Hunting the Noise Floor
The limit on what I can hear is not the antenna or the receiver but the noise. In a block of flats every wall hides a dozen switching power supplies, and each one puts out a faint hash across the shortwave bands that adds up to a floor of noise under which weak signals simply vanish. On the 31 metre band my noise floor at midday was reading about S7 on the receiver meter, which means that anything weaker than a strong broadcaster is gone.
I spent a Saturday hunting the sources with a cheap portable receiver tuned to a quiet frequency around 7 MHz, walking round the flat and listening for the buzz to rise. The results were humbling, because most of the noise was mine. The ranking, from worst to least bad:
- A phone charger left plugged in by the bed, with nothing attached. It alone lifted the noise two S-units when I stood near it, and turning it off at the wall was audible at the kitchen table.
- Two cheap LED bulbs in the hall, which produce a harsh whine that peaks every 50 kHz across the whole of the lower bands.
- The laptop power brick, as expected.
- The fridge, but only when the compressor starts, a sharp burst that lasts a second.
The upstairs neighbours contribute an evening buzz that I cannot do anything about, but after unplugging my own chargers and replacing the two hall bulbs with ones that proved quiet in a test, the midday noise floor on 31 metres dropped from S7 to about S4. That is roughly a factor of ten in power, and it brought three or four weak stations out of the noise that I had never heard from this flat before.
The other change was to the antenna feed. The coax from the window transformer to the receiver was running along the skirting board right past the power strip for the television. Rerouting it along the ceiling, held up with a few hooks, took the last S-unit off the noise on the higher bands. A choke made of eight turns of coax through a ferrite ring at the receiver end helped a little with the common-mode hash from the building, though less than I had hoped.
The lesson is that before buying anything, I should walk round with the portable and turn things off. Most of my noise was within arm’s reach.