DMR distance limit

I have heard ‘traditional wisdom’ that there is a timing limit imposed by the DMR protocol limit that limits radio path lengths to less than 70km.

There is indeed a limit and it is due to the 2ms guard interval between time slots, and half of that time is available for accomodation of round trip radio propagation delay (RTT), so 1ms with c0=3e8m/s gives a one way distance limit of 150km.

To put the ‘traditional wisdom’ to the test, I took a UHF DMR handheld up onto a nearby hill and successfully had a medium length QSO via the VK2RAG repeater netoworked to VK1BGT using the VK1RBM repeater.

The radio I used was a hand held Tytera MD-380 portable, nominally 5W output and using its rubber ducky antenna.


Above is my end of the contact, a radio path of 146.4km with near perfect audio in both directions.

This contact would suggest that near to the theoretical timing constraint of 150km can be achieved and that lower practical limits are not simply due to the timing constraint but include other factors.

From the same site I could reliably reach VK2RCG and VK2RHP, and not trigger VK2RLE or VK2RRW. The problems with the latter sites was not simply timing related but basic coverage issues.