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UHF range

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doogymon

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Hi

Is it possible to hear commercial UHF mobiles 15 miles away?

They are at 1150 ASL and I'm at 1100 with a clear path.

Cheers

Doog
 
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kayn1n32008

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doogymon said:
Hi



Is it possible to hear commercial UHF mobiles 15 miles away?



They are at 1150 ASL and I'm at 1100 with a clear path.



Cheers



Doog
Click to expand...


Easily done. And impossible with vegetation in the way. Depends on many factors.


Sent from my iPad using Tapatalk
 
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doogymon

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Hi Kayn1

No vegetation

They are channelized 50 KHz off. 6 channels in a picket fence amongst our 8 channels.

Intermod hell.

Seems like a classic case of 2 freq 3rd order. And even some 3 freq 3rd orders.

I used the old IM intermod DOS program. many hits right on the receive of our repeaters.

Messing up control and voice channels.

Cheers & thanks

Doog
 
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zz0468

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doogymon said:
Intermod hell.

Seems like a classic case of 2 freq 3rd order. And even some 3 freq 3rd orders.

I used the old IM intermod DOS program. many hits right on the receive of our repeaters.
Click to expand...

So you didn't quite come right out and say it... Are you getting intermod mixes where mobile signals are contributing sources?

I recently dealt with just that situation with a multiple site trunked system. Mobile transmitters were mixing with base transmitters and mix products were appearing on the base receivers. The fix was to remove the duplexers, and run the systems on separate tx and rx antennas.
 
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doogymon

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Duplexer reportedly changed to a BandPass filter on the Transmitter with newly separate TX Antenna. Four receive filters in series on the newly separate Receive Antenna. Symptoms less since going to separate antennas, but the controller channel gets hung up and has to be "rolled".Receive Multicoupler also in the circuit with Preamp at 8 dB and then 14 dB pads. Aren't preamps problematic.
 
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zz0468

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doogymon said:
Duplexer reportedly changed to a BandPass filter on the Transmitter with newly separate TX Antenna. Four receive filters in series on the newly separate Receive Antenna. Symptoms less since going to separate antennas, but the controller channel gets hung up and has to be "rolled".Receive Multicoupler also in the circuit with Preamp at 8 dB and then 14 dB pads. Aren't preamps problematic.
Click to expand...

Sounds like progress.

Can you duplicate the symptoms at will? If so, see if you can measure the level of the mix products, then put a 6 db pad in front of the preamp and measure again. If the difference is greater than 6 db, then your preamp is contributing to the problem.

I assume the preamp is providing 8 db gain. Try setting the through gain of the multicoupler to no more than about 1 0r 2 db and see what happens. Try running without the preamp at all.

Have you made any effective sensitivity measurements to see what sort of desense the site is experiencing? Try checking with and without your own transmitters running.

What's actually causing the control channel to fail? Illegal carrier?
 
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jim202

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doogymon said:
Duplexer reportedly changed to a BandPass filter on the Transmitter with newly separate TX Antenna. Four receive filters in series on the newly separate Receive Antenna. Symptoms less since going to separate antennas, but the controller channel gets hung up and has to be "rolled".Receive Multicoupler also in the circuit with Preamp at 8 dB and then 14 dB pads. Aren't preamps problematic.
Click to expand...

Are the filters on the RX side before or after the pre amp? They should be before the pre amp.

You should have RX antenna, then the filters, then the pre amp. Might still have to put a pad on the pre amp input. Depends on the insertion loss of the filters.

Some pre amps are more prone to overload and intermod than others.
 
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zz0468

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jim202 said:
Might still have to put a pad on the pre amp input. Depends on the insertion loss of the filters.
Click to expand...

Sometimes that's what it takes to settle an overloaded preamp down, but it degrades the system noise figure by the loss value of the pad. At that rate, the NF might be degraded to the point that you're better off just removing the preamp completely.

Making careful measurements and understanding what the results mean is the key to success there. A lot depends on the site's noise floor. A half db noise figure does you no good at all if the site noise is 6 db. Especially if that noise overloads the preamp. You may end up putting 6 db in front of the preamp, have a total noise figure of 7 or 8 db, but the preamp is settled down and stable and you've optimized the receivers for best NF and gain at that particular site. Ugly, but it works.
 
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doogymon

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Will try to duplicate symptoms. Will try pad. 6 dB is the standard size? Sensitivity test using BERT live? Tech says RedFlag after 28 seconds of hung Controller. Says non subscriber RF causing. Filters on Antenna side , before PreAmp. RSSI 100. I'm not clear on RRSI (arbitrary number )is there two(2) RSSIs? One for Clear Channel and one for legit signal?

MTF & Thanks
73s
 
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doogymon

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Another quickie if you please. In TRBO, is the transmitted carrier 12.5 KHz?
Is the 6.25 KHz a subband within the 12.5 KHz?
So the the Receiver front end of a TRBO repeater is it narrow, really narrow, or super narrow?
73's
 
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kayn1n32008

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Narrow. The repeater uses a 12.5KHz channel that is then divided into 2 timeslots giving the equivalent of 2 6.25KHz voice paths. The repeater transmits full time, only the subscribers rapidly key and un-key.
 
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doogymon

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Ok So did a receiver test (mobile keyed for 15 seconds 500 yards away) and measured with a spectrum analyzer before the pre-amp -87 dBM and after the pre-amp -64 dBm. The before pick off used a -33dB sampler. The after pre-amp was from an unused rcv multicoupler pre-amp output. -87 + 33 = -54 dBm
Diff b/w -64 dBm and -54 dBm = -10 dB . Did the test a second time but this time had 10 dB of external attenuation. The result was -18 db. So this appears linear yes no? Therefore the pre-amp not running hot (saturated).

Cheers
 
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doogymon

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Using mobile from "the other" UHF Trunk Anlog system I was able to induce degradation into the TRBO repeater controller twice (once from 7 miles away however the TRBO subcriber radio was in vehicle with me and I was keying the "other" subscriber radio), when the right combo of channels got issued from the controllers. The TRBO controller seems to get hung up and starts its watchdog timer for interference.When this happens the subscribers start getting unsubscribed if prolonged interference. If over 28 seconds the Controller will switch to another radio to become the new controller. This a safety issue. BTW, I can key up the other repeater with one of it's handhelds, inside a SeaCan container from 15 miles away. Receive modulation was 5x5 using a R&S (Rhodes & Schwartz )receiver at the TRBO location. Is this a no brainer poor assignment of frequencies?
 
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