I've learned online some very thin information about how many hams will use 100 Hz CTCSS tones on their APRS transceivers to be able to make voice contacts with hams who are also using APRS.

I've been experimenting with APRS for the past couple of years and have upgraded my car rig to a dual bander that supports APRS on one of the VFOs and it's been a great travel companion.

We're about to engage in a long highway trip (3,000 km) and I'd like to know more. How does this work? I've already configured my APRS this way and the local iGate/digipeater is still digipeating my packets fine and relaying them to the Internet. I assume the remaining stations I hear on the speaker are ones also using tone? Presumably they are able and willing to make voice contact? What's the proper operating procedure to do this?

I think it depends a little on the particular radio in question. On my Yaesu FTM-400 I will receive any APRS beacon within range of my radio that transmits a 100 Hz tone (the digi won't forward the 100 Hz tone). However when another station has a 100 Hz tone set, I will get an audible beep when the beacon comes through announcing that the other station is transmitting with a 100 Hz tone.

I believe the general procedure is to call CQ on the APRS frequency, make contact with the other station, then to agree on another frequency and finally QSY to that frequency for the QSO. That way you only impact the APRS channel momentarily and then are off that frequency so as to not impact other stations.

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