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

Repeater Tweaks

A recent trip to the repeater site was completed to take care of a few details after the initial install. The temporary low capacity backup battery was replaced with a proper one. A Deka Fahrenheit series 140 AH AGM battery was installed. Over a hundred pounds worth of battery is now connected to the Magnum Energy inverter-charger and should provide a minimum of 12 hours of 90/10 service down to 60%-70% depth of discharge.

A second task was to switch the DMR side of the system from Brandmeister master server 3102 to 3103. Ping testing showed about 10ms less latency to the CA based server. Packet loss has been minimal. The master server URL is a parameter that can’t be changed remotely with RDAC.

RF performance is still being evaluated in different parts of the East Valley, metro Phoenix, and beyond , but so far it looks very good. Because of mountains to the South of the site, coverage is compromised toward 180 degrees. This is predicted from modeling, and confirmed by a recent trip to Tucson. Coverage was also compared to other UHF repeaters at the same location, and was as expected. The goal was to have a solid signal in the East Valley including Mesa, Gilbert, Chandler, Tempe, Scottsdale and the Eastern half of Phoenix. This appears to have been largely achieved, but testing will continue. Mountains are both a blessing and a curse, depending on whether you are on the top or bottom, and on which side of them you find yourself. Because of our topography, shadowing of a few areas is inevitable. Commercial services like public safety employ multiple sites to minimize this problem.

Setting a balance between RX and TX range of a repeater is always a compromise. It depends on your target audience; are you looking for HT coverage or mobiles? Is it a 10W, 25W or 50W mobile? Etc, etc. I modeled using a 10W mobile with a unity gain antenna at 1.5M height on Radio Mobile to predict coverage. Not quite an HT, but not a 50W mobile station with a 6 dB colinear either. So anything more is gravy. Is it an Elephant or an Alligator? Not sure yet, but I might be able to make the ears a little larger; I have a 10 dB attenuator between the preamp output and RX input. Whether the extra gain would be helpful is questionable. No plans on adding a power amplifier though. 40W from the TX and 20W to the antenna should be enough RF for the desired coverage area.

Just a few items are left to be done. The most significant is a method of remote battery state monitoring. The charger in the Magnum Energy unit is fully automatic. It does bulk charging at 30A max and then switches to float. It would be nice though, to be able to check on the battery from time to time. The company does offer a way to read the SOC of the battery over IP, but it’s an expensive solution. An alternate way using the RasPi AllStar controller with an A/D converter will be implemented instead.

Dave, K7DMK

K7EVR Digital Amateur Radio