BLUF: Solution for TYT TH-9800, TH-7800, TH-7900 Radios on APRS is to Increase Frame Synch Preamble to 800 Milliseconds - But after changing you have to stop tracking and start tracking again.
I have had issues with my TYT Radios not being picked up on APRS.
I can use an FT60 and a Baofeng with a Nagoya antenna or hooked to the same base and mobile antennas that my TYT Radios are on and in all cases the UV5r and FT-60 work fine. When I transmitted from the TYT and listened on another radio I noticed the preamble was missing. The radio wasn’t going from RX to TX fast enough and the APRS preamble was getting omitted.
In APRSDroid > Preferences > Connection Preferences > Frame Synch Prefix > 800ms
Then back all the way out to the mail APRSDroid Log Screen > Stop Tracking (if you had started already) > Start Tracking.
For me the key was stop tracking and start tracking. At one point I had my Frame Synch Prefix set to 5,000,000,000ms and it made no difference. The Preamble was still too short but converting 5 trillion milliseconds would be several hours and I could tell the preamble was not more than a second at best. When I flipped it to Stop Tracking and Back to Start Tracking the preamble length finally stuck and took significantly longer. APRSDroid comes pre-set at 200ms. I went to 800 and have had successful results. May go to 1,000 just to make sure but if no change in performance, will back down to 800 again.
Note - At no time in this issue and trouble shooting was the Digirig-Lite ever the cause of the issues. It worked perfectly. Issue was with APRSDroid matching the TYT Radio’s ability to recognize a key up command in a timely manner - in milliseconds.