ICI Trunked reception on a Uniden 996P2

[xilix]

I may have gotten a little spoiled by my SDS 100, which seems to have no issues receiving the Glendale ICI cell from Pasadena.

I just acquired a 996P2 for mobile use. Sitting here on the bench, the 996P2 is connected to my external discone and is getting a good signal RF-wise. The SDS 100 is just using the stock antenna.

I only have the Glendale cell active in both radios, so when idle, both scanners sit on the Glendale control channel. Both scanners are in ID Scan mode.

When I receive traffic, the SDS100 instantly makes the jump to a voice channel and receives OK. The 996P2 will continue to sit for about 3 seconds (sometimes more, sometimes less) before making the jump. If the QSO is short, it won't jump at all.

I've looked thru the P25 audio decode wiki page over here. When in the P25 auto mode, the P25 threshold seems to jump around from about 7 to 15. I've tried to manually change it but haven't had much success.

Searching for the secret sauce to make this work!

 

[maus92]

I may have gotten a little spoiled by my SDS 100, which seems to have no issues receiving the Glendale ICI cell from Pasadena.

I just acquired a 996P2 for mobile use. Sitting here on the bench, the 996P2 is connected to my external discone and is getting a good signal RF-wise. The SDS 100 is just using the stock antenna.

I only have the Glendale cell active in both radios, so when idle, both scanners sit on the Glendale control channel. Both scanners are in ID Scan mode.

When I receive traffic, the SDS100 instantly makes the jump to a voice channel and receives OK. The 996P2 will continue to sit for about 3 seconds (sometimes more, sometimes less) before making the jump. If the QSO is short, it won't jump at all.

I've looked thru the P25 audio decode wiki page over here. When in the P25 auto mode, the P25 threshold seems to jump around from about 7 to 15. I've tried to manually change it but haven't had much success.

Searching for the secret sauce to make this work!

This is typical behavior for a 996P2 when monitoring a P25 simulcast cell. You might have too much antenna, i.e. receiving multiple sites at similar power levels. Since the 996P2 is not designed to receive simulcast (the SDS radios are specifically designed to receive P25 simulcast,) you need to try to isolate reception to one tower site using a directional antenna or antennuation. When mobile, sometimes the radio will work fine in certain areas, and not in others. Same principle.

 

[wtp]

switch the antennas...

 

[xilix]

switch the antennas...

This is typical behavior for a 996P2 ...

Sometimes, less is more!

So I ended up putting a rubber duck antenna on the 996P2 and, as the kids say, OMG!

The SDS100 still switches to the voice channel a little faster, but the difference is now less than 1/2 second and is consistent.
The P25 decode parameters are no longer jumping around and I was able to manually dial in a decode threshold.

A follow-up question about that: The P25 decode threshold I dial in for say, the Glendale cell - would that be the same value for all cells in the system or would each cell (or site) have it's own threshold value ?

I need to read up more on simulcast systems - I am guessing that the control & voice channels are the same freq. across multiple mountaintops and I could see how that would be a real problem with our "consumer grade" radios.

Thanks for the help !

 

[hiegtx]

I need to read up more on simulcast systems - I am guessing that the control & voice channels are the same freq. across multiple mountaintops and I could see how that would be a real problem with our "consumer grade" radios.

Thanks for the help !

Start with this article from the Wiki.
Simulcast digital distortion - The RadioReference Wiki

Essentially, you are correct. The same transmissions occur, at the exact same time, on the same frequencies used in the various simulcast site towers. These transmitters are at varying distances from your location. While radio waves move at light speed, and the time differentials for various copies of the same transmission to arrive at your location are extremely small, they are still enough to keep the scanner from reliably decoding the control channel data and getting clear audio to come across from your scanner. By changing to "less" antenna on your 996, a strong signal from one specific tower may be able to override out of sync, but weaker, signals from more distant sites. That might also be the case if you lived very close to one site. The strong signal might drown out traffic from more distant locations. Of course, all bets are off if you were driving around the area. While you are getting good results at home with the antenna swap, you would likely lose that advantage if you were mobile in your area.