[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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