Not My Noise Sources

Most DXers are noise-limited. With modern equipment, what most commonly prevents us from detecting more of the signals we desire is the presence of unwanted signals (noise) rather than a lack of electromagnetic energy reaching us from transmitting towers in distant and exotic lands. Some of the time that noise is of natural origin (static) but, increasingly, the offending radiators are artificial. Some of the latter are simple co-channel or adjacent-channel transmissions but the rest fall into the "unintentional radiators" category. That's a polite way of saying RFI from consumer products manufactured with little regard for anything other than the immediate gratification of the majority of buyers.

On Saturday my local electric utility (BC Hydro) had a scheduled power outage in my immediate neighbourhood. Unlike most people around here, I was looking forward to the event. Never having had much luck at reducing my noise level by selectively turning off my own electrical devices, I was hoping that most of the buzzes, growls, and other irritating sounds found on LW, MW, SW would vanish as 30 to 50 households were forced off the air for a few hours.

As I have mentioned in the past, one of my favourite features of the Tecsun PL-380 is its digital readout of signal strength. Shortly before the outage I took the Tecsun to five different locations in my house and recorded the background noise on a set of 23 frequencies from LW to the 13 m band. Most of those were the ones that I use for my on-going noise survey of local parks but seven of them were ones that I included because they were near the centres of some particularly offensive broadband signals that I found on my Perseus SDR about half an hour before the outage. I repeated the measurements at the same locations during the outage and again after power had been restored.

Was I treated to a few hours of urban DXer bliss? Did I get metaphorically transported to some magical DXpedition site? Did I get to hear the ionosphere as it was before the invention of the square wave? Was I inspired to embark upon a noble quest to purge my home of switching power supplies?

No.

The entire exercise, though informative, was a disappointment. Within the limitations of my experimental technique, the noise levels were nearly identical before, during, and after the event. To me, the most obvious conclusion is that most of my RFI originates from my extended urban environment of millions of switching circuits, rather than from a small number of sources in and near my home. Of course, by using a 6 kHz bandwidth on each frequency, I sampled less than 1% of the RF spectrum below 30 MHz so I could easily have plenty of local sources that do interfere with my DXing but which escaped detection by my simple test.

One thing that surprised me is how little noise I found coming from the UPS that feeds a desktop computer. That observation gave me an idea. What I should do some day is switch off all power to the house and use the UPS to power my Wellbrook loops and the SDR while running the latter as a 40 MHz spectrum analyzer. That might reveal some nasty domestic RFI sources.

Although I do not plan to abandon proper RF hygiene, I do intend to put more effort into finding a quieter site away from home than into trying to make my home quieter. My noise page still shows the results from my early work on that but I have accumulated much more data since I generated its chart so I really should replace the graphic with one where each plotted point is derived from more measurements.