APRS fill-in digipeater 2E0RLZ-11

Radio amateurs use the APRS — Automatic Packet Reporting System — to share a variety of local information in real time. That includes the location of stations or other objects and telemetry like weather conditions. It can be used to transmit storm warnings or other bulletins. It can transmit short messages for other users or onward to SMS or email users. If you have a radio with APRS, you can turn it on and wait half an hour or so, and see what the local activity is at that time. Much of the same data is also relayed to the internet where it can be viewed on a map.

To communicate its status to the APRS network, a mobile radio will regularly send out a little AX.25 packet, beaconing its own information, for other stations to decode. Some of those stations will then retransmit the message far and wide so that it can be heard by as many additional stations as possible. Usually the stations repeating the message like this are situated on hills, or otherwise have a lot of range. This retransmission is known as digipeating ('digital repeating') and it's the process by which packets are propagated around a region, sometimes by repeating the message at several points.

I live just above Bradford-on-Avon, a town that sits partly in the Avon Valley. Mobile radios in dead zones like this can struggle to get a packet out to the network because while there may be an established, 24/7 local digipeater, a radio in a valley may not have a clear line-of-sight link to it. We solve this problem with a fill-in digipeater. These helpful stations listen for packets which have not yet made their first hop, and retransmit them. However, if a packet has already been digipeated by another station, that means that it has already got out there, and so a fill-in digipeater will leave it alone. By concentrating only on packets which might not otherwise make it out, they improve the outbound coverage for lower-powered mobile stations without significantly adding to network congestion.

When I am at my station I often operate a fill-in digipeater under the identifier 2E0RLZ-11. the 11 refers (tenuously) to the APRS path WIDE1-1, the path to which that a fill-in or first-hop digipeater will respond. The station is just a Kenwood TM-D710E radio connected to a rooftop white stick vertical antenna. The digipeater's participation in the APRS network is solely over the airwaves. It doesn't relay packets to APRS-IS. The local, always-on digipeaters like MB7UB (Bath), MB7VV (Lansdown) and MB7USW (Swindon), as well as the local iGates, already do a good job of that.

In line with Ofcom's updated licensing terms, the station uses a transmission power of five watts into a rooftop vertical antenna. Experiments have shown this is more than enough to forward a packet onto the rest of the APRS network. And according to aprs.fi the station regularly relays traffic from stations 70km away or further, especially during unusually good VHF propagation.