Wednesday, 5 June 2019

500kbps packet radio

Packet radio is pretty much all but dead here in the US but it seems not to be that way in France.

I recently saw a message on the AMPR mailing list about a 500kbps modem that someone in France had designed. The design files and software were put up on the Hackaday website. The design uses a handful of development boards cobbled together and has an Ethernet interface for the client computers to connect to. 

So I decided to have a go at this project. I ordered some PCB's from JCPL and searched the usual online suppliers for all the other parts. It took about 6 weeks to get everything together but for about $65 per modem I was able to build 2 and test them across my workbench.

One of the modems under test

I've been playing with them for a few weeks and am very impressed with them. I can sustain my MMDVM hotspot across them as well as attach a WiFi access point and make a WiFi Calling cellphone call through it too.

The downside for us here in the US is the bandwidth and speed. The FCC says that we can only use a symbol rate of 56kbaud and a maximum bandwidth of 100KHz for data modes on the UHF 70 centimeter ham band (where these modems operate). Currently the modem software will only allow links of 200kbps in 270KHz and so do not conform to the rules. 

Interestingly, this is not the only data mode to be found on the 70CMs band. Digital ATV has been around using the DVB-S standard for quite some time here. The ATV crowd erroneously reclassify their data (up to 2mbps!) as pictures/video rather than the data that it actually is. In this way they fool the FCC into allowing the mode without a rule change. So maybe that's how we do it for Packet Radio? Reclassify our data transfers as "image". After all, web pages have pictures in them. :)

Thanks to F4HDK for his design.

The network that supports the modems under test