Home > Consumers & Users > What is Interference? > Radio interference
Document Actions

Radio interference

Radio reception problems are normally caused by a weak signal or an interfering signal. AM and FM types of modulation are used for public broadcasting in New Zealand, these are analogue forms of transmission. In the presence of interference, AM and FM radio receivers exhibit quite different effects on the audio the listener is hearing. Additionally, the frequencies used for AM and FM are quite wide apart resulting in differing interference mechanisms. AM receivers are more susceptible to interference than FM receivers.

What causes weak signals on AM and FM radio?

  • the signal may be reduced by an obstruction blocking your antenna from the signal e.g. trees, hills, or severe weather.
  • multi path signals causing cancellation of the primary signal.
  • you are too far away from the transmitter.
  • your receiver is faulty, or antenna system is a low gain type or faulty.
  • AM radio reception of weak and/or distant stations at night is prone to fading and distortion, caused by the simultaneous reception of both wanted and unwanted signals.


Audio examples of what radio interference sounds like can be found at what does interference sound like?

What causes interference to the signal?

What causes multi-path interference?

FM signals are affected when part of the signal comes directly from the transmitter, while another part has been reflected from a hill, a building or some other large object in the locality. The reflecting obstacle can be located in any direction from the antenna.

On AM radio, in the morning and evening, the changing ionosphere may cause the audio to slowly drop in and out of phase causing distortion (called selective fading). The direct signal from the transmitter may also be affected by the signal reflected from the ionosphere resulting in slow fading of the signal. These effects often happen together.

What causes Radio Frequency (RF) interference?

Transmitters on the same or similar frequency to the one you are receiving i.e baby monitors, cordless phones, amateur or personal radio transmitter, oscillating amplifiers (like audio or radiating aerial amplifiers).
At times you may receive interference from a powerful nearby transmitter that is over loading your receiving equipment i.e taxis, carrier. This effect is normally only temporary.

What causes co-channel interference?

For FM vertical layering of moisture content and temperature in the atmosphere (inversion layers) can occasionally cause signals to travel hundreds or thousands of kilometres further than usual. An inversion layer (or duct) is most commonly observed over high pressure regions and may affect radio signals for several hours to several days. The phenomenon is commonly referred to as anomalous propagation and is more likely in hot, dry weather in late summer.

Disturbance to radiocommunication services from this cause are observed infrequently in New Zealand and reception returns to normal as atmospheric conditions change.
For AM long range co-channel signals may be received at night. The intensity will vary from winter to summer and over the 11 to 13 year sun spot cycle.

What causes electrical Interference?

Is usually caused by power lines, electric motors/thermostats, microprocessors, switch mode power supplies, etc. Indeed anything using electric power may cause interference.

On AM and FM radios, the interference is characteristically heard as a buzzing noise, wine or hiss affecting mains and battery operated radios. AM reception is more prone to interference than FM reception. For AM the source of the interference may be many hundreds of metres away


How can I avoid interference?

The secret to avoiding interference is to:

  • buy good equipment
  • install it properly in accordance with manufacturers installation instructions and sound radio frequency installation practices
  • operate it properly

Reception problems may be caused by faulty devices or connections. A check of your receiver, aerial, cable connectors and cable are good first steps.


How can I register an interference complaint?

The process

Experience has shown that very often the cause of interference will be found in the household affected, and may be solved by the householder. Please ensure your reception problems are caused by external interference and not by incorrectly installed equipment or interfering equipment installed in your home.

If you cannot resolve the problem yourself, we recommend you contact your local service technician in the first instance. If your local service technician is unable to resolve the problem for you, RSM can provide an interference investigation service for radio reception problems within the coverage area of the radio station affected. In this case, please contact us.

Our commitment

Interference complaints will be responded to, and remedial action commenced within two working days of being lodged for 95% of domestic interference complaints.

What does radio interference sound like?

Audio and video illustrations of what different types of radio interference sound like.

Read more about What does radio interference sound like?

How can I fix my radio interference?

A list of solutions to help fix a variety of radio interference problems.

Read more about How can I fix my radio interference?

Last updated 2 December 2014