

1 comment: