Hi, all --
I am on the technical committee of an amateur repeater club. A club in the next county has approached us to ask about sharing one of our 2m receive sites with them. It's a commercial site and we are there at the pleasure of the owner, who is willing to accommodate them and thinks that there are no bad mixes.
The other club has asked to share our 2m receive antenna and feedline, and has volunteered to install whatever equipment we specify so that our site's performance is not degraded.
While this sounds doable to me, I am more of a digital guy than an RF guy. What should we install in order to do this "the right way" without degrading our site's performance?
Our system transmits on 146.88 MHz and receives on 146.28.
We use a Motorola receiver (I think it's a Micor).
The other club's system transmits on 147.345 and receives on 147.945.
Both have links in the 440 band.
Neither system will transmit on 2m from this site.
Any advice or recommendations as to architecture, equipment that works (or doesn't work!), or points to be included in a Memorandum of Understanding, would be appreciated.
Regards,
Bob Koblish
[EMAIL PROTECTED]
Quite doable....
First of all the 2m TXs does not enter into the equation since they
are not at your site.
You will need a 2m receiver multicoupler. There are splitters and
there are multicouplers. Splitters have no gain and are designed
for situations where you have enough signal but need to go two
(or more) places. Picture your cable TV signal going to two (or more)
TV sets. Each TV set gets a little less than half the signal, but
generally that isn't a problem because the cable TV signal has
a lot of headroom.
A multicoupler is a device containing basically a low-noise preamp
feeding a splitter. The preamp makes up for the splitter loss. A
bandpass filter (like a DCI) can be placed between the preamp and
the splitter.
The 2m feedline that now goes to the 146.28 RX will feed the
multicoupler input and one output will go to the 146.28 RX and the
other to the 147.945 RX.
If the budget allows for it I'd definitely get an Anglelinear
multicoupler. Yes, I'm partial to their stuff. I've used it and it's
better than anything else on the market.
Anglelinear <http://www.anglelinear.com> is owned by a ham (Chip
Angle, N6CA, 310-539-5395 or [EMAIL PROTECTED]) who is very
much involved with the weak-signal enthusiast community.
His web site is EXCELLENT and has a large amount of easy-to-read
info. Example: <http://www.anglelinear.com/couplers/couplers.html>
With the above said, as a proof-of-concept test you can just parallel
the RX inputs with a BNC T-connector and a couple of patch cables
but expect a noticeable loss of performance on weak signals.
Running it that way for a few days or even a week will tell you if you
have any deal-killers before you spend any money (example: their RX's
local oscillator intermodding with some TXs at the site and landing on
your input - don't laugh - I've seen it happen - and the TX was a
90.something mHz FM broadcast TX).
And before they install any equipment I'd run the 440 TX freqs
through the an intermod calculator and look for hits. Just do an
inventory of the TX freqs and local oscillator freqs and and look at
<http://www.audiotechnica.com/using/wireless/compat/index.html>
which is an on-line intermod calculator that handles 30 frequencies.
Yes, it's oriented to wireless mics but - hey - it's free.
Or you can go to <http://www.emrcorp.com/imcalcinfo.html> or to <http://www.kesslersystemsinc.com/ampim.htm>. There is one
more at <http://www.tcstx.com/software/Intermodulation.cfm> but
it's in beta (has been for a year) and the help file is not that great.
But it does do 5th order checks...
Or there is ham-written shareware that you can get from
<http://www.repeater-builder.com/downloads/intermod.zip>.
All in all, with the mix and intermod situation handled, your site
sharing should work just fine.
Oh - and make sure that the 440 link TXs have isolators or
circulators on them.
Mike WA6ILQ
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