Answers below....
On 3/9/2010 8:29 AM, Ross Johnson wrote:
Can a dualband antenna VHF/UHF for RX ONLY be fed to two receivers one
VHF, one UHF, without a quote "duplexer" using a T instead?
Yes. Typically performance is better with mono-band antennas, since all
multiband antennas are a trade off in their design, but a "T", or even
splitting multiple times is certainly an option for any receive-only
antenna system, with the caveat that there's loss at each "split".
Pre-amplifiers can help a bit, but once an RF signal is lost, there's no
"getting it back" by amplification.
Here's the idea. This is a remote RX site. The idea is to run
something like a beefed up X500 dualbander at tower top, then 7/8
hardline 100 feet down to the receivers.
So far fine.
Both receivers will have one or two bandpass cavities inline before
the T.
I assume when you say "before" the T you mean antenna -> split ->
bandpass -> receiver. Yes, this is probably a good idea to keep the
receiver from being hammered by other signals that are out-of-band, but
not 100% necessary if this receive antenna is out in the middle of
nowhere with no high-power transmitters nearby.
The bandpass filtering is lossy too, of course. The higher the Q of the
bandpass filter, the less the loss. (High Q bandpass cavities are
typically MUCH larger than BpBr duplexer cans. At VHF they're enormous
and take up a lot of space. Ceiling mounts are common.)
remember also that you're really only adding the bandpass to design for
what the receivers NEED to have filtered to perform at their best. If
the receivers are something like the GE MASTR II or similar with a
cavity helical filter front-end (bandpass filter) built-in, you don't
NECESSARILY need more filtering in front of them. Just sayin'.
Design your filters specifically for your receiver's ability to handle
out-of-band or nearby signals and the signals that you expect to be
present at the site.
The filtering has nothing to do with the "multi-bandedness" of the
antenna, etc. UNLESS your chosen receiver is particularly bad when say,
a 1/4 KW 900 MHz transmitter is 2 feet away from the receive antenna,
and your particular radio doesn't like that. (An example I saw once...
even WITH filtering the amount of 900 MHz "energy" coming through the
filters was enough to piss off a UHF receiver, being it was a 2x
multiple of the UHF's front end and passed through without much loss.
Would a duplexer be necessary in this case. Or could it be done with
proper cable lengths and a T?
A duplexer is a set of filters designed to pass a transmit frequency and
filter it out of a receiver on a nearby frequency. Did you mean
diplexer? I think that's what you're really meaning to ask. And the
answer is no... you don't truly need a diplexer. ESPECIALLY if you're
running separate bandpass filters on each receiver. Think about what a
diplexer does... it passes lower frequencies to one port, and higher
frequencies to another port... if you're already going to bandpass
filter there's no need for it.
As far as cable lengths go, I have no idea what you're asking. Cable
lengths should have no effect on this system at all.
Thanks for your time and for the probably obvious answer I'm not sure
of...
No worries, you're asking the right questions to learn what you need to
know. We've all been there! (GRIN!)
For more "thought exercise" on the topic of multi-band reception, pick
something you know receives multiple bands, and think about it...
Think about a scanner and a discone antenna. Technically inside the
scanner, there's probably multiple "receivers" so to speak (not really,
but bear with me... it'll receive on multiple bands, and what it's
really doing is switching those receivers in and out for each band as
necessary -- kinda... scanners really typically just have really broad
receivers that are ultra-sensitive but tend toward not being very
selective)... you just get the RF to the scanner, it'll hear it.
Because it has a front-end with virtually zero filtering, It'll also get
hammered by close-frequency transmitters and almost always suffer from
"images" where strong out-of-band signals will mix in the scanner's IF
and show up as frequencies you never thought had signals on them. (And
don't.)
The scanner nor the antenna "care" which band they're receiving. The RF
just passes from the very wide-band antenna down the cable, where the
receiver does what it can with the pile of signals that are constantly
present.
Other thoughts to think about:
It is VERY common at busy sites where antenna space on a tower is at a
premium to do things like require site tenants to share either a
"community receive" antenna, and sometimes even a "community transmit"
antenna. The receive antenna setup for a single band is simple...
antenna -> perhaps a wide bandpass high-Q cavity -> perhaps a
pre-amplifier to amplify only what's "left over" (the band desired)
after that cavity -> a splitter, and multiple runs of cable to each
tenant's receiver.
And finally:
If you're going to have TRANSMITTERS at the same site as these multiple
receivers, this changes the ball-game (and the answers) significantly,
because now you're going to suffer from desense from those and need to
filter appropriately (duplexer, or other...).
Hope this helps enlighten a bit...
Nate WY0X
