6: Interference
Sounds you don’t want to hear
Interference is a topic that affects not just the MW DXer but just about every radio listener. In fact it is usually the level of interference rather than any other factor that limits the reception of weak and distant stations on the MW band.
Interference is usually taken to mean any unwanted signal (or noise) that, by adding to the desired signal, degrades reception of the wanted information. It is usually the case that the interference most often encountered on MW is man-made in origin. Whereas there is very little one can do about naturally occurring interference, it is possible, theoretically at least, to eliminate man-made sources of interference. The first step to suppressing interference is in fact recognising it and identifying its origin. Having identified a source of interference it is an unfortunate fact of life that it may prove impossible to do anything about it. The following are the most common forms of man-made interference to affect MW reception:
Co-channel interference
The MW band has been designed around distant channels (9kHz apart in Europe & Asia, and 10kHz apart in the Americas). As there are many more stations than channels there are inevitably many stations transmitting simultaneously on each channel. Radio planners do their best to ensure that the powers and locations of stations are chosen to ensure that a low level of co-channel interference occurs within the target area of each transmitter. However, listeners outside a station’s primary target area will experience this form of interference, which generally gets worse at night as interfering signals propagate further via the ‘sky wave’. In fact it is the acceptable limit of co-channel interference (also known as the protection ratio) that usually defines the target area boundary for a particular transmitter.
Co-channel interference on 1010kHz (WINS & CFRB) – recording by Barry Davies
The best defence against co-channel interference is a directional antenna which favours reception from one direction to another.
Adjacent channel splatter
Splatter or adjacent channel interference can be recognised as unintelligible modulation or programmes heard mixed with the desired programme with the interfering signal originating from a station transmitting on a channel adjacent to that of the desired station. Given that stations are adhering to their local channel bandplan, there are two main causes of modulation splash. Firstly, splash can be the result of a station not limiting the bandwidth of its transmitted audio which results in components of the transmitted sidebands interfering with signals on adjacent channels; this form of splash can also result from a poorly maintained or over-modulated transmitter. Secondly a form of adjacent channel interference can be generated within a receiver with insufficient selectivity or with a poor ability to handle very strong signals.
Radio tuned to 790kHz suffering from splatter from strong signal on 792kHz – recording by Barry Davies
The best defence against modulation splatter is usually a directional antenna, but a good quality receiver can also make a big improvement. Reception in SSB mode can also be a helpful technique.
Heterodyne interference:
A heterodyne is an audible beat note or whistle that is generated in a receiver when two signals on slightly different frequencies are received simultaneously. In a perfect world where all MW stations operated exactly on their allocated channels, heterodyne interference would not be a problem. However since different channel plans are used in different parts of the world it is possible to hear heterodynes on the MW band. Occasionally, within one radio planning region it is possible to find off channel stations either because the station has failed to conform with planning guidelines or a technical problem has arisen in the transmitter.
Unwanted heterodynes are annoying but fortunately they are easily removed with an audio notch filter. DXers often purchase such an accessory since it improves reception and reduces listener fatigue.
300Hz heterodyne – 1467kHz R. Paradijs and IRIB Iran 300Hz up- recording by Franck Baste F4LKC
Electrical interference:
This title covers a multitude of interference sources which will tend to affect listeners living in built up areas, and near industrial zones. Man-made electrical interference comes in all shapes and sizes but can be classified as intermittent or long term. It can be difficult to track down intermittent sources of interference but fortunately their nuisance value is not long lasting. This form is nowadays the biggest barrier to successful MW DXing for most people.
Since the turn of the century there has been a huge growth in domestic electronic equipment much of which can cause electric interference. Widespread use of digital equipment (TVs, computers, internet routers, power-line communications) and switch mode power supplies and low energy lighting has made “electro-smog” an almost universal problem for listeners in urban & suburban areas.
Severe noise generated by power supply of a satellite TV receiver – recording by Max van Arnhem
The best solution is to listen somewhere remote and rural. But even then you might hear electric fences.
Short repetitive pulses created by an electric fence – recording by Max van Arnhem
Failing that put up an outdoor aerial as far away as possible from sources of interference. Then you need to play detective to locate and hopefully suppress interference at its source – not always easy to do. A directional antenna can help if you have a discrete radiated source of interference.
Jamming
This is a deliberate attempt to interfere with reception and is usually a transmission of man-made noise intended to blanket another programme to make it unintelligible. The amount of jamming present tends to reflect the degree of political unrest in the world and today there is relatively little to bother the MW listener. The extensive jamming associated with Eastern Europe and the former USSR is now consigned to history, but jammers are still active in the Middle East and Korea and other areas of instability.
“Wobbling” jammer on 1377kHz likely from Iran (January 2016) – recording by Max van Arnhem
Natural sources of noise
Even if one lived in a world without any man-made interference, one would still notice a whole range of noises that limit reception of very weak signals. Of these the least significant (for the MW listener) is the thermal noise and other electrical noise components actually generated within the components of the receiver. This is because the level of other naturally occurring noise sources picked up by the receiver’s aerial is many times greater.
Common examples of these types of interference are atmospheric static, which manifests itself as a continuous crackling noise and lightning discharges which are heard as a loud crashing noise. The distinguishing feature of these signals is their broadband nature; namely the noise will be heard at all frequencies in the MW band although the intensity will decrease at higher frequencies. It is interesting to note that the radio wave emitted by a lightning flash behaves as any other radio wave and therefore can propagate over considerable distance; in fact one of the great sources of interference worldwide is the noise generated by the large numbers of daytime tropical thunderstorms. It is for this reason that historically many broadcasters in the tropics chose frequencies between 3 and 6 MHz for local broadcasting where the effect of thunderstorms is much reduced.
Rain static building up and discharging – recording by Max van Arnhem
You may even encounter unusual static noises caused by snow and hail and sandstorms depending upon where you live. But fortunately these are short-lived.
Once you have exercised your detective skills as Hercule Poirot and identified the mode of interference and its source, you’ll now need to adopt the role of Arnie Schwarzenegger’s Terminator in order to eliminate the problem. Good luck.