In Reply to: Re: Streetsweeper Antenna posted by Mouser on October 23, 2000 at 16:00:27:
Crossed polarization in the real world is probably worth at least 6 dB,
but it sounds like you won't have line-of-sight anyway. The signal
will squirt out from under the vehicle and stay low - the energy
reaching the Rx antenna will have been randomly reflected by obstacles,
and won't have any consistant polarization, so don't sweat it.
A waveguide is some sort of hollow structure where the signal is
constrained between conductive walls (usually rectangular or circular
for easy analysis and construction). The size is picked to match the
frequency of use. In the vehicle case, you have an upper conductive
surface but the lower one could be conductive/reflective when wet or
absorbant if dry and loose. The sides are mostly unconstrained, so
it's not really a waveguide - I was stretching it a bit. But still,
the transmit signal "pattern" is highly dependent on the geometry of
the objects in the near-field, and it's probably faster to try it and
see than to get the s/w and build the models for computer analysis.
As for Rx antenna height, you want line-of-sight at UHF. Keep it low
if the vehicle is within sight, but raise it if you're counting on
stray reflections. Good luck!