A Conceptual Question About Trunking

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MFD4305

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I hope this doesn't result in the need for an exhaustive review of trunking - which would probably mean I'd 'exhaust' everyone's patience!

I have monitored a local (nearest tower about 5 miles away) Moto P25 trunked system for several months. Reception has been outstanding and I've even had to lock-out certain Talk Group IDs because traffic was almost constant and I missed other, more interesting transmissions.

Suddenly (today) I'm receiving almost nothing on this system. The Control Channel seems to "fade away" for minutes at a time ['NFM' replaces the CC frequency on my BCD996T display]. When I do receive a transmission, it's often one side of the exchange.

Here's the conceptual question: since trunked systems are not the traditional base/mobile pairing where one signal (the base) would be expected to carry farther due to higher power, and the signal from a mobile unit or a dispatch point should arrive at my scanner with the same strength on one of the equal-power voice channels, why might I be receiving only a few transmissions?

Sorry - no prize for correct answers other than knowing you've helped me with a very perplexing question.

Thanks!
 

slicerwizard

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Toronto, Ontario
Here's the conceptual question: since trunked systems are not the traditional base/mobile pairing where one signal (the base) would be expected to carry farther due to higher power, and the signal from a mobile unit or a dispatch point should arrive at my scanner with the same strength on one of the equal-power voice channels, why might I be receiving only a few transmissions?
Because your control channel signal is poor? The scanner can't follow conversations if it can't properly decode the control channel.
 

DickH

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... why might I be receiving only a few transmissions?

Sorry - no prize for correct answers other than knowing you've helped me with a very perplexing question.
Look around your neighborhood for a cell phone antenna site. One have been installed recently. It may not be easy to spot because they often disguise them.
Intermittent reception is because some freqs. are affected more than others when certain cell freqs. are in use.
If your scanner has an attenuator, try turning it on.
 

loumaag

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I will go with DickH's assessment first. The telling sign is that you say the CCh seems to "fade away", it sounds like a reception problem. However, the "one side" of the conversation makes no sense whatever. All of the conversations you hear are coming from the tower (they are all on the output of the repeater). If you have few minutes to spare, read the Trunking Basics article on the RR Wiki.

Based on a recent cable experience here, another possibility since you are using a base station is if you are using an outside antenna, check the coax run from radio to antenna for damage, squirrels love to chew coax for some reason or another. :(
 

Caesar

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Based on a recent cable experience here, another possibility since you are using a base station is if you are using an outside antenna, check the coax run from radio to antenna for damage, squirrels love to chew coax for some reason or another. :(

not to stray from the subject at hand to much, but just think of how much better off we would all be in a world with out squirrels!! :)


but yes, try your ATT on, or if you are using external antenna just put a stock duck on it and see if you get any difference. do you have a mobile are you seeing a difference on it as you leave your neighborhood?
 

MFD4305

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Ypsilanti, MI
Resolution - I Think!

First - sincere thanks for the many thoughtful responses to my query. I tried most; some I had already thought of.

In the end, I apparently "got my system back" in a sort of unorthodox way. I accessed the semi-covert settings that came with the last firmware upgrade for the BCD996T and reset "P25 Decode Delay" from 400 mS to 0 (zero) delay, and reset the "Digital AGC Response" from 0 to -8. Traffic levels seem to be back to about what they were on this busy system - the Illinois StarCom-21 system. However, something's still not entirely back to normal as the display indicates that I'm briefly losing the Control Channel fairly regularly. Maybe I'll drive over to the tower that's 5 miles away and try out "close call...."

Another interesting note: one of the voice channels listed for my closest StarCom-21 site is also a Control Channel for the Michigan State system across Lake Michigan from my location (1/2 mile from the Illinois shore). If I 'hold' on that frequency, the display clearly shows the system and site ID's for the Michigan system - but I still receive regular voice traffic on that frequency. I guess that means the StarCom-21 signal overrides the Michigan Control Channel??
 
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