15 May 2013

EFHW Coupler experiments contd/

I erected a wire in my garden that is exactly 66ft and 8inches long. The shape of this wire is an inverted U. This is also known as a HALF SQUARE. Theoretically this resonates at 7020Khz although in practice it is probably affected by the surrounding trees etc. I have not tested the actual resonant frequency of the wire. I guess I can do by coupling it at the end in some way into my antenna analyzer and using it as a grid dip oscillator. I then coupled it to my T-200-2/RG58U coupler. The Toroid has 18Turns of large stiff wire on the Secondary and the link has about 2.8 turns of the same stiff wire. The Link is not connected to the secondary since I wanted to experiment with a counterpoise. I connected the antenna analyzer to the link coil and was able to tweek the windings on the toroid to obtain resonance on the target freq 7020Khz. I did not find it necessary to change the coax length. Resonance was observed by a slight dip in the SWR meter.

I then connected the counterpoise to the cold side of the toroid winding and by unwinding a length from the coil I was able to observe a change in the SWR as it dropped lower. I noted that the length of the counterpoise did not materially affect the resonant point. Only the SWR. The SWR continued to decrease as I rolled out the wire on the lawn. I placed the coupler and the analyzer on the lawn and on the garden table. In both cases I was unable to detect any difference in readings.

When I rolled out approximately 21 foot of counterpoise I noted that the SWR dropped to 1:1. Upon rolling our more wire I noted the SWR begin again to increase. The optimum point was quite sharp to note. So what is going on?

The counterpoise length has a major impact on the SWR but only minor impact on the system resonant point.    Looking at Fig 3 of the AA5TB graph it doesn't make total sense although the reactive component of the model does stay approx constant between .05 and .45 wavelength at 1000ohms reactive. Perhaps this was the part of the curve I was operating in. The optimum counterpoise length seems to be about .2 wavelength with this set up.

So what? This looks like a practical setup. Can it be repeated in the field with different wire configurations? This needs to be tried next on my next field trip. If the coupler can be adjusted to resonance by tweeking the turns and if the SWR can be adjusted by rolling out an optimum length of wire then we have a practical system that can be deployed and that can be used for high power. This system does not rely on an expensive transmitting capacitor.

Next in my garden I will try to see if I can get the system to stabalize by using a counterpoise of 6ft 8inches. According to the AA5TB figure 3 model that length is the length where zero reactance ocurrs. This is the 0.05 wavelength point. What procedure would I follows?

  1. Attach a 6ft 6inch counterpoise
  2. Bring the system to resonance on 7020Khz by tweeking the toroid coils on the toroid.
  3. Tweek the link coupling for lowest SWR. (This may be easier said than done but let's try).
  4. Check to see if touching the counterpoise or the coax feeder that the system is stable.

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