Henrik (LB5DH) and myself (LB0VG) are building a “fully” automatic SSTV decoder to be put up permanently in our club. The biggest part of this project is building an antenna from scratch.
The goal of the project is to have a SSTV decoder that in theory can be online 24/7 for displaying decoded pictures in realtime. In addition, it is also a rough way to display propagation for the 20m band. This will hopefully drive more attention to SSTV so that our club can become more active on digital modes.
The equipment we are going to use for this task is a couple of Raspberry Pi’s, a RTL-SDR dongle and a home-made vertical antenna. For software we plan to use gqrx and qsstv. The project will be divided into two parts
- The first part will be outside and connected directly to the antenna. This will recieve signal on 14.230Mhz with the SDR and gqrx. From there, an UDP stream containing audio-only will be sent from the receiver unit.
- The second part will be inside and connected to a display. This client will recieve the audio stream, and decode said audio into a picture-format using qsstv.
The antenna we are building is a shortened quarter-wave antenna. This consists of a 4m PVC pipe, approx 6 meters of cable (we chose the thinnest one) which is run inside the pipe. The excess wire that exceeds the pipe is coiled up in the middle of the antenna in approx. 6-7 turns. We used 32mm PVC, but thinner could work just as well. We are currently working with tuning the antenna by cutting off wire and adding turns of wire, but we are holding the final tuning until the antenna is up.
We have also ordered the RTL-SDR dongle, which is what allows us to listen to 14.230Mhz. RTL-SDR dongles usually don’t go below 24Mhz, but this dongle achieves this through a technique called direct sampling. This dongle will hopefully arrive in 3-4 weeks.
Except waiting for the dongle our next task is to find a suitable connector for the antenna. We are currently choosing whether to mount a connector straight to the wire or installing a chassis-mount connector on the antenna itself. The negative part about leaving a connector on the wire is that it can become fragile, whilst installing a chassis-connector requires some adapting to fit to our RTL-dongle. This will probably be figured out soon….
This is currently an ongoing project – new updates are coming soon!
73 de LB0VG