Alexander,
I ran your system's numbers through CommShop to see what kind of isolation
your duplexer must have to show no desense, and I came up with 92 dB. The
Sinclair Q-202G duplexer can barely make 85 dB when tuned on a network
analyzer, so that's the major part of your desense problem. It's only a
four-cavity duplexer, specified at 80 dB minimum isolation, so no amount
of tuning is going to make it operate at an isolation above its design
limit.
Assuming that every bit of the interconnecting cable is double-shielded,
the repeater might work properly if the preamp were preceded by a
two-cavity bandpass filter. The Sinclair Q-202G duplexer, like most BpBr
designs, has practically no bandpass effect, so the preamp is amplifying a
whole spectrum of noise along with the desired signal. One of the
less-obvious sources of noise and signal leakage is the use of UHF
(PL-259) connectors in place of type N connectors, and the use of adapters
and barrel or bulkhead connectors. I strongly suggest that jumper cables
be made up in single lengths with the proper connectors on each end.
It may be instructive to measure the noise floor at your repeater site,
using the techniques described on the Repeater-Builder site. If your site
has a high noise floor, you may not be able to operate a 2m repeater
without some specialized design changes.
Finally, you might try placing a high-quality 6dB or 10dB attenuator on
the output of your preamp. Some preamps have far too much gain at VHF,
and limiting the gain to no more than 12 or 15 dB sometimes makes a world
of difference.
73, Eric Lemmon WB6FLY
Alexander Tubonjic wrote:
> Hello,
> Well here is one that can't be figured out locally, thought I would
> throw it to the experts. My high school radio club has recently made a
> few upgrades to our VHF repeater. We replaced the Ringo Ranger antenna
> with a Comet GP-9 Antenna, replaced the 9913 coax with Andrew 1/2''
> Heliax, and put a Mirage Preamp inline. The problem we are having is
> mainly with the preamp (I think)
> I will describe the setup and give all the specs first. The repeater
> Rx and Tx is from a 110 Watt Motorola Mitrek power out from the Mitrek
> is 50 watts. The power supply is an Astron RS-70M, Duplexer is a
> Sinclair Q202G (yes, it is properly tuned and working as it should)
> the antenna is rooftop on the schools theater building (the roof is
> about 75 feet AGL and the antenna is on an 8' mast pipe) there are no
> obstructions for at least 5 miles around. The preamp setup is as
> follows, it is after the duplexers and before the receiver. We have
> the preamp directly connected into the duplexer and a jumper going
> from the preamp to the Rx bulkhead on the repeater cabinet. The preamp
> has two settings, a higher gain and a lower gain setting, initially we
> had it setup to the higher gain setting but I went up today and
> switched over to lower gain and turned the power output to 30 watts.
> The problem we have is the Rx is not what is should be. It is a tad
> worse than when we were running the repeater at 15 watts without the
> Preamp. I would really like to get the preamp to work like it should
> in the repeater system, I just don't have any ideas on what to do next
> to get better sensitivity on the machine. Any thoughts and ideas would
> be appreciated. Thanks.
> Alexander KG4OGN
>
> P.s, all jumpers are made with double shielded ridgid coax, so thats
> not the cause of the problem.
>
>
> Yahoo! Groups Links
>
>
>
>
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