I believe I have tried both, but I'll try again to make sure. Yes, a Uniden.
I'm intrigued what @buddrousa has done as well and may try that method too to see if there's a difference.
How would one monitor radio IDs on the input frequencies?
- Thread starter BinaryMode
- Start date
I believe I have tried both, but I'll try again to make sure. Yes, a Uniden.
I'm intrigued what @buddrousa has done as well and may try that method too to see if there's a difference.
AB5ID
Member
Just wanted to throw this into the mix as food for thought. It comes from ChatGTP so take it for what it is worth.
You can determine the inbound (uplink) frequencies of a trunked APCO P25 system if you know the outbound (downlink) control channel and voice frequencies by using the standard frequency offsets for the system's band. These offsets vary based on the frequency band in which the system operates.
So in this case, the radios would be transmitting on 808.200, 809.100, 809.300, and 809.500 MHz.
You can determine the inbound (uplink) frequencies of a trunked APCO P25 system if you know the outbound (downlink) control channel and voice frequencies by using the standard frequency offsets for the system's band. These offsets vary based on the frequency band in which the system operates.
Step 1: Identify the System Band
P25 trunked systems operate in several frequency bands, each with a standard uplink/downlink offset:| Band | Typical Downlink (Tower TX, Radio RX) | Typical Uplink (Radio TX, Tower RX) | Offset |
|---|---|---|---|
| VHF (150 MHz band) | 159-162 MHz | 152-155 MHz | ~7-9 MHz |
| UHF (450-512 MHz band) | 453-470 MHz | 458-475 MHz | ~5 MHz |
| 700 MHz | 769-775 MHz | 799-805 MHz | 30 MHz |
| 800 MHz (Public Safety NPSPAC) | 851-860 MHz | 806-815 MHz | 45 MHz |
| 800 MHz (Rebanded & Non-NPSPAC) | 851-869 MHz | 806-824 MHz | 45 MHz |
Step 2: Apply the Offset
If you have the downlink frequencies (control & voice), you can determine the uplink frequencies by subtracting the standard offset for the system's band.Example: Determining Inbound Frequencies
Let's say you are monitoring an 800 MHz P25 trunked system and you find these outbound (tower-to-radio) frequencies:- Control Channel: 853.200 MHz
- Voice Channels: 854.100 MHz, 854.300 MHz, 854.500 MHz
| Outbound (Tower TX, Radio RX) | Inbound (Radio TX, Tower RX) |
|---|---|
| 853.200 MHz (Control Channel) | 808.200 MHz |
| 854.100 MHz (Voice Channel 1) | 809.100 MHz |
| 854.300 MHz (Voice Channel 2) | 809.300 MHz |
| 854.500 MHz (Voice Channel 3) | 809.500 MHz |
Step 3: Verify Using Radio Software or SDR
If you want to confirm these frequencies:- Check FCC Databases – The FCC’s ULS (Universal Licensing System) can provide licensed trunking frequencies for a given area.
- Use SDR (Software-Defined Radio) – A scanner or SDR (like an RTL-SDR, SDRplay, or Uniden scanner) can be tuned to possible uplink frequencies to listen for bursts of data or low-power radio transmissions from nearby units.
- Trunking Decoders – Software like Unitrunker or DSDPlus can help decode the system and confirm inbound frequencies dynamically.
Key Takeaways
- The inbound (uplink) frequencies are usually 45 MHz lower in 800 MHz systems and 30 MHz lower in 700 MHz systems.
- Other bands (VHF/UHF) use smaller offsets (~5-9 MHz).
- Once you have outbound control and voice frequencies, simply subtract the offset to find the inbound frequencies.
- Tools like SDR, trunking scanners, and FCC databases can help verify.
For VHF it's worth nothing unfortunately.
AB5ID
Member
Why is it worth nothing? I'm not disagreeing with you just wanting to learn more
nd5y
Member
Same for UHF.
ecps92
Member
AI = Always Incorrect
in the US of A there is no band plan for input/outputs for Part 90 VHF
from what Tom added, AI is wrong
Some UHF = -5 (does not apply to the Broadcast "BA" allocations" which can be + or - and not always 5
UHF T = -3
nd5y
Member
AI is useless for anything but entertainment. Only a fool would use it for learning.
You want to know about uplink/input frequencies then read this:
Offset - The RadioReference Wiki
AB5ID
Member
Fair enough, I read through the Wiki; Thanks for the link.AI is useless for anything but entertainment. Only a fool would use it for learning.
You want to know about uplink/input frequencies then read this:
and that article wasn't written by AI.Offset - The RadioReference Wiki
wiki.radioreference.com
Would you help me understand how I could figure out the input frequencies for this local non-simulcast trunk system, or is not possible to know with info available online?
Missouri Statewide Wireless Interoperable Network (MOSWIN) Site: Independence
Missouri Statewide Wireless Interoperable Network (MOSWIN) Site: Independence Details
www.radioreference.com
Systems the days use multi-block TSBK's, so the input frequency is transmitted in the VC grant. Look into one of the 9600 CC decoders as they should display the info.Fair enough, I read through the Wiki; Thanks for the link.
Would you help me understand how I could figure out the input frequencies for this local non-simulcast trunk system, or is not possible to know with info available online?
Missouri Statewide Wireless Interoperable Network (MOSWIN) Site: Independence
Missouri Statewide Wireless Interoperable Network (MOSWIN) Site: Independence Detailswww.radioreference.com
nd5y
Member
For VHF systems, even conventional repeaters, there is normally no standard offset and the FCC doesn't require specifying frequency pairs for each repeater.
There could be 6 repeaters and the license will show 6 FB frequencies (outputs) and 6 MO and/or FX frequencies (inputs). Normally there is no way to tell which input is paired with which output. You might get lucky and find that the number order of the frequencies on the license matches if the person who filled out the application entered all the data in order.
Here is the license for the site you posted.
FCC Callsign WQPV891 (MISSOURI, STATE OF)
FCC Callsign WQPV891 (MISSOURI, STATE OF)
www.radioreference.com
The paging and marine channels, which this site doesn't have, are paired so those are easy to figure out.
The only thing you can tell in this case is 151.34 is the input to 154.935.
The 163-172 MHz frequencies are federal. All you can tell from the license is 163-166 MHz FX1 and MO8 are inputs and 169-172 MHz FB8 are outputs. There is no way to tell how they are paired without direct monitoring.
AB5ID
Member
Thank you. I really appreciate your time and the explanation. My head started spinning trying to understand VHF/UHF part of the Motorola trunking section on the Wiki page.For VHF systems, even conventional repeaters, there is normally no standard offset and the FCC doesn't require specifying frequency pairs for each repeater.
There could be 6 repeaters and the license will show 6 FB frequencies (outputs) and 6 MO and/or FX frequencies (inputs). Normally there is no way to tell which input is paired with which output. You might get lucky and find that the number order of the frequencies on the license matches if the person who filled out the application entered all the data in order.
Here is the license for the site you posted.
MOSWIN uses a mix of public safety, paging, auctioned marine and federal frequencies depending on the site.FCC Callsign WQPV891 (MISSOURI, STATE OF)
FCC Callsign WQPV891 (MISSOURI, STATE OF)www.radioreference.com
The paging and marine channels, which this site doesn't have, are paired so those are easy to figure out.
The only thing you can tell in this case is 151.34 is the input to 154.935.
The 163-172 MHz frequencies are federal. All you can tell from the license is 163-166 MHz FX1 and MO8 are inputs and 169-172 MHz FB8 are outputs. There is no way to tell how they are paired without direct monitoring.
Many thanks!
Steve
AB5ID
Member
Thank you. Do you know if Pro96com still a viable option? I have a 396xt with serial cable that I'm not using.
I would think so. You may have to dig around to find the VC grant messages, I haven’t used it in several years and don’t recall the option to see them.
nd5y
Member
This assumes that all the repeaters are in the same location. Usually the location for mobiles is a radius around the repeater or base. If there are multiple locations with only one repeater per location then you can usually match the inputs with the repeaters.
Sometimes the mobile location is a state, county or other area and you won't be able to match them if there is more than one repeater.
