UHF P25 Antenna Issues

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Steven_024

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If this is in the wrong forum, please let me know!

I monitor a Fire Department that is on a UHF P25 system. The problem is my BCD436HP will not “decode” clearly. The repeater site is approximately 4 miles away from my scanner. I have an multi band Tram-1089 magnetic car antenna that I ran from my basement (where my portable is located) to the roof of my house. There is approximately 8-9 feet of BNC (3 Dbi) that is being ran. I have tried making a "conventional" system and a "P25 One Frequency" system on the BCD436HP. Neither of the systems will bring in audio clearly and constantly. It comes in garbled, in pieces, and sometimes not even at all. I am using Proscan to record the audio and UID's. I can normally hear audio clearly when a stubby antenna is connected while driving. I know the scanner is not picking up traffic because I have a Motorola XTS5000 that picks up all traffic clear.

I also monitor our county/statewide system on the same antenna and scanner with that audio coming in clearly.

My question is, is 8 feet of cable too much? Ive heard of signal loss but does that happen in BNC cable too?

Is there a special antenna I need to monitor a P25 UHF system?

Just trying to troubleshoot and find out what is causing the issue and resolve it ASAP.

Any help will be greatly appreciated!


73's

Steven
 

ko6jw_2

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Neither the cable nor the antenna care whether they are receiving P25 or FM or AM or any other modulation method. Does the system you are trying to hear have more than on site? Receiving signals from multiple sites can cause phase distortion in the radio. This could be causing the problem you describe. Paradoxically you may need to reduce the signal strength. Some have tried directional antennas. An attenuator may help. Your radio has a built in attenuator. Try switching it on.
 

Steven_024

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Neither the cable nor the antenna care whether they are receiving P25 or FM or AM or any other modulation method. Does the system you are trying to hear have more than on site? Receiving signals from multiple sites can cause phase distortion in the radio. This could be causing the problem you describe. Paradoxically you may need to reduce the signal strength. Some have tried directional antennas. An attenuator may help. Your radio has a built in attenuator. Try switching it on.
No, this system only has 1 repeater. According to FCC database, the repeater puts out 110 watts. With the antenna being only 3 miles away, I would have to think It couldn't be an issue with receiving a weak signal. I will try the attenuator and see what that does.

I know with HAM you need to worry about cable length due to signal loss, is that not the case with a scanner?

Is there anything else that you could think off that could use adjusting?
 
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jaspence

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Even large objects or buldings can cause radio waves to be reflected, which can give similar effects to multiple transmitting towers.
 

ko6jw_2

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Antenna and feed line are just as important with scanners as with ham radio. However, they don't have anything to do with P25 which is just an FM signal with digital modulation. If you take the radio outside with a whip antenna does it work? I've never owned a 436. My old 396T had some optimizing capability for P25. I don't run across P25 very much. None in this county. Only one or two trunked systems for that matter which is why I never upgraded my scanner. A another person suggested, there may be some weird path loss. Moving the radio around might help. Also don't overlook the possibility of some local interference (computer or other electronic device) that might be causing problems in the front end of the radio.

At 3 miles from the repeater losses in the coax of the antenna are not likely to be a problem.

Keep experimenting.
 
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