W8EMV - Whiskey Eight Echo Mike Victor

Tuning an MMDVM hotspot to minimize BER for multiple radios

My RD-5R gives about 0.4% BER when talking to my MMDVM hotspot, but my GD-77 with the same configuration gives nearly 1.0% BER. Audio quality is still good, but of course it's worthwhile to tune things to minimum error rates regardless.

Some notes on tuning the TX level on the radio:

• N6AMD, How to tune your MMDVM repeater with an SDR for low BER. Focuses on getting your audio levels correct; "If the raw audio is either too quiet or too loud, subscriber radios will not decode the digital signal properly." Uses pistar-mmdvmcal and an SDR to calibrate audio levels (TXlevel).

• F5UII, Install MMDVMHost on Raspberry Pi and tuning MMDVM for DMR digital mode . N6AMD's article is derived from this one, which uses both SDR and oscilloscopes for tuning. "The goal is simply that the audio levels are not too strong and do not saturate the analog input of the arduino".

Some notes on tuning the RX offset on the hotspot

• Ailunce, How to set RX TX offset. "Also every DMR radios may get a different BER when using the same hotspot. If you can't find a good balance for the BER between the different modes, it may be better to use different hotspots for each different mode."

Thanks to Christopher M0YNG on Feditalk to help me with some tuning ideas.

What is the effective coverage of my DMR hotspot?

My new MMDVM DMR hotspot shipped with a little SMA antenna, and has a power output measured in milliwatts. I wanted to figure out just what kind of effective range I had for it, so that I could get away from my desk while connected to it.

My radio is a DM-5R with low power at 1 watt and high power at 5 watts. At about a block away from home, transmitting on low power, I can get a clear echo reply to my hotspot with Parrot. At four blocks away, the low power TX doesn't get into the hotspot, but my RD-5R still picks up some fraction of activity on an active channel.

The next test was to change to high power (5 watts) on the radio, and to get a better antenna setup for the hotspot. I have a 2 meter ladder line antenna that also works on 440, so I connected that up to the SMA port to see how it would work. My best distance was about 137 meters, as measured by GPS, from my house to the bus stop. That's almost line of sight with just one building in the way using high power (5 watts) on the RD-5R into the hotspot, and Parrot to verify transmissions.

Some other DMR range experiments from Youtube:

• K0LWC measured 0.7 miles (1100 meters) with an Anytone 878 into a 10 milliwatt hotspot, with clear line of sight across a park and no buildings obstructing his view.
• KG4VDZ hooked up a SharkRF OpenSpot 20 milliwatt hotspot to a 50 watt amplifier, and claims a DMR range of 15 miles using a vertical antenna on top of a tower.

dmrconfig - a configuration utility for DMR radios

I have a Radioddity / Baofeng RD-5R DMR radio. I got it back in 2018, and have recently added an MMDVM hotspot for it to my collection.

One of the downsides of programming this particular radio is that CHIRP (which is used to program analog radios) is not available for DMR, and that the supported radio software only runs on Windows. And while Windows is fine, the rig I have to run that at home is an ancient Eee PC with a small screen that makes using the programming software awkward at best.

An alternative to the vendor software is dmrconfig, a configuration utility for DMR radios by Serge KK6ABQ. It supports a pretty good set of DMR radios including my RD-5R, and it runs on MacOS and on Linux. It lets you read and write codeplugs to the radio, and it helpfully converts your codeplug to a human readable file so that you can edit it in a text editor.

So far so good with dmrconfig - I've only used it once, but the programming looks like it took just fine. It's certainly a lot more helpful to have a format that can be backed up easily! It does however make me want to get a radio that can have more user entries in the database, the RD-5R is skimpy on that account.

Also noted but not tried yet: qdmr, which uses the same codebase as dmrconfig for radio codeplug writing but that offers a GUI for it. Worth a test another day.

Thanks to @[email protected] for the reference to dmrconfig - they are using an Anytone 878 radio with this particular setup.

Radioddity RD-5R "RX Group List" - or, why can you hear me and I can't hear you?

A programming quirk of the Radioddity RD-5R.

If you want to add a DMR talkgroup to talk on, there are really four things you need to do:

1. Add a digital contact, listing call ID and name.
2. Add a channel, listing that contact along with frequencies.
3. Add that channel to a zone, allowing you to select it on the radio.
4. Add the channel to the "Rx Group List" so that you can listen to it.

If you miss step 4, you get the following: people can hear you on a channel, but you can't hear them.

Debugging this was tricky. I had added a few extra talkgroups to my hotspot configuration, so I thought I had checked everything - and it turns out I had everything correct for transmitting. What could be different about the Feditalk group? Well, it's UK based, so I tried connecting to the UK Brandmeister server and listening to some other UK talkgroups, and that didn't work.

The AHA moment was of course an RTFM situation. Section 3 of the Radioddity DMR Quick Start Guide referenced a screen I hadn't seen before, the "RX Group List". Adding the channels to that list (and noticing that they weren't there already) fixed the problem.

Thanks to the following for helping me through this bit of troubleshooting:

• Nick W8XM for the ARROW RD-5R codeplug and some timely consulting on RX offset programming
• Patrick KC3OWU for encouragement and debug help and a first successful QSO on Feditalk, TG 23514
• Christopher M0YNG for running mastodon.radio and prompting this exercise in the first place

Having figured all this out, I'm now intrigued by OpenGD77 which is open source firmware for this RD-5R radio and for other DMR radios. Perhaps a next project!