[TowerTalk] 160 meter vertical on sloping ground

[Brian Beezley]

 >>> The most significant issue is the polarization.

This was one of the first drone issues I identified. I figured I could 
get away with crossed dipoles or orthogonal loops, whichever turned out 
to be more compact and lighter. I know nothing about drones, but I 
assume you can orient its attitude so that the antenna is broadside to 
the test site during measurement. That eliminates the third dipole. I 
don't think these antennas need to be efficient. They need to be small 
and lightweight.

If a waveform's polarization is twisted 15 deg by a terrain reflection 
or diffraction, the original polarization is down 0.3 dB while the 
orthogonal is down 11.7 dB. If you ignore polarization shifts, the power 
you accumulate is 0.3 dB low. Maybe worse, the -11.7 dB orthogonal may 
get re-reflected at an angle where you see a null. That's a large 
relative error. What if you get many of these polarization rotations? 
Now, 15 deg is a pretty steep slope. I'm not sure how often you'd see 
that in actual terrain. I suspect most errors would be smaller. Maybe 
just do one polarization and accept the inevitable errors due to 
polarization rotation. It sure would simplify things. But I'd want to 
study some real terrain examples more carefully and think about the 
significance of null filling before committing to one polarization.

The other big drone issue is that all significant terrain must be 
between the drone and the site. Terrain behind the drone will reflect 
signals that will cause errors. It will also cause errors simply because 
its forward reflections and diffractions are missed. That's what would 
kill it at my QTH. I have prominent hills in most directions many miles 
away. No way to get a consumer drone on the far side of them.

I'm sure those aren't the only issues, but by the time I identified 
those two I had given up. I think using a drone to measure antenna 
patterns is a really neat idea. I just don't see an easy way to do it 
accurately. Much of the data collection could be automated, maybe even 
in conjunction with the drone flight pattern. Maybe someone will think 
it all through and offer some hardware and software for sale. Then 
anyone who wants to measure a sited antenna pattern won't have to solve 
all the problems himself. If it's expensive, a ham club could own a 
system and pass it around to its members.

Brian