Interference to radio equipment

Part of the reasoning for managing the radio spectrum is to separate users in terms of the frequency, geographical location and time of operation. This is achieved through licensing. We issue licenses to individuals and create licence exemption regulations for specific apparatus. In both instances, there are prescribed terms, provisions and limitations for lawful use.

Anyone who uses radio apparatus outside of the terms, provisions and limitations of a licence or licence exemption could be guilty of a spectrum offence.

Electromagnetic disturbances, also referred to as electromagnetic emissions, are normally considered to be unwanted emissions in the radio frequency spectrum. They can occur naturally or be generated artificially.

An electromagnetic disturbance may affect the performance of or even stop radio communications apparatus from functioning.

Apparatus for sale or put into service should be designed to function compatibly within the environment in which it is going to be used. This is called electromagnetic compatibility.

Apparatus should not emit excessive levels of electromagnetic disturbance and it should be suitably immune from external sources of electromagnetic disturbance.

If your apparatus is affected by an electromagnetic disturbance from an external source, then you need to ensure that your apparatus is suitably immune.

Read more about immunity below.

To protect against electromagnetic disturbances, radio receivers rely on adequate filtering to make sure only the desired signal is received and unwanted transmissions or electromagnetic disturbance are rejected.

Interference can occur because of inadequate filtering or immunity from an external source of electromagnetic disturbance. In some cases, additional filters might be required to provide adequate immunity or selectivity.

The level of filtering needed to reject artificial electromagnetic disturbances will often depend on the proximity of the affected receiver to the source of electromagnetic disturbance. It may not always be possible to eliminate the effect of interference by filtering alone, in which case specialist advice is recommended.

Multiple sources of electromagnetic disturbances can result in a cumulative effect. For example, the level of background levels of electromagnetic disturbance is likely to be higher in industrial areas than urban or rural areas. When planning and installing a radio system, the environment should be considered. Careful siting of the apparatus is essential to minimise the risk of interference.

In the case of artificial electromagnetic disturbances, it is important to determine if the frequency of the disturbance is within the passband (reception window) of the affected receiver. If it is, physical separation may be the only option. In extreme cases, it may be necessary to relocate the affected station or apparatus to increase the physical separation from the source of electromagnetic disturbances.

Your installation should be engineered so that it complies with the terms and provisions and limitations of your licence or licence exemption.

You should take care with the type and positioning of antenna (if it is not an integral part of the apparatus). The coaxial cable and connectors should be of a suitable type for the installation and frequencies. Where an installation is co-sited with others, you can prevent problems with both the receiver performance and the transmitter spectral purity by using double-screened cable, as well as components like filters and isolators.

Congestion occurs when multiple users are operating on the same frequency or frequency band at the same time in the same area.

Where a licence is designated as ‘shared’, it will be necessary to refer to the terms, provisions and limitations of the licence. Where the problem persists contact Ofcom’s licensing team for allocation of a new frequency.

To minimise interference from other users on a shared two-way radio system, many systems employ squelch coding such as Continuous Tone-Coded Squelch System (CTCSS) or Digital-Coded Squelch (DCS). It is important to check that this is functioning and that the correct code is used.

In the case of interference experienced to licence-exempt apparatus, you can try switching channels to avoid congestion.

If your apparatus uses an antenna, you should situate the antenna away from potential sources of interference.