More on Elephants and Alligators

Many DMR radios have a hidden feature that allows a user to enter a “service menu” through a series of key presses that offers a few features helpful for troubleshooting and adjustments. Both Motorola XPR gen 1 and gen 2 series radios as well as Hytera HT’s and mobiles have this facility.

One of the functions present in this service menu is RSSI , or received signal strength indicator. This gives you a fairly accurate ( to be determined), way of measuring RX signal strength in dBm. A kind of a poor man’s measurement receiver. An alligator meter for a repeater transmitter, you might say.

As far as the elephant part of the equation, the live Brandmeister last heard list can display the measured RSSI value reported to their network by Hytera and Moto repeaters receivers. Sounds like an elephantometer to me.

So by monitoring the RSSI values from the rig and from the last heard simultaneously, what we have is a way to compare RX vs TX performance of a repeater from a particular location. We can use either a HT or mobile radio for the test, and evaluate the performance at a variety of remote TX power levels.

The accuracy of these RSS indications is somewhat unknown without additional measurement verification, but that’s pretty easy to do. Just put your rig into the service mode, display RSSI and feed in a calibrated signal generator, and see what it reads. Similarly, a measured DMR signal, from say a HT with an attenuator, connected directly to the repeater RX input will give a known RSSI value to be compared to the reading on the last heard. Ideally, a couple of power levels should be checked on both the subscriber radio and repeater.

Some initial trials on the new K7EVR machine look promising as being a valid measurement technique, but calibrated tests have not been performed yet. The results will be updated here, if significantly different. As an example, using a MD782G mobile radio programmed for low power (5W), a quarter-wave vertical antenna @ 4M AGL, and a distance of 11 KM (LOS), the RSSI values were within about 3 dB of each other, with the RSSI from the repeater TX being the better number. That seems to indicate the ears and mouth are about the right size for say a 10-15W mobile station.

More testing to be done, but interesting..

Dave, K7DMK

K7EVR Digital Amateur Radio