[Elecraft] AX-1 -- with and without counterpoise

Brian Hunt huntinhmb at coastside.net
Mon Sep 26 17:22:04 EDT 2022


Ha!  I did the same thing last night only with slightly parameters:

0.2 inch diameter 4 ft. rod (average diameter of AX1?) with the base 5 
ft above average ground.  Counterpoise (#18 wire) drops 4 ft from the 
feed point then trails out 9 ft to about 2 inches above ground. The base 
loading coil was 11.3 uH with a 6.7 ohm series resistor for a Q of 150.

Not surprisingly, the gain plot showed best gain in the direction of the 
counterpoise: -0.37 dBi at 30 degrees elevation.  Gain in the opposite 
direction was -4 dBi.  So if you are in a QSO with someone, you should 
be walking away from them. :-)   SWR at 14.050 MHz resonance was 
calculated at 3:1.

Interesting exercise!

72,
Brian, K0DTJ

On 9/26/2022 12:15, David Gilbert wrote:
>
> I modeled something similar to an AX1 with EZNEC last night.  I used a 
> base loaded 4' length of 1/2 inch diameter tubing for the whip, a 
> roughly 10 uH coil with a Q of 200 (25 turns, 1.5" diameter, 3" 
> length), and a 13 foot #14 counterpoise wire.  It was oriented 
> vertically with the bottom end of the counterpoise wire being 2 feet 
> above "typical" ground.  The frequency was about 14.1 MHz.
>
> It looked surprisingly decent for both SWR and field pattern ... only 
> about -2 dBi ...  although of course we should be suspect of the 
> accuracy for a model that severe.
>
> The calculated field strength without the counterpoise wire was about 
> -10 dBi, but that's clearly unrealistic because the SWR was 700 (no 
> exaggeration).   In practice I'm sure that other things (human body, 
> coax shield if used, proximity of the rigs case to ground, etc) have a 
> moderating effect, and they all likely introduce additional loss.  I 
> don't think there is any way that EZNEC can accurately model something 
> like that, and we also have the issue of how much loss there might be 
> in the tuner trying to match such a difficult load.
>
> Thus the urge to do a real world test, although I'm coming to believe 
> that no matter how hard I try, it probably isn't possible to control 
> proximity effects well enough for the case without the counterpoise 
> wire to get a truly definitive comparison.  That SWR of 700 in the 
> model was because the feedpoint had a capacitive reactance of 35,000 
> ohms (!!).  It is certainly debatable whether that figure is accurate, 
> but there can be no doubt that it is very high.   A ballpark 
> comparison is therefore about the best I expect whenever I find myself 
> able to do the test.
>
> 73,
> Dave   AB7E
>
>
> On 9/26/2022 4:05 AM, Gerry leary wrote:
>> I have used my AX2 both ways, and I have never found it to be 
>> advantageous to use that antenna without a counterpoise.
>>
>> Gerry Leary Wb6ivf
>
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