2E0RLZ-10: My APRS iGate

The Automatic Packet Reporting System (APRS) is a way to share local information of immediate value in real time. This information is often the position of its users, but can also include short messages and announcements, telemetry data like weather reports and storm forecasts.

You don’t need the internet to use it, so it doesn’t rely on there being a good mobile phone signal wherever you happen to be. Instead, it uses established packet radio protocols on various amateur bands. A handheld transceiver can receive information packets for a computer to decode and display, and also send encoded packets out for others to receive on the same shared frequency. A network of digital repeaters re-transmit the message, transporting it further than the handheld transceiver can reach. Internet-connected receivers, called iGates, also collect the data and relay it into the APRS Internet System (APRS-IS) to appear on sites like aprs.fi.

2E0RLZ-10 is one such iGate. It is an attempt to improve APRS coverage in Somerset.

You can make one yourself using an SDR dongle and a Raspberry Pi. It is fairly easily done since there is an SD card image available which requires little configuration to get things going. The result is a Direwolf-based APRS decoder which sits and listens to the APRS frequency via the SDR dongle. Thanks to aprs.fi, you can see from where 2E0RLZ-10 is hearing stations.

The Raspberry Pi I use was kindly lent to me by Jeff Belcher. Thanks Jeff!

After being down in Somerset for a few weeks, I set up my iGate as 2E0RLZ-10 and hung the antenna so that it points towards the M5 and the A38. Hopefully I'll receive the APRS of a few passing stations, though coverage is pretty good around here, thanks to iGates like G7LEE-1 covering the levels, moors and coast from Glastonbury.

Testing it, and using my radio down here, it's fairly common to receive APRS packets direct from France or even Ireland. I've seen this before, particularly when I've been using my radio up on the hills. This is likely due to tropospheric ducting, the result of climatic conditions causing temperature inversions, which in turn cause a higher refractive index in the atmosphere, and bend the signal. Taking reports from many iGates, it's possible to build an idea of the current propagation from stations operating near 144MHz from the last hour.

Yesterday I installed the latest image onto my iGate setup. While I was playing with it, I noticed that sometimes the frequency drifts quite significantly and sometimes it stays pretty much spot on, especially if it's had chance to warm up a bit.

Sometimes the iGate picks up lots of APRS traffic and sometimes it's silent. For a long time I thought this might be due to a dodgy connector or a poorly-constructed antenna. Now I can see that it might be due to frequency shifts. Anecdotally, I also seem to be having better results on clearer days and when there was some tropospheric ducting happening.

Any station I hear seems to be inevitably also heard and reported by G7LEE-1, a much more capable setup on the other side of the levels in Glastonbury. That means coverage is very good around here, and hopefully my iGate is helping with that, even as a backup station.

The iGate has started to pick up passing stations. One of them was M0LXD-9, 63km away. I'm rather sceptical, to be honest.

I noticed that the iGate relayed a dozen packets last month from G7NVZ-7, a PicoAPRS tranceiver. These are tiny – about seven grams – with a transmission power of a watt. It's cool to see that they work, even with the fairly ropey antenna that I've got set up to receive these packets.

G7NVZ-7 passed again yesterday. When I compare each of the traces, I can see there's a range of about 3km in each direction on the M5. This is more likely due to my antenna than the PicoAPRS. I should look into getting a better antenna and asking for permission to install it on the roof.

On Friday I was asking in #lhs-radio about ways to improve the iGate's antenna. I remembered I had a loft, so far unexplored since moving to my new place in January. Depending on the roof material and the availability of power, a colinear antenna up there might work quite well. Once I get a ladder I will go up there and investigate...

The iGate has been packed away in preparation for a visit from some good friends. It should be back in a week or two. By that time my APRS radio should have arrived back from its second round of repairs and I can see what the range is like first-hand.