Interference has a working definition of any disturbance in reception that spoils the enjoyment of a broadcast programme. We can do little about barking dogs, the mother-in-law, cellphones in restaurants and social workers who have defined the word differently, our interference is of an electrical nature. And there is a lot to choose from.
At long and medium waves where the radio has a long-wire antenna, this is most likely due to direct radiation from over-head powerlines, the higher the distribution voltage, the more of it there is.
The wetter the weather, the more humid the atmosphere, the more of it there is - as any report on 80m through the Stygian gloom of a November fog will tell you.
Every effort is made by the power companies to limit this where it occurs in the main AM bands, happy in knowledge that a domestic radio is a lot less sensitive than what we may be using and the average set has a ferrite antenna which, in working only with the magnetic part of a radio signal, neatly avoids the noise-bearing electrical part.
Most mains-powered radios have a delta-suppressor, the three points connected to line, neutral and MOST IMPORTANTLY the earth. This will reduce the chances of mains-borne interference getting into the set, but with receiver sensitivities on the increase, the effect appears to be marginal.
The legal requirement for suppression is that reasonable care has been taken with the appliance to reduce - not kill off completely - any noise which may be heard on a domestic radio below 1600kHz and above 88Mhz. The suppression will have some effect on frequencies between these limits - shortwave radio frequencies - but does not have to be effective here.
You can experiment with the spike-killers used by computer enthusiasts, but if there is a PC in the house then you will have special interference problems all your own, most of them without a cure. To be fair, the computer industry has done a lot of late to clean up its act on the Electromagnetic Compatibility front, but to suppress at all frequencies adds cost and no accountant will stand for it in such a competitive market-place.
Experience shows us the worst offender in the home computer set-up is the monitor. Most go for cheapness if graphics are not important and in so doing, throw out time-base and inverter noise to the far-flung corners of the radio speculum. Sorry about that. The Editor expects it of me.
Depending on the type of printer used, these can be a tremendous source of noise. Apart from the nuisance factor, they usually are only in intermittent service so less of a problem to us. If the computer is your own, the answer to the problem is time-management.
Allocate some time to radio, some other time to computing.
If the computer is the decoding element in some form of Data-by-Radio system, then all we can suggest is meticulous earthing as per User Guides and a coax-fed antenna with the receiving part as far as possible from the terminal.
Most standard Office packages now have a Sound Recorder. Record the recovered audio so you can fine-tune the decoder over several playbacks.
Add to this heady mix multiplex noise from anything in the house with a digital display, thermostat clicks from fridge, freezer and central heating with a squeeze of timebase noise from the TV and there you have it. The noise cocktail below 10Mhz. Serve slightly chilled with liberal wide-band noise from the microwave to taste.
Those who have a noise blanker - the NB button - will note, or indeed may have already written in to say, that theirs has no effect on the interference dealt with so far. Quite true.
These are usually designed to be proof against impulse-type noise such as may be heard on an AM radio in a car. These wide-band products mix so well with the wanted signal that they become a part of it. And that, sadly for us, is how the radio sees them.
The latest generation of English receivers has their noise blankers on all the time. So, if you can hear it, it means the radio can't deal with it, thus saving the frustration of switching the noise blanker in and out to see by just how much the poor thing cannot cope.
Above 10Mhz there may still be the odd car on the road with a dying ignition condenser - your NB works well here. There may be the nth harmonic of yours or your neighbour's fridge thermostat - your NB works well here.
It's All Quiet on the Eastern Front for a while yet, but as we climb out of this radio recession, they will start using Over The Horizon Radar again.
The Russian Over-the-Horizon HF radar got the pet name of The Woodpecker in its heyday at the turn of the century. Sounding just like that noisy bird, some NBs can deal with it, but with its long complicated phase-related pulse chains don't expect too much.
And don't believe all you read in the spec sheets. Some manufacturers claim they - and only they - have the technology to deal with Woody Woodpecker. We will believe it when we don't hear it. Not forgetting the computer. It will be making its own special contribution to carpeting the noise floor.
Not a lot, really. By the time the interference is in the air winging its way to destroy AFN's football coverage, it is already too late. The cause of the problem must be suppressed at source. Hamlet, always one for a good laugh, expressed it thus;
Find out the cause of this effect;
Or rather say, the cause of this defect,
For this effect defective comes by cause.
Sorry about that. Too clever by half. By all means use a portable radio to track down the problem, but on finding the offending appliance TAKE NO ACTION YOURSELF. Refer the fitting of suppressors to a qualified electrician. Suppression is the appliance of science.
Your Editor is old enough to remember long overs on 80m AM when the topic was receiver noise. This strangely comforting hiss, a form of pink noise created in the first stage of our receivers never bothered us as we never chased the DX that could be so easily lost in it. Did anyone else try to replace the first RF valve of their AR88 with an EF183? Everybody did, surely?
We still take an hour on a Saturday morning to hear AM live in the form of the VMARS Net on 3615 kHz. But now it's almost impossible to hear them. A wideband hiss sometimes referred to by G3s as sharsh covers the whole of 80m at about S7.
Like receiver noise back in the days when it was an issue, this S7 hash is now my noise floor. To hear anyone, they have to be a generous S8 and for that wonderfully evocative armchair copy, stations have to be S9 Plus to offer any reasonable signal to sharsh ratio.
It has not taken too much detective work. Thanks to the iPod Generation, it's not too weird to walk the streets wearing headphones nowadays. It's just that mine are plugged into a TRIO R11, a shortwave portable given to me by radio guru John Wilson.
The interference is everywhere now, propagated by electrical wiring. The source? Wireless hubs in countless homes. Don't get me wrong, Wi-Fi is the future but there is major EMC issues with the power supplies and processors radiating noise across the radio spectrum.
We SWLs are in the minority but at my QTH the QRN only tails off at about 76MHz allowing reasonable FM reception. Pulse noise from Freeview set-top boxes even go high enough to punch holes in DAB reception.
We can ask but we are second-class spectrum users and really don't have a voice. All we can do is keep campaigning and hope that The VMARS Saturday AM Net, GB2RS and G3LYW make it above the noise. So far, the noise is winning. Sorry, chaps.