Saturday, September 1, 2018

Steep Learning Curve

Since I can't afford to get an HF (High Frequency) radio plus antenna at this point in time, I'm looking into Digital Mobile Radio aka DMR. This mode uses low bandwidth digital transmissions over UHF (Ultra High Frequency) and VHF (Very High Frequency) radio frequencies to a local repeater. The repeater is connected to the Internet and your digitized voice is sent out as an IP data packet. You use globally assigned talk groups to communicate kind of like a digital party line. Anyone who is keyed up on that talk group can communicate with the voice data traversing the Internet.

To use DMR, I needed a radio that has the proper digital capability. Two Chinese companies, Radioddity and Baofeng, worked together to bring out a very affordable tier 2 DMR radio. The RD-5R. The case looks like the very common (and cheap) Baofeng UV-5R but the innards, programming software, and commands are Radioddity. It is dual-band i.e. 2m and 70cm and supports analog in addition to DMR. Right now, I'm just monitoring the Ester Dome analog repeater at 146.880 MHz. One of the challenges for me is that the Radioddity programming software is Windows only. And DMR is too cumbersome to even try and do manually. If that’s even possible.

The other necessary piece is access to a DMR repeater. Unfortunately, there are no DMR repeaters in Fairbanks. In fact, the only one in Alaska appears to be in Homer. UHF frequencies are strictly line of sight so not a chance of reaching more than 20 miles or so. The solution is a hotspot such as a Raspberry Pi with a radio "hat". The Raspberry Pi is a small, cheap single board computer that was developed to teach computer programming skills. It has built-in Ethernet, HDMI video, 4xUSB ports, audio out, and a camera connection. The Pi 3 has built-in WiFi and Bluetooth. Since I only plan on running DMR, I'm using an older Pi 2 and a WiFi USB dongle that I've had lying around. The "hat" is a daughter card which, in this case, has a 70cm radio and is made by DVMEGA. The transmit power is only a tenth of a watt so this is strictly a local device. But local could be in the RV tethered to my phone.

The software image that I'm using is called Pi-Star and the download includes Raspian, the Linux operating system for the Raspberry Pi hardware so all I needed to do was copy the bootable image to a micro SD card and plug that into the Pi. With the DVMEGA hardware, it supports not only DMR but also D-Star and Yaesu System Fusion. These are two other digital modes. DMR seems to be a more open standard i.e. supported by cheaper radios, that's the one I opted to mess around with try. I now have the Raspberry Pi/DVMEGA hotspot running but needed to order a small 70cm antenna for the daughter board. The screenshot on the left is the Pi-Star software running my hotspot. 

DMR is somewhat confusing since it is a commercial system being adapted for amateur radio. It's far from a perfect fit. It took me a couple of days but I think that I finally have the digital part of the radio configuration worked out. On Thursday afternoon, I connected through the hotspot to talk group Tac-310, which is listed as North America, and talked to several guys in MN, CA, AZ and TX. And this was with the hotspot paired with my phone. I had the hotspot sort of working on Tuesday but couldn't hear any audio from the radio. At that point, I wasn't sure if it was the configuration of the radio or the hotspot. It turned out to be one checkbox on the channel config of the radio. There isn't a lot of documentation or information on this stuff except on the Internet. The last picture is the completed hotspot. I found a nice aluminum case on Amazon for only $7 and I just needed to cut a small hole for the antenna.

I haven't tested the range of the hotspot but it's now usable throughout the house. The limitations to the range are the gain of the antenna and the output power of the hotspot radio.

Sunday Afternoon Update - I updated from the Raspberry Pi 2B to the Pi 3B which has built-in WiFi. The CPU load is now about 1/10th of what it was but the metal case was limiting the WiFi range. This isn't really a problem as I'm planning on having this sit right next to the AP anyway. I plugged in the little USB dongle and the link quality jumped up to 100/100 from 40/100. The data needs of this system is pretty low but the reduced load and temperature is significant.

2 comments:

redlegsrides said...

Cool stuff, what one can do with a Raspberry Pi....

RichardM said...

I have five in use right now. Solar charge controller data collection. OpenVPN server. Media server. And this DMR hotspot. Plus I have one more with a wireless keyboard and 3 1/2" touchscreen to display the charge controller status. They are very handy little computers.