Heinz and Chris
Of course you are right… I mixed the principles of 1/4 and 1/2 lambda… of course it doesn’t work that way.
73 Armin
Not exactly that, but something similar.
I used a 6m tall mast which is definitely not tall enough. The antenna was configured as inv-L and I used a 110uH inductor that I had with me for my 80m EFHW configuration (20m wire - 110uH coil - 2.1m wire).
For 160m the configuration was: 20m wire - 110uH coil - 10.6m wire. The sag of the horizontal part was pronounced and therefore the SWR was not that great (maybe just below 2) and the bandwidth pretty narrow. A 18m tall mast would be definitely better.
It worked somehow, even it was still daylight, and I made at least one contact, but the efficiency was probably pretty low.
Don’t worry about the counterpoise, a wire of about 8m length will be just fine.
The coupler needs more attention. I would go for at least a 1:64 or 1:81 coupler with enough primary inductance.
Antenna experiments are fun!
73 Stephan
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@Heinz and Stephan
Maybe I didn’t express myself clearly enough. My consideration relates to an end-fed antenna with a 1:49/64 transformer, which practically does not require radials.
With a perfect counterweight, a radiator of any length can theoretically be tuned to any frequency using an appropriate extension coil. The coil should have the highest possible quality (q>>100). In addition, the impedance drops from the usual 37 ohms to values of up to a few ohms and there are reactances that have to be compensated.
The radial of the AX1 represents one part of the dipole with 1/4 wavelength, the Telescope whip with the extension coil represents the other. The radial therefore contributes a very significant proportion of the radiated energy. This also explains the very good effectiveness of the antenna in rocky high mountains. The rock acts as an insulator and the power emitted in the radial is not absorbed in the ground.
73 Chris
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I was reading an old issue of sprat (issue 185) the other evening. There was an article about a vertical for topband. The author made a helical with a 30ft fishing pole and 273ft of wire wound round it.
Not exactly portable, though no radials, just an earth spike.
Article was by David G4LDT
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James,
Just want to tidy up a couple of points.
If you have an antenna that will be an electrical half wave but is barely physically a quarter wave you should put the loading coil near or at the centre.
End placement is the worst place for a loading coil on a half wave.
Adding capacitance hats at both ends will reduce the coil size and increase efficiency.
Inductors are currenty things
Capacitors are voltagy things.
So put them where the max current or max voltage is expected respectively.
Shortening an antenna not only reduces it’s efficiency but also it’s bandwidth. Prune carefully.
The advantage of a half wave is that you do not need an extensive counterpoise to get a good match and avoid common mode currents on the feeder.
On the other hand if you can create something approximating a infinite ground plane you will pick up some gain due to the image and low ground losses.
Let us know what you finish up with.
73
Ron
VK3AFW
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