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UHF repeater possibly needing receive preamp

KC4YIN

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Joined
Jun 29, 2003
Messages
404
Location
Kinston NC
I'm posting this question mainly for educational purposes.
The current installation is an XPR 8400 UHF repeater operating on the ham bands with a Motorola T1500 duplexer and the antenna is just over 100 feet above ground.
Receive performance is fairly acceptable with areas we feel could do better.
We have available to us an MTR3000 repeater without preselector with a Celwave 6 cavity duplexer to replace the XPR8400.
We also have an extra filter cavity we can tune to the receive freqeuncy for added isolation on the repeater input. We are thinking of adding an Advanced Receiver Research receive only preamp to help with receive.
The main question is: Would moving from the T1500 4 cavity duplexer to the Celwave 6 cavity duplexer plus one additional cavity filter provide enough isolation for the preamp?
 

prcguy

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Jun 30, 2006
Messages
9,760
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So Cal - Richardson, TX - Tewksbury, MA
I would say in general with can size being equal and both duplexers being Bp-Br, going from a 4 can to 6 can would probably give you as much isolation as the gain of an average preamp. Adding another 5" cavity with minimal loss like .5dB should give at least another 10dB isolation at 5MHz.

I've had a number of ARR preamps and they are not very good. For $30 less you can get the MiniCircuits ZX60-P103LN which has about the same gain (you would want to attenuate some), better noise figure and a 10dB higher compression point.

Is the repeater site a major one with lots of other UHF repeaters on air or is it a quite low RF site? The noise floor at a busy site can be 10-20dB higher than a site with no other transmitters and you may not improve anything at a busy site with a preamp.

As a coincidence and just a few weeks ago I was playing with my Yaesu DR-2X repeater in UHF at my house with 6 cavity Bp-Br duplexer and I added an Angle Linear high level PHEMT preamp and single Motorola 5" can with a transfer switch to see the immediate differences. The Angle Linear preamp is probably the best ever made for repeater use.

Using my service monitor generating into an antenna on the same property I set the level to get about 12dB SINAD from the repeater receiver and switching the preamp and filter in and out of circuit did basically nothing, maybe 1dB improvement at best but it was hard to really tell. If this were an old Micor or Master II I would probably see a more significant improvement as their sensitivity and internal noise figure are not as good as a modern repeater receiver. The MTR3000 is a really nice repeater but I'm not up on its sensitivity or noise figure.

The place where a good preamp really shines is a receiver multicoupler/distribution for many repeater receivers. With a good tight and low loss preselector filter the preamp will usually give a better noise figure than any repeater receiver alone and it will drive the power splitter to supply each receiver with 0 to a few dB gain to overcome splitter loss.


I'm posting this question mainly for educational purposes.
The current installation is an XPR 8400 UHF repeater operating on the ham bands with a Motorola T1500 duplexer and the antenna is just over 100 feet above ground.
Receive performance is fairly acceptable with areas we feel could do better.
We have available to us an MTR3000 repeater without preselector with a Celwave 6 cavity duplexer to replace the XPR8400.
We also have an extra filter cavity we can tune to the receive freqeuncy for added isolation on the repeater input. We are thinking of adding an Advanced Receiver Research receive only preamp to help with receive.
The main question is: Would moving from the T1500 4 cavity duplexer to the Celwave 6 cavity duplexer plus one additional cavity filter provide enough isolation for the preamp?
 

KC4YIN

Member
Premium Subscriber
Joined
Jun 29, 2003
Messages
404
Location
Kinston NC
Thanks PRCGUY that really answers our questions. As to your question the noise floor is rather low considering it's a local hospital. We really rather not depend on a preamp if possible. Our biggest issue right now is access to the repeater as it is on the roof of our hospital and gaining access to it due to Covid 19 restrictions is tricky. I appreciate your comments and will heed them.
73's
 
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