[Towertalk] Low-Angle Scattering

[Pete Smith]

At 08:16 AM 4/1/02 -0500, K1MK wrote:
>...

>I'm guessing this what you're seeing with YT and TA and it's real. I know
>the method used by YT is valid for computing the reflection from an
>arbitrarily rough surface; I assume the same is true for TA. And as an
>educated guess, I suspect what you're seeing is much more likely to be the
>result of random scattering from a rough surface rather than coherent
>diffraction from periodic structures.
>
>A question to consider is what are the absolute gain values in the new
>models at these small angles and how do those gains compare to the free
>space gain of stack or the individual antennas? Are you seeing something
>comparable to the free space gain or has the depth of the null on the
>horizon been reduced?


Interesting... Here are the numbers on 14 MHz for a 2-high stack of 
2-element yagis at 97 and 69 feet, as computed by YT (this is close to my 
antenna system, which uses C-3Es):

At the very-low-angle peak, the modeled signal is 17.8 dBi at 1.75 degrees 
above the real horizon.  At the same elevation angle above flat terrain, 
the same antenna shows approximately 1.8 dBi.

Over flat terrain, YT shows this stack to have a peak gain of approximately 
14 dBi, so the low-angle peak is significantly above the maximum unassisted 
gain of the stack.  Moreover, at every angle from about 7 degrees on up to 
the first null at 24 degrees, the signal level over my terrain is higher 
than the curve for the same system over flat terrain, by typically .5 dB 
(with one peak at 3 dB higher).  From 16-24 deggrees, the increase appears 
to result primarily from the shifting of the null one degree higher (that 
is the shape and slope of the curve are roughly the same as the original, 
just one degree higher.  Is this still consistent with your analysis?



>




73, Pete N4ZR

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