Radio interference

Radio reception problems are normally caused by a weak signal or an interfering signal. AM receivers are more susceptible to interference than FM receivers.

Causes of radio interference

Radio reception problems are normally caused by a weak signal or an interfering signal. Possible causes could be:

  • The signal may be reduced by an obstruction blocking your antenna from the signal, such as trees, hills, or severe weather.
  • Multi-path signals causing cancellation of the primary signal.
  • You're too far away from the transmitter.
  • Your receiver is faulty, or your 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.

There's more information below about some of these.

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, 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.

Radio Frequency (RF) interference

Radio Frequency (RF) interference is caused by transmitters on the same or similar frequency to the one you're receiving — for example, baby monitors, cordless phones, amateur or personal radio transmitters, oscillating amplifiers (like audio or radiating aerial amplifiers).

At times you may receive interference from a powerful nearby transmitter that's overloading your receiving equipment (eg, taxis, carriers). This effect is normally only temporary.

Co-channel interference


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.


Long range co-channel signals may be received at night. The intensity will vary from winter to summer and over the 11-13 year sun spot cycle.

Electrical interference

Electrical interference is usually caused by power lines, electric motors/thermostats, microprocessors, switch mode power supplies, etc. Anything using electric power can cause interference.

On AM and FM radios, the interference is characteristically heard as a buzzing noise, whine 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.

What you can do

The avoid interference, make sure you:

  • 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.

Registering an interference complaint

Very often the cause of interference will be found in the household affected, and can be solved by the householder. Before you do anything else, we recommend you complete the checks on the page below to confirm that your reception problems are caused by external interference, and not by incorrectly installed equipment or interfering equipment installed in your home.

What to do about radio interference

If you can't resolve the problem yourself, the next step is to contact your local service technician. If they're not able to fix the problem for you, we can provide an interference investigation service for radio reception problems within the coverage area of the radio station affected.

What radio interference sounds like

AM and FM types of modulation — used for public broadcasting in New Zealand — 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. In addition, 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.

See What radio interference sounds like for more information.