[TowerTalk] 160 meter vertical on sloping ground

[Brian Beezley]

"A good way to think about this kind of 'antenna on the side of a hill' 
is to rotate the pattern for flat ground to the angle of the hill."

That's how I think of it, too, Jim. It's a good first approximation, but 
there's more to it. In the simplest case, the far field includes a 
direct ray, one from sloping ground, one from flat ground after the 
slope quits, and diffraction from the bottom of the slope. There might 
also be a secondary bounce from sloping to flat. Although sloping ground 
can certainly concentrate power at low angles, you never get power right 
at the horizon due to the inevitable phase cancellations. W6NL has an 
old photo me of explaining this very geometry to W7EL by drawing rays in 
the dirt with a stick at his hilltop QTH!

My TA program will calculate the resulting pattern for a vertical 
antenna at the edge of a slope, including all possible reflections and 
diffractions. But I don't have a working copy.

One thing to note about these ray-tracing programs is that they all do a 
one-dimensional analysis along a given bearing. For mountainous terrain, 
this is often inadequate. Power can easily reflect or diffract from 
terrain to the right or left and wind up on the bearing of interest. The 
magnitude of these off-bearing paths can be quite large. This really 
restricts terrain for which results are accurate.

I've been thinking of resurrecting TA for Windows. If I do, it will only 
be for a full 2D analysis. That should be possible in a reasonable 
amount of time with modern computers, especially with multiple 
processors gnawing away at all of the potential paths. But the main 
problem is getting reliable data to check the calculations. What I used 
for TA was the original WA3FET IEEE article. I had to extract data by 
hand from the curves. But as I recall, the helicopter that took the data 
flew directly over a hill with the RF source so it is not a good test of 
2D terrain. I would not write a 2D program without having reliable data 
to check it with. There are just too many ways to make ray-tracing errors.

Brian