[Towertalk] (no subject)

[Pete Smith]

I've been using YT for years.  Recently, I upgraded my terrain profile to 
Europe to one with much higher accuracy than before.  The profile is 
essentially flat within about 50 feet out to 2 miles, but the 
higher-resolution plot reveals many small ups and downs of 10-20 feet in 
magnitude.

When I ran this terrain plot against my antennas (a tribander stack), I was 
tremendously surprised to discover that the resultant elevtion pattern 
includes a VERY strong, very low-angle component.  Compared to the same 
antennas over flat ground, the pattern is approximately 14 dB better at 1.5 
degrees elevation on 21 MHz.  The enhancement at that same angle is 
approximately 12 dB at 28 MHz, and at 14 MHz, the enhancement at 1.75 
degrees is approximately 17 dB.  On 7 MHz, with a single yagi at 104 feet, 
the 1.75 degree enhancement is right off the chart, at over 20 dB.

Just for fun, I ran the same antennas in TA, with the same terrain profile, 
and the results are at least qualitatively identical.  Of course, I have no 
way of knowing whether both programs use the same or similar 
reflection/diffraction methodology.

I would welcome advice on whether what I'm seeing here is
real, or just a modeling artifact.  It's been suggested that my washboard 
terrain might be acting to enhance signals at very low takeoff angles, in 
the way that I gather a diffraction grating can scatter light arriving 
almost parallel to the plane of the grating.  Anyone out there with a 
physics or optics background care to comment?

By the way, before anyone asks, I really can't judge from practical 
results.  On most bands, the percentage of signal arriving at these angles 
is very low, and I haven't had sufficient experience with high-performance 
antennas to give me much of a baseline.