> Where can you experience a situation where you have
> line-of-sight, but poor
> fresnel clearance, and have no communications path (assume
> VHF and UHF, not
> microwave)? Is there ever a case?
A couple of simple examples:
1. If you have an exactly-wrong height as far as Fresnel clearance goes,
you have complete cancellation. Hard to realize in the real world though,
at least not a consistent basis, but you can get close...
2. Ducting and inversions that "bend" the signal, either horizontally or
vertically. I have 950 MHz STL paths over water and marshland with full LOS
and Fresnel clearance that, when conditions are just right, will go from >
1000 uV to below squelch threshold (about 5 uV), about a 50 dB drop.
3. Multipath. Signals arrive out-of-phase at equal amplitude.
4. Antenna pattern issues, nulls, etc. Probably self-explanatory, and
obvious.
> We've got a link path that an analysis indicates "absolutely
> no way" yet it
> works just fine (UHF). The obstructions are numerous and
> large, but they are
> some distance from either site. Then I've seen other paths
> (UHF) where a
> somewhat mild obstruction causes grief, but the obstruction
> is close-in to
> one of the sites.
As far as path obstructions go, diffraction is one of the biggest factors.
The further from either endpoint the obstruction is, the more of a chance
diffraction will work in your favor, "bending" the signal over the
obstruction. Think of it as a "shadowing" or "shielding" effect - if you're
up close to the obstruction, the shadowing effect is much more pronounced as
compared to if you are further away from it.
In a bit of a hurry, sorry for the short responses.
--- Jeff WN3A
