Need Help Understanding P25

[scannerowner]

I'm not all that familiar with P25 technology, so maybe someone can help me. I'm monitoring the MOSWIN VHF P25 Trunked System in Missouri with two P25 capable scanners. The site closest to me is only 10.5 miles away from it. My problem is both scanners are barely able to lock onto the control channel at the short distance. They go between "LNK" and "DAT" a lot and I have only 1-3 bars, and most of the time nothing. The next closest P25 trunked system is 800mhz and it's about 30 miles away and I can pick it up with one maybe three bars without any problem in the decoding. I figure knowing that it's VHF I would pick up the control channel at 4-5 bars.

I guess my questions are, do a lot of tweaks need to be done before running full power with a control channel? What all needs to take place before I can recieve this control channel at nearly full power? Do I need to setup band plans for the scanner for it to improve, from what I understand I don't need to, but not sure if I would in this case?! If that is what needs to be done, how would I setup the band plans? In general layman's terms, how does a P25 System work???

Thanks to anyone that can provide any input!!

 

[nd5y]

What scanner are you using?
In my experience with the local VHF P25 multi site non-simulcast system on a BCD396T I can tell you that it is normal for it to alternate between LNK and DAT when parked on a P25 control channel.

If you are getting any bars at all you should have no problem receiving the system.
I tried the various Uniden P25 level tweaking instructions that can be found in the wiki and none of them seemed to make it any better. Setting it to Auto or default (which is the same as auto on the 396) made it worse in some cases. I leave it set on Manual and 8 and it seems to work best.

If you have a late model Uniden or RS/GRE you don't need to mess with band plans. The newer scanners get the band plan from the control channel so there is nothing to adjust.

I don't know what you mean by receiving at full power. If you are upset because you don't have a full scale reading on your signal strength meter there is not much you can do except get a better antenna or move the antenna and that might make it worse if you are getting inteference from strong local signals. If the signal strength fluctuates on a local signal it could mean your antenna is on the edge of a null and moving it vertically or horizontally at least 1/4 wavelength may help. It could also mean your receiver is being desensed by a strong local signal.

Try listening to the control channel audio in FM and see if it noisy or full quieting, sounds distorted, drops out, or is being interfered with. That will tell you more than the useless bar graph meter.

When the repeaters are installed they are most likely set to whatever power they are licensed for. Usually that is around 100W for a system that is designed to cover large rural areas. They probably won't adjust the power levels or antenna patterns unless they have an interference problem.

 

[scannerowner]

I'm using a BCD996T and 396XT and so far I do hear some interference, but not much. I notice it going between LNK and DAT on the other P25 system, but not as much as this one. It alternates about every second, and there are currently no talk groups being used on it yet, they're still in the process of completing the system. I'm only 10.5 miles from the tower and I get nothing!! I'm wondering what the deal may be... I hear other Control Channels from other sites that are 30-40 miles away depending on skip conditions and they're at 1-2 bars. I don't understand P25 at all...Within the programming for P25 I hear static on the control channel and then it will show me the SYSID etc after the signal strength gets back up. They one license on the site, and no other frequencies assigned besides the control channel and it's input.