Interference
From Freeradio
Interference is when one radio signal causes problems with the reception of another, or when a Transmitter causes other radio users reception problems. For receiver related interference, see QRM, QRN and Reception issues.
Contents |
Avoiding causing interference
The main reason cited by the Establishment for taking (often heavy handed) action against Pirates is that they allegedly cause interference. Sadly, a few pirate stations have interfered with other stations and other frequencies - which is most unwelcome, because it gives the DTI a reason to crack-down on pirate radio.
The first, and most obvious, way of avoiding interference with other stations is to find a clear frequency, well away from that of any other stations. At least 200kHz spacing should be allowed between one station's frequency and that of another, more if the adjacent station is local or specified as serving the area.
Don't over-modulate
Over-modulation can cause severe interference on both AM and FM, both to adjacent stations and to other radio bands (such as those used by Emergency Services). To avoid this, keep the modulating audio no louder than that of other stations on the band, preferably a shade quieter (most legal stations use audio compression, which makes them sound louder while keeping the audio signal within modulation limits).
If broadcasting in Stereo or using RDS, special attention must be paid to the output level of the stereo encoder or RDS encoder.
A limiter (or compressor-limiter) is ideal for keeping the modulation within limits.
Use a clean transmitter
A cheap or poorly engineered transmitter may drift widely, causing interference to adjacent stations (and annoying the listener who has to keep re-tuning). Lack of harmonic suppression can cause interference at frequencies well outside the broadcast band, at multiples of the transmitter frequency.
Transmitters which involve the mixing of signals (eg PLL transmitters) must be engineered to remove "spurious emissions" caused by the mixing process. Mixing of signals also presents a challenge when two transmitters on different frequencies are installed at the same location.
Link transmitters
Some pirates use a Link frequency, which is not on the broadcast band. The Link frequency must be carefully chosen, to avoid causing interference to other radio users.
This is especially a challenge on the Aircraft band (where some pirates have had their Link frequency), due to the long range of signals receivable at high altitudes (eg in an aeroplane).
High power levels
The use of high transmitting problems can greatly increase the risk of causing interference, for two main reasons.
The first is that the transmitter's amplifier may amplify not only the wanted frequency, but also any unwanted signals such as spurious emissions.
The second is caused by the receiver suffering the interference. A very high signal being received can cause "receiver overload", where the receiver's input RF amplifier becomes non-linear and creates its own spurious frequencies. For this reason, it's perhaps best to avoid using high transmitter power in built-up areas or near to communications Antennas (eg those used by Emergency Services or Taxis).
In severe cases, interference can be caused to audio equipment which is not designed to receive radio signals. This is because the transistors used in audio amplifiers have "junctions" which act as diodes, and "demodulate" strong RF signals. in the case of FM this tends to manifest itself as an annoying hum, in the case of AM it can cause the transmitted audio to be heard on the audio amplifier's output. This can cause challenges for a station's studio, in avoiding interference from the station's own transmitter (often referred to as "RF Feedback").
