Control Frequency Help (Maryland, USA)

[studysession]

Hi -
I am having trouble figuring out control frequencies. Someone on this forum helped me figure out which frequency to use for Carroll County, Maryland, USA.

I type that one 852.9375 in SDRSharp and I can see the signal in the display.

Going through county after county across Maryland (Baltimore, Howard, Prince Georges, Fredrick, etc..) entering all the control frequncies one by one and cannot see anything for any of them.

https://www.radioreference.com/apps/db/?stid=24&tab=trs

What am I doing wrong? Thanks

 

[ka3jjz]

Well first off it would help if you were in the right county - all you need to do is click on the name, viz.

https://www.radioreference.com/apps/db/?sid=8201

Now see those freqs in red and blue? They are the primary (red) and secondary (blue) control channels.

Concentrate on just this one- the other counties are, by and large, out of range unless you have a very good antenna nice and high outside.

Mike

 

[studysession]

Ok so might be out of range - that makes sense. Will try it again when I am down near Baltimore County.
That county is not far from me but the antenna might be. I do not have an outside antenna so this makes a lot of sense.
Thanks

 

[ka3jjz]

Besides the lack of antenna, once you get one county going, you will then know how to get the others. Don't try to swallow all of this at once, it's easy to get lost in the mud

Just curious, what are you using? Mike

 

[studysession]

RTL-SDR.com dongle and telescopic antenna

All indoors - so when he commented about not being close to the antenna it made sense. I will be in those counties on Tuesday, so will try it from my truck at that time.

I made me and my brother each a Trunk Digital Police / Emergency Radio scanner using Raspberry Pi, SDR Dongle and touch screens. I now want to make config files for the rest of the counties in Maryland and Washington DC.

 

[studysession]

Also I am using boatbod's branch of op25

 

[boatbod]

First things first, I'd recommend firing up op25 in non-trunking mode and see what the fft plot (#1) shows.

./rx.py -P fft --args "rtl=0" --gains 'lna:36' -S 1440000 -v 1 -q 0 -f 852.475e6 -2 -U 2> stderr.2

If you are lucky you'll see a spike near the central tuning line and this would indicate you have some sort of signal present. After that you can think about setting up a proper trunk.tsv and seeing if you can enable trunking using -T trunk.tsv

 

[studysession]

Thank you but not at all

Just tried N. Baltimore County.

https://www.radioreference.com/apps/db/?sid=7117

And I just realized that it says Phase I - is that a diff command?

If not - then I must be too far from the antenna

 

[boatbod]

Phase 1 / Phase 2 really doesn't matter for the purposes of seeing a signal on the fft plot.
I suggest retrying with each of the frequencies listed just in case the control channel is not active on the freq that it was originally entered into RRDB.

 

[studysession]

thanks and will do

 

[studysession]

I have now figured out the control frequencies for couple other counties. Is it any different command if they are on Phase 1 or is that automatic?

Thank you

 

[boatbod]

I have now figured out the control frequencies for couple other counties. Is it any different command if they are on Phase 1 or is that automatic?

Thank you

All P25 control channels are essentially the same regardless of whether the audio is phase 1 or phase 2. The key difference you'll see for phase 2 traffic is the presence of a slot id ("Slot 0" or "Slot 1") for voice traffic whereas for phase 1 there is no slot.