How can I tell if someone is mistakenly transmitting on a repeater output frequency, instead of the (offset) input frequency?

Question

I sometimes run a VHF radio net for our club, on a 145.170 MHz channel served by a repeater with a -600 KHz offset. People who program their transceiver with the proper repeater offset mode use it with no problem.

But some people fail to use repeater offset mode. They transmit simplex on the repeater's output frequency. I hear them, because they happen to be close enough to my station that their signal reaches me simplex. But they aren't using the repeater, and others further away from them don't hear them.

Assuming I have spare radios for that band, how can I set them up to detect this situation? I want to find out that it's happening and let them know to fix their transceiver mode. I can imagine setting a spare transceiver to listen, simplex, on the repeater's input frequency. If I see that spare transceiver receive at the same time as I hear the station on the repeater's output frequency, then it's likely that station is configured correctly.

But if I hear a station on the repeater's output frequency, but don't see the spare transceiver receive, then it could be one of two situations. Either the station is configured wrong, transmitting simplex on the repeater's output frequency, or the station is configured correctly but is distant, so their transmission makes it to the repeater but not to my spare transceiver. I don't see how to tell these two cases apart.

To be clear (because repeater offset notation always confuses me), we publish the channel as "145.170 MHz -600". That means the repeater transmits on 145.170 MHz, and receives on 144.570 MHz. Our transceivers complement this: receive on 145.170 MHz, and transmit on 144.570 MHz.

Answer 1

Some ways, roughly in order of increasing difficulty but also automatability:

1. Most repeaters have a "courtesy beep" they transmit when their squelch detects the end of an incoming transmission, followed by a period of silence. A direct transmission from a different, normal, transmitter will not have the courtesy beep and silence unless it was deliberately imitated.

2. The repeater will have a generally consistent signal strength and the other transmissions will likely be weaker, or at least not match exactly:

   • You can observe the difference on your S-meter, if your receiver has one.

   • You will hear more of the noise that is characteristic of a weak FM signal than from the repeater. Of course, you can also get that noise on the way to the input of the repeater, but in my experience the noise will sound different in the two cases (because of the audio-frequency characteristics of the repeater).

3. Have a direction finding system. Observe that the direct transmission is coming from a different direction.

   This could be as simple as a Yagi antenna pointed at the repeater and a second "omnidirectional" antenna. Connect the second antenna to a second receiver on the output frequency and notice when the ratio of signal strengths does not favor the Yagi by the normally observed amount.

4. Some work has been done on "fingerprinting" transmitters using, if I remember correctly, the frequency deviations at the beginning and end of transmission as the transmitter powers up and down. You could implement this on SDR receiver hardware, and look for transmissions that do not match the characteristics of the repeater's transmitter.

Answer 2

Some repeaters filter the CTCSS tone. If that's the case, then if you detect one, then it isn't going through the repeater. Some repeaters use a different tone on the output than on the input...also a way to tell.

Answer 3

It is highly unlikely that the station transmitting on the output of the repeater will have exactly the same signal strength as the repeater transmitter. Simply monitor the signal strength of the transmission and if it is not typical for the repeater output, the station is likely transmitting on the output.