Causing interference FAQ

Please note: This information is intended for members of the Radio Society of Great Britain, but is also made available to non-members. Any information is given in good faith and the Society cannot be responsible for any misuse or misunderstanding.

If while you are transmitting, you try to operate a nearby receiver on an adjacent frequency then you will soon understand that the relatively high field strengths generated by an amateur band transmitter are readily capable of causing breakthrough and interference.

It is therefore not surprising that your transmitter can cause interference or breakthrough on domestic equipment in your own home, or that of a neighbour.
There are two main sorts:
  • Interference to a receiver (such as a domestic radio, terrestrial television, satellite television, Wi-Fi network etc) typically caused:
    • by the receiver not adequately rejecting signals that are outside of its designed frequency range
    • the high field strength from the transmitter overloading the receiver and driving it into a non-linear range of operation; or
    • harmonic or spurious signals from the transmitter occurring within the band that the receiver is operating
  • Overloading of other types of electronic devices such that they fail to work properly in the presence of the high electric field strengths from a local transmitter, e.g. LED lights that come on or go off unexpectedly, solar power systems that either stop functioning or switch modes, loss of network operation of an ethernet or Wi-Fi system, etc.

Currently, with modern high-performance transmitters, the largest majority of EMC problems suffered by neighbours are breakthrough, caused by the susceptibility of many devices to high transmitted field strengths. Fortunately, most can be cured, or can be significantly reduced, by some relatively simple changes.

Interference to a receiver can take many forms. In the past, with AM (amplitude modulated) transmission and analogue receivers it often was intelligible interference, i.e. the amateur's speech could be heard. These days, with ssb and digital mode transmissions, and primarily digital receivers (e.g. DAB radios, DVB digital televisions), any interference is likely to take more subtle forms.

With AM or FM radio receivers, an SSB transmission could be semi-intelligible, equivalent to listening to SSB with the BFO turned off. For digital (e.g. DAB) radios, any interference is more likely to cause the received signal to break up, or for the radio to stop working for the duration of the transmission.

For televisions, the effects depend upon where the interference is being picked up. With analogue interfaces such as SCART, it could take the form of moving dark and light horizontal bands across a tv screen. With a digital terrestrial TV (DTTV) or satellite TV receiver, interference below a certain threshold level may have no effect, but once this threshold is exceeded then the interference may manifest itself either as pixelated or frozen pictures, or by the complete loss of picture. With the loss of pictures, many TVs revert to a blue screen with a text message indicating loss of signal. In some situations, the loss of signal may not necessarily occur on all channels simultaneously, but may only affect certain channels or multiplexes. TVs are prone to pick up interference on the braid of the coaxial downlead from the aerial or dish, or via the mains lead or other connections such as an HDMI cable or network cable.

With Wi-Fi systems, the effect of interference could be to slow down or completely halt the transmission of data across the network. It may affect just one device on the network, or it could affect the whole network

When the interference occurs in other electronic equipment then almost anything is possible; i.e. the equipment may stop functioning, it may function intermittently, or it may experience un-commanded events i.e. it could unexpectedly turn on or off or change mode.
Today all electronic devices must meet national and international standards concerning immunity to radio frequency interference. This covers the rejection of out of band signals, overload performance and immunity to high electric fields. This should prove adequate in most situations. Nevertheless, with sensitive receivers in TVs and radios, and with most modern devices being controlled by digital processes operating with low voltage, high impedance controls, inevitably there remains some susceptible to high electric fields which can, for example, potentially overload the controls and cause spurious events.

The two primary immunity specifications for equipment in a residential or light industrial environment are EN 55024 and EN 55035. These specify radiated immunity up to 3V/m which is adequate in many situations. However, it should be noted that with a high-power transmitter, particularly where that is connected to a high gain antenna, an amateur transmitting station can generate fields well in excess of this limit. Therefore a piece of neighbour's equipment may meet the requirements of the appropriate immunity specification, but still suffer from interference.
If you are causing interference or breakthrough the first thing to determine is whether the transmitter is performing properly, i.e. is the interference caused by the fundamental transmission frequency within the amateur band allocation, or is it caused by harmonics or spurious emissions from the transmitter? These days it is unusual for the problem to be caused by a faulty transmitter, but if you have any suspicions in this area it is worth checking the output of transmitter. A simple passive or active wavemeter, a wideband SDR receiver (even a cheap dongle) or a spectrum analyser will show whether there are significant emissions on frequencies other than the fundamental frequency that the transmitter is tuned to. If necessary, try to solicit the help of a nearby experienced operator to examine the output of your transmitter.
If a neighbour complains that equipment in their house is suffering from interference from your transmissions, then you need to verify that the interference really is only present when you transmit and is not there when you are not transmitting. The neighbour, in all innocence, may believe that because you have a large aerial in your garden that you must be the culprit. Ask the neighbour to keep a log of when the problem occurs and compare this with your own log of when you were transmitting. In some situations, you may find that you are not the cause.
If a neighbour complains about interference, it is worth investigating whether anything in your own home (preferably analogous equipment) is similarly affected. If you are causing interference to your own equipment, then you start from a poor position in discussions with your neighbour. Therefore, if you are experiencing problems in your own home, then it is worthwhile spending some time in developing cures for your own equipment before attempting to resolve any problems with your neighbour.
Having checked that the interference is not caused by a faulty transmitter, there are still several things that you can do at your end to try to reduce or even eliminate the interference. Key measures you can take are:
  • Use high quality coax for any connections between the transmitter and the aerial
  • Check that any connectors in the aerial lead (including connections with an aerial tuning unit) are well made, and are not suffering from corrosion
  • Check that the antenna is presenting a low SWR
  • If appropriate, use a 1:1 choke balun (sometimes known as an "ugly balun") near to the antenna feed point to reduce radiation from the feeder cable
  • Look to move the antenna further away from the neighbour's house, or direct it in a way that reduces the field strength in the direction of the neighbour's house
Further information can be found in EMC Leaflet No. 10: Avoiding Interference to Nearby Electronic Equipment (4-page/544KB PDF)
Before you speak with your neighbour, you may find it useful to consult your RSGB Regional Representative, or email RSGB EMC helpdesk at helpdesk.emc@rsgb.org.uk. Resolving EMC problems calls for great diplomacy with your neighbour. Be sympathetic, be cooperative, and, as far as possible, provide guidance and advice. Often the initial reaction of the neighbour is that the problem must be with your transmitter, as without it, their reception is faultless. Read EMC Leaflet No. 1: Radio Transmitters and Domestic Electronic Equipment (4-page/338KB PDF), and, if appropriate, let your neighbour have a copy to read.

It is very important that you do not, in any way, make the accusation that the problem lies with the neighbour's radio, TV, etc. and is not the fault of your transmitter. Explain that you are willing to help, and that a few simple steps involving no direct modification to their equipment, may resolve the issue. Inform them, maybe even demonstrate to them, that the interference does not occur on similar equipment in your own home.

You should not consider any internal modifications to your neighbours' equipment. But you could offer to try various filters on the leads in and out of their equipment, e.g. toroids on speaker leads, telephone leads and mains leads of affected equipment, or a braid-breaker on the TV antenna feeder. If these basic steps fail, then it may be necessary to contact the equipment retailer for some help.

Finally, your neighbour has the option of complaining to Ofcom who may offer advice and assistance, or who may send out a spectrum engineering officer to carry out an investigation. However, Ofcom will only investigate if the problem is causing "harmful interference" to the reception of radio or TV signals. Your neighbour will need to complete the Ofcom form Report of Interference to Domestic Apparatus. Alternately they can contact Ofcom directly by telephone or email.
We have a wide range of EMC leaflets available to provide additional help and guidance on many EMC matters.