> What spacing do you have between the mast and the antenna element,
> and how tall is the mast Gary? I've never seen that effect either in
> a model or in real-life antennas.
>
> I'm puzzeled by the large reported change, since it also makes no
> sense from a theoretical standpoint unless the mast is a significant
> fraction of a wavelength tall, perhaps .1 wl or more. But then there
> is always something to learn, if you can get to the bottom of
> disagreements. 73, Tom W8JI
--------
Tom
I first modeled the K9AY loop on 160M with the published dimensions
(25 ft overall height), with the terminating resistor adjusted for the
deepest null. I use the "real ground" option because it shows the best
agreement with observed behavior.
Next, I placed a 25-foot high conductor a few inches away from the loop,
grounded at the bottom to simulate a supporting mast. The previously
well-defined sharp null was no longer present, although F/B was still
good enough to consider useful.
As a sanity check I modeled the system over perfect ground. The
effect of a grounded support on F/B is still present, but smaller (2-3 dB).
Although the antenna and mast dimensions are relatively small in terms
of wavelength, it makes sense that there is eome degree of coupling
due to the proximity of the conductors.
As I noted before, because the model suggests that a metal mast
may have a significant effect, I simply felt that it was prudent (and no
extra work) to avoid the possibility of reduced performance by using
insulated supports.
73, Gary
K9AY
k9ay@k9ay.com
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