Minimalist urban setup, part 4.

A quick recap: the major challenge in an urban environment is signal reception. Due to heavy city RF pollution – the result of hundreds of electrical devices in close proximity to a receiving antenna – only the strongest signals are audible and decodable.

The best way to think of it: the difference between the number of stars visible from within the city versus the majestic night sky in the middle of a desert. The bad news for urban amateurs is that the reception in urban areas will continue to worsen.

The good news: transmitting is relatively easy. Getting out is not a problem, as long as you are able to erect a quarter wave antenna above the roof line, or, at least, above the gutter. A modest output power of 100 watts is sufficient for worldwide coverage on 20m and above.

Here is an actual log of contacts made this morning running 60 watts into a quarter wave vertical with two radials, on FT8 mode:

There are plenty of juicy DX out there!

Back to reception.

My radio of choice is SunSDR2. For one good reason: it is a perfect radio for a challenging environment. With two receivers and a wide band scope, as well as the ability to zoom in and out of a band, this radio is essentially a precision spectrum analyser.

It allows me to literally see what is going on, in real time, to monitor the noise floor level and to see any interference appearing on the band, degrading the receiver’s performance. All these things – and countess more – an ordinary analog radio can not help with.

Yesterday I talked about a massive interference caused by my fridge’s inverter compressor: multiple spikes consisting of multiple signals spread 8.5KHz apart, over the entire HF band. With SDR, identifying an intruder is easy and instant.

Here is another example of an RF polluter: switch mode LiFePO4 rapid chargers.

Obviously, I knew straight away that this charger would cause interference, but without an SDR receiver it would be impossible to even guess what exactly is going on.

Here are two screenshots of 20m band.

The first one was taken with the charger turned on. The noise level is -95 dB.

The second screenshot shows the moment when the charger is turned off. The noise level is -110dB.

A massive 15dB difference! You can see that with the charger on (1), only a few of the strongest signals are above the decoding threshold, however, once the charger is turned off (2), a dozen or more signals are visible in the spectrum.

Getting off the mains, grounding, filtering and paying attention to every electrical device on your small city lot is crucial and will make a significant difference between hearing a weak signal or not.

My minimalistic setup is almost completed and the logbook is filling fast with two way contacts. Currently, FT8 only, but the Bencher paddle is to be plugged in soon. And there is one remarkable difference between SunSDR2DX and the old trustworthy IC-7700: a complete lack of fan noise, perfect silence! With that massive fan over the heatsink you would expect a mini typhoon in the shack! On the contrary, often, I find myself checking if the fan is actually working. The only sound that could be heard in the shack is a firm antenna relay click at the end of every 15 second FT8 transmission. Truly amazing.

[to be continued]