On Tue, 30 Aug 2011 13:04:48 -0700, "Sal" wrote:
If your intent is to experiment with the cheap duplexer, what I have to
offer is irrelevant. However, if you aim to build a repeater, consider
getting your isolation over distance (much greater than 10m). Put the
transmitter and receiver a few km apart and link them using another band
authorized in your area (440?).
I encountered this in Key West Florida some years ago. Worked fine there.
How fine is fine?
That last time I did that (about 30 years ago), on a commercial
system, there was a huge difference in transmit and receive footprint.
Some locations could hear but not talk. Others were the other way
around. Either way, the customers were not thrilled. We went back to
one antenna per radio.
We then repeated the same mistake with a common receive antenna at the
very top of the tower, followed by an RF amp, and then an 8 way
splitter. Attach 8 receivers and you only need one RX antenna.
Unfortunately, the amplifier was too easily overloaded, and the
splitter did not provide sufficient isolation to prevent the local
oscillator leakage from creating new receiver spurs. I later added
cavities and isolators to solve that problem, which increased the cost
sufficiently that 8 TX/RX antennas would have been cheaper. (Except
for the tower space rental, but we owned the building and towers).
Also, with a single RX antenna on 8 radios, it makes a great single
point of failure for lightning hits. It was easy to tell if the RF
Amp had taken a hit. The office would simultaneously get dozens of
irate service outage calls (These were community repeaters with up to
15 customers per repeater). I would never want to be the top antenna
on a tower, no matter what the range benefits.
9 repeater in one rack. Notice the lack of duplexers.
http://802.11junk.com/jeffl/pics/Old%20Repeaters/slides/Santiago-01.html
Here's the corresponding transmit antenna intermod generator:
http://802.11junk.com/jeffl/pics/Old%20Repeaters/slides/LoopMtn02.html
The receive antenna is behind me, on top of another telephone pole.
Yet another great idea was to physically separate the transmitter and
receiver buildings on a mountain top. That was Verdugo Pk in the San
Fernando Valley. Sorry, no photos handy. This had some real
advantages, especially at low band (30-50Mhz). The problem was that
with all the transmitters jammed into one building, with little
physical isolation among the antennas, there's was considerable
intermod caused by the various TX mixes. Since the original
justification for this great idea was to solve the intermod problem,
this was also loser.
Much as I don't like duplexers, isolators, cavities, lightning
arrestors, fat coax, and omni antennas, the combination is the
solution that seems to work the best. All the other great ideas are
far worse, more expensive, or deficient in some manner.
http://802.11junk.com/jeffl/pics/Old%20Repeaters/
Guess what's happening with this antenna problem?
http://802.11junk.com/jeffl/pics/Old%20Repeaters/slides/LoopMtn03.html
Incidentally, that's where I was cleaning up the mess and almost
picked up what I thought was a piece of black coax. It was a snake.
--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558
#
http://802.11junk.com
# http://www.LearnByDestroying.com AE6KS