Chuck:

The basic equation for path attenuation is Attn. = 36.6 + 20 log F(MHz) + 20
log d (miles) (in dB).*

The log function dilutes the effect of frequency change, so tripling the
frequency from 147 MHz to 441 MHz increases the loss by 20 log 3 ~ 10 dB.
If the UHF antenna gain is increased by 5 dB on each end it is a wash. A 20
foot long DB-224 antenna has 6 dB of gain on 144, and a 20 foot long DB-410
has 10 dB of gain on 440, so the overall loss going from VHF to UHF with
comparative size antennas is about 2 dB. Not really noticeable except under
marginal conditions. Mobile antennas on VHF run from unity to 3 dB, and UHF
run from unity to 6 dB, so the system loss difference for base to mobile can
be from 3 dB to 5 dB, which approaches noticeable, but not remarkable.
Distance difference will be in the same category, hard to notice. UHF tends
to diffract better, so it will go "over the hill" a little further. while
VHF will go a little further on flat ground. There is no one exact answer
for which works better. Give me an exact path description and I can
calculate loss, but paths of the same length with different profiles will
vary greatly.

Another rule of thumb is doubling antenna elevation at VHF produces about 6
dB gain, so my antenna on top of my monster truck roof will work 6 dB better
than the one on your motorcycle.

That what you are looking for?

* ITT Handbook, Chapter 28 in most recent editions.
--
Crazy George
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