I am experiencing interference

House and signal tracesThere are many potential sources of interference to low-level radio signals, particularly in the urban and suburban environment. With a little care and research, it is generally possible to trace these, and for corrective action to be taken.

Introduction

Most amateurs are well aware that even in the quietest locations the level of noise on the HF bands is far above the thermal noise produced by the receiver front-end. In general the level of natural noise exceeds the thermal noise by a few S-points (10 to 30dB). This naturally occurring noise varies with time of day and season of the year. In addition to this, for a resonant antenna, the antenna factor—the ratio of the field strength to the actual signal at the receiver input—is proportional to the wavelength so that a tuned/matched antenna such as might be used by a radio amateur is more “sensitive” at lower frequencies. Most man-made sources of radio interference (other than PLAs) tend to fall off significantly as the frequency rises so the overall effect is that the received noise is worse on the lower frequency bands.

EMC Leaflet 4: Locating Sources of Interference to Amateur Radio Reception describes many of the potential sources of interference.

Where interference is experienced, you should proceed as follows (we will consider each step in detail later on)

  1. Confirm you are not responsible
  2. Try to identify the source of the interference
  3. Consider speaking to the “owner” of the interfering equipment
  4. Tell the RSGB of your findings
  5. Make a formal complaint to Ofcom

Confirm you are not responsible

You should first check that the equipment in your own house is not the source of the interference. EMC Leaflet 4: Locating Sources of Interference to Amateur Radio Reception describes the steps you should take here.

Make a note of the amateur bands affected, and the strength of the interference as indicated on your S-meter. You might also like to record the interference level on some of the short-wave broadcast bands.

Try to identify the source of the interference

This can be tricky. If you have a portable receiver which covers the affected frequencies, see if you can hear the interference on its internal antenna. Often you will be able to, but equally often not. If you have a directional antenna associated with your station, try rotating it to peak (or null) the interference. This should give you some idea of the direction of the source. Again EMC Leaflet 4: Locating Sources of Interference to Amateur Radio Reception provides more detailed guidance.

Consider speaking to the “owner” of the interfering equipment

This will depend on your relationship with that person. It may be that you can resolve the problem with a little cooperation from the person concerned. At the very least you will be able to confirm for sure that you have identified the correct source of the problem (remove its power source and make sure the interference ceases). But it may be difficult to go beyond that.

Tell the RSGB of your findings

Complete the simple form on the RSGB Intereference Reporting page, pressing “submit” when you have inserted all the information. The RSGB cannot investigate your complaint, but may offer advice and in some cases assistance in preparing the information for a formal complaint to Ofcom. Just as importantly, the database which we build up from complaints will help us in our discussions with Ofcom.

Make a formal complaint to Ofcom

If we are to register the full effect of interference with Ofcom, it is really important that everyone affected complains. You can do this by going to the Ofcom website and completing the “Report interference or abuse on an amateur radio system” form. When you get to the free-text box at the bottom, headed “Please describe the interference and give any other comments” you should include the following:

  • “The interference affects the **details** Amateur Radio Service and/or **details** short wave broadcast frequency bands.”
  • Add, if you believe you know the source: “As far as I can ascertain it is caused by [a nearby data-over-mains device (PLA/PLT)] [a central heating thermostat] [a switch mode power supply] [a local plasma television] [other]” You should select the appropriate source [ ], if you are reasonably confident that you have identified it.

You should report all factual information that will help Ofcom in its duty. For example, any information that you have regarding the possible source of the interfering signals, and any investigation that you have carried out.

You will see that at the end of the form there is a statement about liability for costs of the Ofcom investigation. If the interference does not originate in your own home, Ofcom currently does not make a charge for investigating. It is for this reason that you MUST ensure that the problem does not originate in your own home!

Note that if the interference concerns the broadcast shortwave bands, then your first complaint should be to the BBC using the reception problems form on their website.

EMC leaflets that will help you in your work include:

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