"RST Engineering" wrote in message
...
Um, that's just not true. SInce the radiation pattern is reduced at
higher
altitudes, the chance for frequency congestion is every bit as probable.
If
all signals are reduced proportionally, then the RELATIVE signal strengths
remain constant.

Jim

Under normal atmospheric conditions, excluding anomolies, as the frequency
or distance increases, the required transmitter power increases while the
recieved signal strength remains the same. At some point, even if you use
the same frequencies, a VOR or any radio signal will vanish into the
background noise. The reciever sensitivity is limited by background noise.
If you like, I'll look up the specific math and we can really get technical.
All but a few would understand it or care. This would be the other limit
that
I didn't explain previously.

All you have to do is space VOR's of the same frequency far enough apart
and they won't interfere. The fact that the radiation pattern is reduced for
higher altitudes seems to imply that the radiation pattern was designed
to reduce transmitted power and limit reception distances at those altitudes
where line of sight would not be a factor.

For reference you might want to find:
Schoenbeck, Electronic Communications Modulation And Transmission
It's one of the books I used when I was working on my electronics degree.

Since the radiation pattern is reduced at higher altitudes, there is
less
chance
of frequency congestion and receiving a signal you don't want.
With limited frequencies available, you have to depend on other
limits to prevent unwanted reception of other signals.