Andy Fletcher Homebrew

On the Bench

Omnidirectional Satellite Aerial

While HF is in the doldrums I thought I’d look skywards. See Satellite Aerial which is a Cebik Moxon aerial.

Brushing up my Electronics Knowledge

I was given ‘Learning the Art of Electronics’ by Thomas C. Hayes for my birthday. This is a hands-on lab course associated with the Horowitz book ‘The Art of Electronics’ (AoE). I am working my way through this and am hopefully filling some of the holes in my knowledge. It’s certainly made me think more deeply about some of the electronics I thought I understood! It doesn’t just rehash the AoE book – it approaches the theory in its own way. It’s good fun, so far.

Satellite Comms and RTL-SDR

I’m playing with satellite communications at the moment. My FT–60 handheld covers the main bands needed (2m and 70cm), but I can’t receive SSB or CW with it as it is FM only. So I wondered if an SDR receiver could do that job, while I scan eBay for suitable transceivers.

For background, I read an excellent introduction to SDR theory Software Defined Radio using MATLAB Simulink and the RTL-SDR written by Bob Stewart et al. at Strathclyde University here in Scotland. It covers modulation and demodulation theory and you can do exercises to help you better understand the theory using a cheap RTL-SDR USB stick. What is more expensive is MATLAB and Simulink. A quick calculation shows that you would need to buy MATLAB, Simulink, DSP System Toolbox, Comms System Toolbox and Signal Processing Toolbox. With a Home licence I make this add up to £245 with VAT included (here in Scotland). So it’s not a cheap option. As it is I already had the required MATLAB modules so it only cost me £23 for the RTL-SDR NooElec NESDR Nano 2.

I’ve thoroughly enjoyed the hands-on approach the authors have employed. The chapter on PLLs was a bit heavy on the theory with a good stretch of maths. I feel I understand the theory better (even PLLs). This may help with my PSK Modem project (see below) which is languishing a bit because the PLL on the coherent demodulator isn’t working.

The last three chapters use PLL blocks in the Simulink diagrams. I found the audio was jerky with my set-up. I guess that the Simulink ‘accelerated mode’ is needed. However, you need to buy two more MATLAB add-ons to do so. I didn’t bother as the course has videos showing how these Simulink models work.

At the same time as reading the book I’ve been playing with the RTL-SDR and Gqrx to look at satellite transmissions. I’m using gpredict by Alexander Csete, OZ9AEC to find out when the satellites can be seen from my QTH and the frequencies they are using. I’m using the MacPorts version of gpredict running under XQuartz on my Mac. I’ve only had limited success so far as I haven’t built a decent aerial yet. It’s also winter here and the weather doesn’t tempt me out into the garden, so the aerial is indoors which won’t help.

Here’s a screenshots of Gqrx showing a 1kHz sine modulating a carrier at 433.9 (into a dummy load).

And the same signal being demodulated by Simulink. The transmitter (see the RTL-SDR book appendix E.2 - Building an ‘RT4’ 433.9MHz AM Transmitter) is built on a breadboard so it’s pretty noisy.