At 10:40 PM 1/1/99 -0800, George Fremin III wrote:
>
>George Cutsogeorge writes:
>>
>>
>> In using YT to try and determine the best separation for a pair
>> of 40 meter beams and my terrain, the following happened: I
>> started with a spacing of 100 feet and gradually raised the
>> bottom antenna while observing the peak gain. To my amazement,
>> it kept increasing with each increment. Surely it would decrease
>> at some point as the spacing got smaller and smaller. Well, it
>> kept on increasing all the way to zero spacing and that's where I
>> quit.
>>
>> It does the same for flat ground or my terrain. What's going on
>> here?
>
>
>Maybe this:
>
>(from the YT manual)
>
> The internal Yagi model in YT is simple and does not compute
> interactions between individual Yagis in a stack -- YT assumes
> that each antenna is a point source. For antennas stacked less
> than about a half wavelength apart this is not a problem.
> For example, you should be cautious specifying spacings less than
> about 20 feet on 20 meters (and proportionately scaled on other
> bands) because of mutual-coupling effects between real antennas.
In other words, the absolute gain figures are not consistent with reality.
It sounds as if the program simply adds the amounts of radiation arriving
at any given point in the foreground terrain, rather than taking phase into
account. I wonder what this means, then, for the amounts of energy
refracted and reflected, and the quantitative accuracy of the computed
effects of uneven terrain.
I also wonder if there is a misprint in that manual. First it says, "for
antennas stacked less than about a half wavelength apart this is not a
problem." Then it says "... you should be cautious specifying spacings
less than about 20 feet ...." Whaaa?
Dean, where are you??? We need this explained.
Wonder if TA has the same limitation?
73, Pete Smith N4ZR
n4zr@contesting.com
Loud is good.
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