Pandemic life, RFI tales and other projects

Posted on 2021-08-22

As with everyone else in this planet, life took a turn last year March. It is August 2021 and still doesn’t show any inkling of slowing down, at least in certain parts of this country.

One of the regular things I used to do before pandemic for the past several years was to visit my parents in Kerala every month. That had to be abruptly stopped and with no means to even do video calls (they didn’t have a smart phone, to start with), I started getting anxious about their well being. Once the first wave subsided and the lockdowns lifted, I made a trip by car in September. What began as a casual visit turned out to be a long affair of being in and out of the state, that ended in me doing eight RT-PCR tests for crossing borders, several flight trips, innumerable hospital trips and stays.. and so on. After the nightmarish eight months, I am back in Bangalore.

And back home means, I can get on the air with my radio, do other homebrew projects etc, at least occasionally. Afterall, it is a hobby.

Unfortunately, all of this meant, I had to miss a few of the PLFA group reading sessions. I sort of lost interest in it, but have regained some interest off late, now that mind is slowly getting back to its normal state.

Last few months has been a great education to me and taught me about the system, about the people and so many other things that I took for granted that would hopefully help me take informed decisions, if I get into a similar situation.

Projects

I started writing some Go at work. I am liking it. It feels like C with a lot less bookkeeping tedium. I miss algebraic data types though. Algebraic data types are great thinking tools and a lot of situations map nicely to it. That said, writing Go has not been too bad. There is a nice property testing library in Go called Gopter, which I like a lot. End of the day, what really matters is whether the end result satisfies the user needs. “Written in ” is not a feature most people are going to appreciate. If using a particular programming language results in lesser bugs and lesser maintainance, then it implicitly maps to better user experience. At this point, I doubt whether such “silver bullets” exist.

A few ham radio workbench projects in the works:

RFI hunting

Ever since I re-started my amateur radio hobby from 2018, I have been noticing a buzz sound, every 20khz on the 7 MHz band. One huge advantage of SDRs is that you can not only listen to the signal, you can also view the spectrum. I even use notch filters to mute these spurious signals.

Last few weeks, I have been trying to find the source for this signal plus other sources of RFI in and around this household. These are rough notes from that hunt and it is still ongoing.

So, there are a few types of noise I am seeing.

  1. Periodic peaks on the spectrum, 20dBm above noise floor. The periodicity is about 20khz. I have traced the source of this. Read below.
  2. Noise that I see only during certain periods of time.
  3. Noise bands that repeats every few khz, but moves slowly.
  4. Noise that occurs only in evenings mostly and raises the noise floor by ~3-4dBm every few seconds. This is heard as a tapping noise. Is this what people call as “woodpecker” noise? I don’t know.

Anyway.. I started with the easiest of all, the first one and the strongest one on my shack. This is all over the lower bands and repeats every 20khz, 7005, 7025, 7045, 7065… khz. If there is a station (SSB) on this frequency, the best bet is to turn on the Automatic Notch Filter on the radio and hope that it does its job (sometimes poorly).

With a Grundig shortwave radio borrowed from a friend, I walked around the neighbourhood after people are asleep and figured out that it is coming from my next door neighbour’s home, most likely from his inverter. I got access to his inverter room and found that the earth wire is not soldered into the Earth pin. There are probably other problems with it, like the controller board PCB trace emitting RF etc. Some schemes to contain such noise is described in KA7OEI’s blog. But to implement those schemes, I will need access to my neighbour’s house, tinker with his UPS setup etc, which he may be uncomfortable doing. I too feel uncomfortable doing it because I may get blamed for even unrelated future problems.

OK, so that is a dead end. A household Netgear switch’s SMPS was found to be emitting RF, every 335khz. That has been isolated.

All other RFI problems remain the same. A cheap solution would be to use VK5TM noise canceller. A not so cheap option would be a true “diversity” reception. It is not cheap because one needs a second receiver and another antenna to catch the local noise alone and then digitally nullify it – not unlike the way noise canceller headphones work.

OK, that’s it for now.