Frequency Question P25 TRS

[PHOENIX_SCANNER]

At the risk of asking something that may seem obvious, here goes.

Ok, so a system has a dozen frequencies let say, with two alternating control frequencies.

I hear a transmission on talk group x, and the control frequency is obviously in play, as well as one other, lets say it is 868.9375.

What frequency is the patrol cars mobile radio transmitting on? Is there a separate repeater input frequency, or are both sides sent on 868.9375?

Thanks.

[DaveNF2G]

All of the communications required to set up a call are transmitted on the CC. Mobiles and portables transmit to the repeater input side of the CC, just like they do on the voice channels.

[AZScanner]

The patrol car is transmitting on the mobile side, exactly 45MHz lower. In this instance it would be 823.9375

I've confirmed this by creating a simple VB control program that emulates the close-call functionality for my BC250D. I programmed the mobile side of the system I wanted to scan as a series of conventional channels, then when the scanner stops on one, the program tunes the scanner exactly 45MHz higher. Worked like a charm to monitor any close by activity on the system while ignoring other, farther off transmissions.

-AZ

[DaveNF2G]

Cool. Sounds like the "repeater finder" function on some newer scanners.

[AZScanner]

Yep. I always knew when Phoenix PD was nearby, even if they were on an encrypted channel.

-AZ

[PHOENIX_SCANNER]

Originally Posted by AZScanner

The patrol car is transmitting on the mobile side, exactly 45MHz lower. In this instance it would be 823.9375

I've confirmed this by creating a simple VB control program that emulates the close-call functionality for my BC250D. I programmed the mobile side of the system I wanted to scan as a series of conventional channels, then when the scanner stops on one, the program tunes the scanner exactly 45MHz higher. Worked like a charm to monitor any close by activity on the system while ignoring other, farther off transmissions.

-AZ

That is a darn cool idea. Good shot.

And thanks to both of you for answering. So there is the offset just like the old school stuff, makes sense.

[qt31005]

Originally Posted by AZScanner

The patrol car is transmitting on the mobile side, exactly 45MHz lower. In this instance it would be 823.9375

I've confirmed this by creating a simple VB control program that emulates the close-call functionality for my BC250D. I programmed the mobile side of the system I wanted to scan as a series of conventional channels, then when the scanner stops on one, the program tunes the scanner exactly 45MHz higher. Worked like a charm to monitor any close by activity on the system while ignoring other, farther off transmissions.

-AZ

Ok let me see if I understand you right. I take 773.1312 and down grade it by 45 maken it 728.1312. That would make it so i only hear cars clost to me. Ok now dose this work for everything even the APO-25 system?

[AZScanner]

Originally Posted by qt31005

Ok let me see if I understand you right. I take 773.1312 and down grade it by 45 maken it 728.1312. That would make it so i only hear cars clost to me. Ok now dose this work for everything even the APO-25 system?

The key is having a computer program running to tell the scanner to tune to the base frequency when there's activity. Without that, you're kinda hosed because you will ONLY hear the mobile side, not the base side which is often just as interesting if not more so than what the mobiles are saying. (Although you CAN push the reverse button on a BC250 and it will hop over to the repeater side for as long as you hold down the button... kewl).

First, you program ALL the mobile frequencies of the target system into a bank on your scanner. Then you start scanning that bank and fire up the program.

Here's what the program does:
1. Scanner stops on 728.13125 - PD officer on his portable is talking to dispatch near you.
2. The program detects that scanning has stopped on the 728 freq and issues a command to the scanner to tune to 773.13125 - 45MHz higher.
3. Now both sides can be heard. The program continues to monitor the scanner, waiting for the transmission to end and the squelch to close.
4. The transmission ends and the system releases the frequency. The scanner resumes scanning.

It was a quick and dirty little proggie I wrote. Only took about 15 minutes or so to write once I thought of the idea.

-AZ

[mikey60]

Originally Posted by qt31005

Ok let me see if I understand you right. I take 773.1312 and down grade it by 45 maken it 728.1312. That would make it so i only hear cars clost to me. Ok now dose this work for everything even the APO-25 system?

Unlike the 800Mhz band, where the input is exactly 45Mhz below the repeater frequency, in the 700Mhz band (Frequencies below 806Mhz), the input is exactly 30Mhz *Higher* than the repeater frequency, so for 773.13125, the input frequency would be 803.13125Mhz.

Mike

[qt31005]

Thanks mikey60 it seams to be working cuz my scanner would not work with it being 45 lower. I will give it a shot.