Brats club logo

Bredhurst Receiving and Transmitting Society

ILC title

7. EMC

7a Basics of electromagnetic compatibility

7a.1 Understand that all electronic equipment is capable of radiating and absorbing radio frequency energy.

Recall that the basic principle of electromagnetic compatibility is that apparatus should be able to function satisfactorily in its electromagnetic environment and without causing intolerable electromagnetic disturbance to other apparatus in that environment.

It is a fact of life that all electronic equipment is capable of radiating and absorbing radio frequency energy.

The whole idea behind the principle of Electromagnetic Compatibility is that equipment should :-

  • limit the amount of RF radiation to below a specified level and also

  • be able to withstand a certain level of RF that tries to enter it.

The levels involved are stated in EMC regulations which thankfully you do not have to learn but must be aware that it is EEC EMC directives in which the limits are stated.

Modern electronic equipment

What you need to appreciate is that even modern electronic equipment, such as televisions and other domestic electronic equipment, may not be able to withstand strong (or high) local RF transmission.


The computer you may have in your shack, when switched on will be a constant source of RF, even though at very low RF levels, and this can stop you hearing the very low RF signals. Thus for serious listening turn off your computer and hear the difference.

7a.2 Understand that transmitters in domestic environments may give rise to RF fields stronger than the specified limits. Appreciate that transmitters in domestic environments are not 'normal' situations and special measures may have to be taken.

The radio transmitter used for amateur radio is not a "normal" domestic appliance in so far as not every home has one BUT nearly every home has a TV and broadcast radio receiver.

Thus the radio amateur is bringing into an environment a piece of equipment which under normal circumstance would not be there. This places the onerous responsibility on the radio amateur to ensure that the transmitter does not cause undue interference to others nearby.

The transmitter may, well almost certainly, give rise to RF field strengths that are stronger if not much stronger than the specified limits.

It is the amateur radio licence that permits the Radio Amateur to transmit RF power levels above the Directive limits.

However the Amateur Radio Licence also has the requirement that if neighbours equipment does suffer interference then the field strength must be reduced to an acceptable level. Good EMC practice coupled with simple remedies should avoid the need for an enforced reduction in power.

So if you want to keep the likelihood of problem to a minimum then use as low power as possible to ensure communication is achieved (even though you may think your licence permits higher power levels it does but only when it does not cause undue interference).

7a.3 Understand that new electronic equipment should meet the European EMC immunity requirements but that existing equipment and poorly installed equipment may not.

With a great deal of new equipment in homes anything made since 1996 must meet the EMC directive on immunity requirements and thus problems associated with lack of EMC immunity is lower. However there is still a great deal of older equipment out there and the regulations are not retrospective so it is still being used and still likely to suffer EMC problems.

Even new equipment badly installed can have problems so it is best to solve your own EMC problems by good housekeeping of the radio installation and using the lowest power which gives good communication.

TV downleads do have a limited service life, like all equipment and especially if out in the open where it may be subjected to ingress of water (entry of water) and a poor quality lead has very little copper in the braiding and thus has poor screening qualities from your RF signal.

7b Good Radio Housekeeping

7b.1 Recall how to interconnect the transmitter, microphone, power supply, SWR meter and band or low pass filters, using appropriate cables, to minimise EMC problems.

With all of the EMC you have to consider where the RF is entering the system.

So far as this section of the syllabus is concerned RF can enter through:-

  • the Microphone lead so a clip on filter may be needed adjacent to the transceiver

  • via the power lead whether that is the Mains or the DC supply so clip on filter may be needed adjacent to the transceiver

HF station

On the route between the antenna and the transceiver the order of connection is as follows:-

Transceiver - SWR meter - ATU - Band / low pass filters - then the antenna

VHF / UHF station

in a VHF / UHF station the LPF is substituted for a band pass filter as you are either on 145 to 146MHz or 430 to 440MHz, single bands thus an ATU is really used.

Ensure that the coaxial feeder and connectors used are all of the best quality that you can afford. Coaxial feeder is used due to its screen properties of the outer braiding keeping the signal contained within the cable are much as possible.

Also you need to keep RF and audio and power leads apart from each other else you are making it easy for the RF signal to enter the other lead especially if they are not screened.

7b.2 Understand that filters can be fitted in the leads from the power supply to the transmitter to help minimise RF energy entering the mains wiring.

Use ferrite rings on 12v leads and mains leads

It is important to understand that the power lead from the mains socket from the rig or from power supply is susceptible to allowing RF entering the cable and for this to be transferred to the mains power in your property.

Proper filters must be used with consideration given to safety when using mains power. Consideration could be given to using ferrite rings at both ends to the mains lead - near the socket to stop RF entering the property supply and near the Rig or power supply to stop it entering the that equipment.  The filter acts as an RF choke thereby stopping RF.

Items connected to the transceiver

any items that you have connected to the transceiver such as a computer and TNC can also give rise to EMC problems and thus all interconnecting lead need the Ferrite ring treatment !!!
Ferrite ring and cable It is hopes that you recall this picture form the FLC course. Whilst the cable wrapped onto the ferrite ring in this case is RF feeder it could just as easily be the 12v lead - the main lead - the sound card leads etc.

In any question asked on this part of the syllabus the answer will lie in stopping the RF going where it should not and generally the answer will be " by the use of a filter " or by the use of a ferrite ring !!!

7b.3 Recall what constitutes a good RF earth, its purpose and use.

If you get a question in the exam referring to a good earth for a transmitter then the question is relating to the RF earth and not the Mains power earth.

A good RF will comprise an earth rod driven into the ground plus a number of radials so as to form an "EARTH MAT" under the antenna. The ends of the radials could also be attached to earth rods.

The radials can be for safety just placed under the surface of the grass and allow the new grass to grow over or through and obscure. Do consider the safety of anyone walking over the lawned area and tripping up.

The RF earth should be connected to all of the equipment in the shack to ensure as far as possible the effectiveness of the various screening, cables chassis of equipment excludes unwanted RF energy.

The RF earth and the mains safety earth serve different functions and must not be confused.

7b.4 Recall how to use a suitable general coverage receiver to check for spurious and harmonic emissions from the station.

General coverage receiver is also a piece of test equipment

Students are forgetting that a general coverage receiver is not only used for listening to stations but is a valuable piece of test equipment for checking for harmonics.

Your amateur radio station is not complete without a general coverage receiver. Few will however consider it as a piece of test gear but as far as EMC is concern that is exactly what it is.

The general coverage receiver can be used it to check out your station for spurious RF emissions and RF harmonics by, whilst transmitting a very low power, tune the radio receiver from below your transmit frequency up the range of the receiver. Remember to calculate the harmonic frequency to particularly check these frequencies as you tune through the frequencies.

If you find an oddity then you can stop the transmission and if the oddity stops you know that your transmission was causing a problem if it does not stop then it is not your transmission at fault.

Of course there is a limit to the frequency range of general coverage receivers so they are limited in their testing ability.

However if you were looking for harmonics of 1.96MHZ then on a general coverage receiver of 0 to 30MHz you would be able to check up to the 15 harmonic   1.96 x 15 = 29.25MHz !!!!

You will want to check all the modes you use as FM gives you least problems with EMC then comes CW and worst of all is SSB.

7b.5 Recall and understand that siting a transmitting antenna close to mains wiring, TV or radio aerials and downleads is a potential problem exacerbated by the use of a loft or indoor transmitting antenna.

So what can pickup your RF with regards to proximity to your antenna:-

  • mains wiring

  • TV and radio aerials and downleads

whilst not mentioned in the syllabus at this point so can also include alarm cables.

Thus if you have decided for what ever reason that you have to locate your antennas in the roof space (loft) or any other indoor antenna you are in fact exacerbating (making worse) the EMC problem.

The field strength from your antenna reduces with distance and therefore you always want to have as much distance between your antenna(s) and ALL wiring.

7c Interference sources and simple remedies

7c.1 Identify the forms of interference caused by amateur radio and other radio transmissions: patterning on the TV screen, loss of colour, voice on TV sound, radio, telephone or audio systems. Recall that interference to digital televisions is different. The picture may freeze, become jerky or disappear.

There are several distinctive effects of RF EMC problems on Analogue Equipment whether it is amateur or other radio transmissions - these are :-

  • herring-bone patterning on the TV screen usually caused by an FM transmission,

  • loss of colour,

  • voice on TV sound, radio, telephone or audio systems usually caused by an SSB transmission

When it comes to digital televisions the situation is different. RF interference may cause :-

  • the picture to freeze,

  • the picture to become jerky,

  • the picture to disappear.

7c.2 Recall other sources of interference and their effects: arcing thermostats and vehicle ignition systems, electric motors in vacuum cleaners, fans, drills, sewing machines etc.  computer and peripherals.

Recall that this gives rise to spots on TV or a buzz on sound radio.

Transmitters are not the only sources of interference to TV's and their effects are different.

The key sources and effects are :-

Electric motors

  • On vision (TV picture) -- Spots on the TV screen

  • On Sound -- the possibly of a buzzing or some other continuous noise


  • On vision -- a regular appearance every 5 to say 10 minutes of a burst of spots and lines on TV screen which are quite intense which last say 2 to 10 seconds

  • On sound -- the screen interference is accompanied by short period of a noise like screwing up paper or a sort of rasping buzzing sound.

Some how students seem to miss this part where thermostats are mentioned maybe is it because they do not understand that a thermostat is used in the residential environment to regulate the temperature of the room whether it is by switching on and off an electric heater or the central heating pump. Thus there will be semi regular operation of the thermostat as the room cools down to turn the heating on and when it reaches the selected temperature switches the heating off. The operation of the switch causes the electrical interference in the TV as indicated above.

Vehicle ignition

  • On vision -- spots / lines on the TV screen

  • On sound -- with clicks which rise and fall with engine speed.


Analogue TV transmissions in the UK have ceased although there will be some analogue TV's with external "Set top boxes" still in use for awhile. These remaining analogue sets may continue to show some signs of local interference from radio transmissions, electric motors and car ignition systems. Common effects include wavy lines, loss of colour, dotted lines, loss of sound and audio  breakthrough.

7c.3 Recall that direct pick-up in affected devices tends to be independent of the transmitted frequency.

When we are dealing with direct pickup we are not dealing with an interference that has passed though any tuned circuits but interference that is having a direct effect on components within the piece of electronic equipment.

The direct (internal) pickup and conducted pickup is by loudspeaker leads and occurs over a wide range of frequencies. So tuning a radio receiver from one end of the band to the other and listening for a received signal has minimal usefulness. The action of the tuning is merely a diagnostic tool. If there is no adverse signal detected whilst tuning then the most likely culprit is direct pickup, in other words you are eliminating other RF causes via cables etc.

7c.4 Understand that masthead and downlead TV amplifiers are broadband and so they amplify a wide range of frequencies, including amateur frequencies. Understand that this can result in overloading of the amplifier and/or the TV input.

One of the biggest problems to overcome, should you or more especially your neighbour suffer from interference from your transmitter, will be if there is a mast head amplifier on the TV / radio antenna.

There will be many occasions where the amplifier is just not needed but may have been installed as a "fad". The problem you face as an amateur is that the amplifier to do its job property has to be very broadband and in being broadband amplifiers a great many signals including amateur radio signals. The effect of this is that the TV's input stage is overloaded which causes distortion or cross modulation as it is called (but you do not need to remember that word "cross modulation" for the exam just remember distortion !!!)

If a mast head amplifier really is needed then a filter at the TV antenna input will help the situation as will a "band specific" mast head amplifier.

7c.5 Recall the use of ferrite ring filters for minimizing unwanted RF on aerial downleads and mains leads to affected equipment.

Recall and understand the use of high-pass filters to reduce the level of HF and VHF amateur transmissions into TV systems.

Understand the use of mains filters to reduce RF, electric motor and thermostat interference to TV, radio, and audio systems.

One of the most useful assets that the radio amateur can own are ferrite rings. In value for money terms they are wonderful. The ferrite ring is effective for RF signals on mains and audio leads and for RF signals picked up by the screen of coaxial cables and downleads. However if the signal is picked up by the antenna and conveyed to the device via the inner conductor then the ferrite ring are ineffective and a filter tuned to the appropriate frequency is required.

In-line mains suppression filters are effective for interference conducted along the mains wires however if the source of the problem is know it is better that the item is repaired or replaced, but at least try filters first before throwing away a piece of equipment.

Please note that spike filters sold for computers often do not have any RF suppression just a VDR (voltage dependent resistor) for spike suppression.

7c.6 Understand that transmitting into a dummy load is a good test for any unwanted RF being conducted out of the transmitter along its power supply leads and into the mains.

Elsewhere in the course discussion has taken place about the DUMMY LOAD and that it is important for an amateur radio station to have one. The dummy load is a very good piece of test equipment as it is used in place to the radiating antenna and so long as it is of sufficient power handling capability will remove the RF field whilst you are trying to establish if you are causing the RF problems.

It the problem does not disappears when the dummy load is used then the problem is likely to be that the transmitter is sending RF down through its power supply into the mains lead and thence into the mains supply. If the problem can be solved by fitting ferrite rings to the mains lead of the transmitter and then when removed the problem returns, then the transmitter is almost certainly sending RF into the mains.

If when the antenna is used the interference returns then put the ferrite ring on the mains lead to the affected device as the mains lead of the device may be acting as an antenna!!

BE WARNED - EMC problems can be very difficult to cure

7d Social issues of interference

7d.1 Understand that the station log will be of considerable assistance in dealing with complaints of interference, and that this is a good reason to keep a log of all transmissions.

In addition to being a good record to look back over at a later date your station log book is valuable in establishing the likelihood that your system is causing the interference or not. If your neighbour complains about interference and your log book is up to date, (as it must be and ALL entries correct), then if the time that the interference was detected does not link in with your operating then quite obviously you cannot be the cause.

If Ofcom becomes involved with the dispute that they will expect both parties to keep logs of the respective interference and operating so that they can compare.

Never claim to be able to solve all problems but do be sympathetic to the complaint - never offer to delve inside their equipment to "fix it" it might just cost you a great deal of money if, rather when, things go wrong!!

7d.2 Understand the merits of the amateur and the complainant keeping a log of the instances of interference. Understand the merit of conducting tests in co-operation with the complainant in instances of interference.

So if your neighbour does complain show the neighbour your log book and suggest that he too keeps a log of the interference and that you will be happy to try and track it down with him.

If however it is plain that your are the source of the interference then you want to be as helpful as you can be to solve the problem and if possible avoid the Ofcom becoming involved as cost will occur and bad blood develop quickly between you. If it can be seen that you want to help, the good neighbourly relationship that you have had to that point can be saved !!

When you have your neighbour on your side and understanding that you want to help then it will be possible to conduct suitable tests to establish what is causing the problems and you are on your way to resolving them.

7d.3 Recall that RSGB and Ofcom produce information leaflets on EMC and interference.

Recall that advice is available from the RSGB EMC Committee and the role of local Ofcom officers in dealing with cases of interference.

RSGB EMC committee has local experts

You must be aware that the RSGB EMC committee has local experts who may be able to assist with suitable EMC advice.

RSGB and Ofcom produce helpful leaflets

Both the RSGB and Ofcom produce helpful leaflets with regards to EMC and EMC problems.

If the need arises for the Ofcom to be called in then it is likely that both parties will have to keep logs for about a month and for the amateur radio station to be inspected. The end result is often that the equipment affected lack immunity to RF, and even if the inspectors find a problem their primary aim is to help you cure it.

RA copyright logo

brats copyright logo