One of the great unverified assumptions of low-angle propagation is
that smooth, sharp decrease of gain from a horizontal antenna as the
angle goes down in the last handful of degrees.
This lack of verification is easy to assume because of the bewildering
variety of terrain, and what it would take to measure 2 or three miles
downwind. E.g. a ground-independent measuring device without trailing
wires being hauled up on a balloon to a height of 2000 feet. And a day
perfectly still so the balloon on the tether is mostly straight up.
There are times (for a few days after a real general area rain storm,
for one) when I hear stuff better than I should. Talking about signals
that probably OUGHT to be low angle. Vertical *AND* horizontal. What's
going on? Dunno, but it happens, and happens again. Got NO way to
quantify it, NO way to prove it, just know that I hear it, and it
keeps me suspicious. I know that I HAVE NOT read anything that
explains what I hear.
73, Guy.
----- Original Message -----
From: "Pete Smith" <n4zr@contesting.com>
To: <towertalk@contesting.com>
Sent: Monday, April 01, 2002 8:35 AM
Subject: Re: [Towertalk] Low-Angle Scattering
> 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
>
> Check out the World HF
> Contest Station Database at
> www.pvrc.org
>
>
>
>
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