Page 003HomebrewŠiauliai
A tin box, a ferrite ring, and the noise that went away
Two of the most useful things on this desk cost nothing but evenings: a choke wound on a ferrite ring, and a dummy load built into an old paint tin. Neither is clever, and both taught me more than the money would have.
- Date
- 5 May 2025
- UTC
- 1605
- Band
- 40 m
- Mode
- SSB
- Typed up
- 30 July 2025
- Reading
- 6 min
The noise I had decided to live with
By the winter of 2024 I had stopped noticing the noise, which is the dangerous stage. The receiver sat five points up the scale on forty metres every evening, and I had built a set of habits around it: listening through it, asking for repeats, avoiding the weak end of the band altogether. When somebody on the other side of the country described his own noise floor as barely a point I assumed he lived somewhere unusually quiet. He lives a hundred metres from a railway.
The difference turned out to be mostly inside my own house, arriving along the outside of the feed line and getting into the receiver through the back door. That is not an unusual diagnosis and it is not a hard one to test: unplug everything else in the room, run the radio from a battery for ten minutes, and listen carefully. Half my noise left with the desk lamp, and the other half moved about when I moved the coax.
Winding a choke without instruments
A common-mode choke at the feed point is a coil of the feed line itself wound through a ferrite ring, and the whole art lies in choosing a ring of the right material and getting enough turns onto it. Having no way to measure impedance, I did what the older books suggest and copied a recipe known to work rather than inventing one: a large ring of the mix that behaves across the high-frequency bands, twelve turns of thin coax, wound evenly and taped so that the turns cannot shift.
The making took an evening at the kitchen table with a roll of self-amalgamating tape and a bad radio programme for company. The awkward part is persuading coax to bend round a core smaller than you would like, and the only trick I can offer is to warm the cable indoors first. The finished choke went into a plastic box with a drain hole at the bottom, and the box hangs a little below the feed point so that rain runs off it.
Noted in the margin
- Choose the ferrite mix for the bands you use, not the biggest ring in the drawer
- Wind the turns evenly and tape them so they cannot move
- Warm the coax indoors before bending it round the core
- Box it with the hole at the bottom: condensation wins otherwise
A dummy load in a paint tin
A dummy load is a resistor that turns transmitter power into heat instead of radio waves, and it is the first thing anybody newly licensed should build, because it lets you test the rest of the station without annoying a soul. Mine is a fifty-ohm resistor of the flat sort meant to be bolted to a heatsink, mounted on the underside of the lid of a one-litre paint tin, with the tin filled with mineral oil and a connector brought out through the lid.
It takes a hundred watts for a couple of minutes and gets warm rather than hot, which is everything I need from it. The tin lives under the desk and comes out whenever anything changes: a new cable, a different tuner setting, a fault to chase down. Being able to transmit into a known load and hear nothing at all on the receiver in the next room is how I found that one of my patch cables had a broken screen.
Measuring the difference honestly
The temptation after building something is to declare it a success, and the only defence against that is writing down numbers before and after. I had four weeks of evening noise readings from before the choke went up, taken at the same hour, on the same band, with the same setting on the receiver. The average beforehand was a fraction over five points. Across the following month it ran between three and four, and the worst evenings were noticeably less bad.
That is a real improvement and a modest one, and it is worth saying plainly, because the accounts I had read beforehand promised a good deal more. The choke did nothing whatever about the electric fence two fields away, and nothing about the band being poor. What it removed was the share of the problem I had carried indoors myself, which came to roughly half. The rest is a longer job, and some of it is not mine to fix.