Noise has always been the bane of the Amateur Radio Operator, and as our technology has developed our urban environment has become an endless source of annoying noise!
If you can somehow figure out how to “boost” the signal high enough above the background noise without producing any other noise – you’ve boosted the Signal to noise ratio (S/N) and your problem is solved! Trouble is, that solution is limited by many factors that may be beyond the limits of we poor urban operators. Oh, for a monobeam Yagi on a 200 foot tower!
“Noise” can be anything that isn’t the signal you are interested in. It can be interference from another human being – QRM, either malicious or accidental, or it can originate from nonhuman sources (QRN), like static, or car ignition noise. QRN can have numerous causes. Many are caused by our modern technology – light ballasts, poorly designed (wall wart) power supplies, or power converters, poorly shielded internet routers. This category seems to be growing every day. Other forms of QRN noise can’t be blamed on human technology – lightning, which can be happening a thousand miles away, static discharge, space based random energy. Hell, everything that isn’t at absolute zero degrees Kelvin emits some amount of electromagnetic radiation noise!
The background noise created by random motion of electrons, or by thermal noise emission, is called Gaussian Noise and is usually perceived by us as white noise by the time it gets to our ears. Impulse noise is a different thing entirely – usually due to some man made device, with its own peculiar spectrum and interval rate.
Very often it is the random noise that is the most bothersome. Part of it is produced by our antenna system, and our receiver! Those factors can be ameliorated by using the shortest, lowest loss coaxial cable possible. In some cases in the UHF range, low noise pre-amplifiers very close to the antenna have been very useful. Since random noise is equally distributed over the frequency spectrum, “packing” your signal in the narrowest acceptable spectrum improves your signal to noise ratio! This is one of the reasons why low power enthusiasts (QRPers) tend to use CW with its 100 Hz bandwidth rather than SSB’s 2.7 KHz or worse, full bandwidth FM! Most people are well aware that using filters designed for the bandwidth of the mode they are receiving improves their S/N. Try listening to someones’s SSB signal with a 4,000 Hz filter and then with a 2,700 Hz filter and hear the difference! Another important thing to remember is NOT to set your gain settings all the way UP – set it so that the background noise is less obvious while the signal is still understandable – all things in moderation! There are simple ways of lessening random noise and then there are much more high tech ways… but we’ll get to that later.
Impulse noise is relatively easier to deal with as radio technology has developed. Initially receivers employed an extremely simple Noise Limiter – a circuit that clipped noise signals that are higher in amplitude than radio signals. It didn’t so much remove this noise, as it did limit it to the same level as your received signal. Further improvements to this technology brought the Automatic Noise Limiter (ANL) which could vary the clipping by the strength of your received signal and not clip your signal along with the noise! These circuits were fine in the 1930’s, but as later receivers were designed with better selectivity the noise spikes were made wider and wider by the time they got to the ANL circuits at the audio output stage and the ANL circuits were unable to “tell” the signal from the noise! With the advent of Automatic Gain Controls (AGC) the impulse noise was being detected as a received signal and causing the AGC to lower the gain on your actual signal! Another solution was necessary.
This led to the development of the Noise Blanker (NB). This circuit is usually designed to operate at a stage much earlier than the old ANL circuits and is unaffected by AGCs and spike widening circuits seen by ANL circuits. These NB circuits are still with us in modern receivers and usually have on/off settings as well as a threshold knob to tweak. Use them at the lowest acceptable setting – as with all other noise reduction, they introduce artifacts that can be just as annoying as the noise that they mask!
DSP notch filtering (NF) relies on QRN that has a steady carrier signal, like some Lid tuning up on the frequency that you are using, and is very good at limiting this form of intrusion. There are trainable NF’s and automatic NF’s. They offer a good solution to a specific problem. Learn to use them when necessary.
Random noise is much more difficult to deal with, and there’s little that could be done until the advances in microelectronics that gave us powerful Digital Signal Processors (DSP) at reasonable prices!
DSP based solutions seem to be the future of noise reduction (some would say that fully Software Defined Radios – SDRs – are the future of the entire hobby). Initially, receivers had relatively simple DSP chips stuck on to the audio output of their receivers. These are not unlike present options available for owners of older radios offered by external monitor speaker manufacturers, which include some rather inventive DSP algorithms to reduce random noise superimposed onto the audio signal (BHI, West Mountain, and Timewave make some very useful devices using this technology). Everyone’s algorithms seem to be different, and each one lessens the noise while inserting its own type of distortions. Purchasers should make an effort to test drive each manufacturer’s product before spending their money on one.
A far better solution is to get the DSP to work on the signal, long before it reaches the final audio stage – somewhere in the IF stage (if your receiver still has an IF stage). Most modern radios tap the signal at the IF stage and with the additional information available at that point offer a better solution to random noise. Again, this is NOT a complete solution – noise is lessened to a degree and distortion is introduced, to a degree. Fortunately, most times, these circuits are adjustable, and can allow you to find the best setting that you can live with.
If at all possible limit the local noise creation at its source at your shack’s location and take all possible measures to feed the strongest signal to your radio. Like most things, it’s much better to fix a thing before it becomes a problem, rather than after.
Lotsa luck, and don’t forget to clean that wax out of your ears!