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Author Topic: Motorola VHF MSF-5000  (Read 1552 times)
N2ZTE
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Posts: 2




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« on: February 05, 2009, 08:33:45 AM »

Hello everyone -

Recently my club set up a VHF MSF-5000 atop a tall building here in the NYC area.  The transmit coverage seems excellent, however, our receive is pretty terrible.  It seems that the transmit is desensitizing the recieve I think.  The machine is recieving very noisily, and I think it may due to interference by the simultaneous transmit.  Distant stations are especially susceptible to poor receive, though they're opening the machine well.  

Does anyone have any suggestions for this? Is there a filter or pre-amp that I might be able to use? PLEASE ADVISE!! Thank you very much.  

The machine is a Motorla MSF-5000 (CXB version) and the duplexer is a Sinclair Q-202, tuned by Sinclair themselves.  
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W0FM
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Posts: 2056




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« Reply #1 on: February 05, 2009, 12:06:42 PM »

On a tall building in a high RF environment like NYC, I would make sure the MSF5000 has a circulator and an isolator installed.  Both are standard or standard options on MSF5000.  I would check for the inclusion those items first.

73,

Terry, WØFM
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N8EKT
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Posts: 371




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« Reply #2 on: February 05, 2009, 03:06:21 PM »

Try keying the repeater with the LEAST amount of power.

At that point if you can't "hold" the repeater, increase your power level until you can.

THIS will be the amount of desense (IF you have it.)

Preamps will only make things worse.

In your location, I would reccomend installing a preselector filter like the DCI 145-2H for $169.95
It can be installed AHEAD of the duplexer and will dramatically attenuate out of band signals from reaching the transmitter as well as the receiver.
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W3LK
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« Reply #3 on: February 05, 2009, 03:31:03 PM »

You managed to find an open frequency pair in the NYC area and get it coordinated?

Congratulations.

73,

Lon - W3LK
Naugatuck, Connecticut
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A smoking section in a restaurant makes as much sense as a peeing section in a swimming pool.
K2YO
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Posts: 436




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« Reply #4 on: February 08, 2009, 01:06:02 PM »

Lon,

This is dated by 18 months, but I spoke to the freq. coordinator for the NYC area and he said there are 440 pair open. Two meter was near impossible but VHF was potentially available.

I thought that was odd after leaving So Cal, there 2M was available, but 440 was impossible.

Bernie
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K2YO
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Posts: 436




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« Reply #5 on: February 08, 2009, 08:52:21 PM »

Whoops, I ment to say UHF was possible.
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N2ZTE
Member

Posts: 2




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« Reply #6 on: February 13, 2009, 08:34:40 AM »

Thank you to everyone so far for the advice. We're are trying numerous different things right now in an effort to try an eliminate the desense.  

- we're swapping out all of the jumpers with new, clean jumper cables.  Both on the duplexer itself and the ones that connect the machine.

- lowered the output power of the machine back to about 60W.  

We were also considering ditching the duplexer and adding a second antenna - one for recieve and one for transmit.  Will this eliminate the desense problem??
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KC9GMX
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Posts: 40




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« Reply #7 on: February 20, 2009, 08:47:49 PM »

I would absolutely keep a duplexer. But make sure it is pass/reject type, with a minimum isolation of 95db.
The duplexer not only keeps your repeaters transmitter out, but also keeps other crap out too.
Desence can also come from other nearby transmitters.
Steven
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