Pi Hotspot

Purpose

One of my initial concerns when getting my Tech license and a VHF/UHF handheld transceiver was that I might not have any repeaters in range and everything would be silent. I considered a DMR hotspot to be a key fallback that would allow me to make QSOs right away, no matter what.

In practice it doesn’t carry the no-infrastructure magic of purely RF communication, but it makes sure you can always talk to someone and get practice being on the air. Without it you might be waiting for a weekly net, or just calling “listening” into a local repeater and hoping someone comes back.

As it turns out, the hotspot also gave me an opening into learning basic Linux and web hosting—but that’s for another article.

A hotspot works roughly like this:

Handheld radio → Hotspot → Wi-Fi → Internet → BrandMeister network → Other radios

Your handheld talks to the hotspot over RF, and the hotspot connects you to the wider DMR network through the internet.

Equipment

There are many pre-built hotspots available, but in the DIY spirit I decided to assemble mine. The ingredients were:

MMDVM Hotspot

https://www.amazon.com/dp/B0BP6ZYF18

You can buy a simplex (one antenna) or duplex (two antennas) version. The general consensus online seemed to be that simplex was simpler and would accomplish most of my needs, so I didn’t spend too much time researching beyond that.

Some versions include a small OLED screen. I haven’t found it especially useful, but it does look cool if you use a transparent case.

Raspberry Pi

Finally a hobby that gave me a reason to buy one and have a purpose for it.

After some deliberation I went with a Pi 3, which seemed like a sweet spot between processing power and power/cooling requirements.

A Pi Zero will also do the job. However, if you want a bit more flexibility to run other things beyond just the hotspot—like Python scripts or small utilities—the Pi 3 is a better call.

One thing to note if you buy on Amazon: check what your Pi actually comes with. Mine did not include a 5V power adapter, so I had to buy that separately. You will also need a microSD card.

If you don’t want to solder, buy the Pi with header pins already installed so the MMDVM board can mount directly on top.

Case

I bought a case from C4 Labs:

https://www.amazon.com/dp/B07MQBYLGW

A solid case would work just fine, but I liked the transparent look for style points.

Approximate Cost

Roughly speaking the whole setup comes out to:

• MMDVM board: ~$40–60
• Raspberry Pi: ~$35–60
• Case: ~$20
• Power supply + SD card: ~$15

Total: about $110–150, depending on what parts you choose.

Assembly

Assembling the Pi, hotspot, and case was a fun little project. Everything went together easily and took about an hour.

With no soldering it was really more of an assembly than a build, but the directions from C4 Labs were straightforward—very much in a LEGO or IKEA style.

I was very happy with the end result.

Pi-Star Setup

Pi-Star is a software image built specifically for Raspberry Pi hotspots rather than the standard Raspberry Pi OS.

There are plenty of YouTube videos that walk through flashing Pi-Star to an SD card and doing the initial configuration, so I won’t repeat the entire process here. The ones I found helpful were:

[add links]

The only thing that wasn’t obvious was the need to add 01 to the end of your DMR ID when configuring the hotspot. You’ll first need to register for a DMR ID here:

[add link]

Most tutorials I saw skipped over that detail.

A Small Technical Note

One interesting thing about simplex hotspots is that they only operate on one DMR timeslot.

A full DMR repeater can support two simultaneous conversations using Time Slot 1 and Time Slot 2. A simplex hotspot, however, can only handle one at a time.

In practice that doesn’t matter much for personal use, but it’s helpful to understand why hotspots behave a little differently from repeaters.

Operation

So far I’ve mostly explored the following talk groups:

• 3100 – USA
• 93 – North America
• 91 – Worldwide

Worldwide (91) is a constant stream of QSOs. The trick there is listening to a QSO and being ready to jump on the PTT once it finishes.

A much easier way to make international QSOs is 93 North America. Despite the name, there are plenty of non-North American stations on it, and it’s far less crowded so you don’t have to be quite so quick.

3100 USA is usually quieter but still active enough that someone will often respond if you put a call out.

Lessons Learned

A few things I noticed after getting things running:

• A simplex hotspot is more than enough for most personal use
• Talkgroup 93 is one of the easiest places to get started
• The OLED screen is mostly cosmetic
• A Pi 3 gives you room to experiment with other projects later

Conclusions

The hotspot has really accomplished exactly what I hoped it would: there’s always someone to hear and communicate with on the radio.

That’s a big advantage when you’re getting into the hobby. You can operate without spending money on a more expensive mobile or HF rig, or putting up a large antenna.

Does it have the same magic as purely RF communication? No.

Does the sound quality and ease sometimes make it feel a bit like using the internet? Yes.

But it’s a great compromise that keeps you active on the air.

The other unexpected benefit is that having a reason to finally buy a Raspberry Pi opened the door to experimenting with some of my other interests, like Python and data analysis.

While Pi-Star doesn’t automatically store a lot of call or talkgroup data, it still provides enough information to play around with—and it makes me all the more likely to buy another Pi soon.