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8. Electro Magnetic Compatibility EMC

7. EMC

7a Basics of electromagnetic compatibility

7a.1 Recall that electromagnetic compatibility (EMC) is the avoidance of interference between various pieces of electronic equipment.

EMC is an abbreviation for ElectroMagnetic Compatibility. If one piece of electronic can operate normally and not cause an other nearby piece of electronic equipment to malfunction then those pieces of equipment are said to be compatible. Interference between such piece of equipment would be caused by lack of ElectroMagnetic Compatibility.

So EMC is the ability of one piece of electronic NOT to interfere with each other.

How can the interference be recognised ?

If equipment does not operate normally then in addition to any other malfunction if could be affected by other nearby equipment.

In a piece of equipment that has a speaker or head phones as it output then you might hear any of the following:- a buzzing sound, a clicking sound, muffled speech or clear speech on a loud speaker system of a radio or hi-fi system or even a TV

If the piece of equipment has a screen then patterning on the screen may be observed.

Question

Which piece of equipment is most likely to be affected by RF? A television because the RF signal going to a TV has to give good picture and sound both of which can be affected by RF interference. (Here we are excluding television signal received via cable TV system)


7a.2 Recall that radio transmitters can cause interference to nearby electronic and radio equipment.

The Amateur Radio Transmitter is designed to send out an RF signal that is many times higher than any other piece or equipment that would exist in any residential home of a "non" amateur.

With this in mind, owning and operating an amateur Transmitter means that you will be generating signals that without due care could deliver into other equipment a great deal of interference.

The best way to reduce the likelihood of interference is to ensure that all the RF signal is radiated from your antenna and none are radiated from your feeder.

The strength of your RF signal is measured as an RF field strength and it is important to know that field strength reduces with distance and therefore the greater the distance from your antenna to any other equipment the better will be the EMC and the reduced likelihood of interference occurring.

The amount of the field strength is also dependent upon the amount of output power thus it is easy to understand that the lower the output power from your transmitter where you can still maintain a good signal to the station receiving you the better as this will also reduce the likelihood of interference.

An easy check of your system is to switch on your HiFi / domestic radio and any garbled speech in sync with your transmission will mean that not only are you causing interference to your own equipment in the home but almost certainly causing interference to others !!


7a.3 Recall that radio receivers can also suffer from interference from local sources.

So what are local sources which are not from a Radio Transmitter.

Any item that has a switch in it that is switching mains electricity can have the ability to cause interference which will be heard as clicks.

The following items are often found in the home :- kettles, electric blankets, central heating systems and sometimes oil storage tanks.

Each of these items have switches and it is the operation of the switch that can cause a spark. The spark was the very first type of transmitter that was used in the birth of radio communication and as such creates RF signal of an infinite number of frequencies. As they are not connected to an antenna the interference is relatively local.

The other type of equipment where sparks can occur are in electric motors and you may well be familiar with the DIY electric Drill, the vacuum cleaner, washing machine, hair drier.

Many of these items have what is called a commutator which is a segmented ring of conducting material which are contacted by usually carbon brushes. Each time the brushes moves across the commutator segments a spark can occur and rather than a clicking would be heard as a buzzing noise which may rise and fall in harmony with the speed of the motor. Again the interference is relatively local.


7a.4 Recall that interference occurs through local radio transmissions being conveyed to the affected equipment through pick-up in house wiring, TV antenna down-leads, telephone wiring etc., and (particularly at VHF/UHF) by direct pick-up in the internal circuits of the affected equipment.


There are many ways that neighbour's house might pick up amateur transmissions and the following are examples telephone wiring, mains wiring, hifi loudspeaker wiring and of course a TV antenna.

The following explains more.

The interference can transfer from one piece of equipment to another in various ways:-

  1. Be radiated from that antenna

  2. Be induced between wires that are running in close proximity to each other

  3. by conduction along a cable which is linked to both the sending and receiving equipment

The interference can be picked up by ALL of the following :-

  1. the television aerial and downlead / feeders and aerial and downleads / feeders Radios and video recorders

  2. telephone wiring both directly from the radiate signal and induced between several wires running close together

  3. the mains wiring in the home

  4. Hi-Fi loudspeaker wiring

    and at VHF/UHF by direct pickup by the internal circuits of the affected equipment itself.


7b Station design for EMC.

7b.1 Recall that EMC problems can be minimized by siting antennas as far away from houses as possible, as high as possible, and using balanced antennas at HF.

Recall that, at HF, (horizontal) dipoles are less likely to be a problem and that end-fed wires present significant EMC problems.

Recall that information on the avoidance of interference by the correct choice and siting of antennas and suitable operating procedures is readily available from several sources.

What can you do to minimize problems related to antennas?

When you are considering where to place your antenna follow these guide lines :-

Locate the antenna as far away from homes (buildings in general) as possible,

Place the antenna as high as possible, but remember the possibility that in a storm the antenna support might crash to the ground with your aerial !!

Use a balanced antenna at HF - the centre fed dipole (with a balun when fed by coaxial cable) is considered the best antenna as far as EMC is concerned.

Avoid the use of a "long end fed wire" as these have a greater possibility to cause interference than the horizontal dipole. The long end fed wire must be properly matched to the Transmitter by an ATU.

Radiocommunication Agency leaflet RA 179 gives information as to how someone should report interference and RA 234 EMC and the Radio Amateur (Rev. 5, March 2000) are useful sources of more information. You should be aware of these publication.


7b.2 Recall that the more power a station runs, the more likely it is to cause interference.

Recall that some types of transmission are more likely to cause interference to TV, radio and telephones than others.

Recall that SSB is the one of the poorest in this respect. FM, CW (Morse) and the some of the HF data modes (such as PSK31) are much better.

Some operators who have migrated to amateur radio might well think that operating at 100 watts rather than 10 watts will have no affect on the possibility of interference. The reason for the limit on the operating power level in much of the Foundation Licence is that the higher the operating power output level the greater the possibility of causing interference.

The Higher the power = the higher the likelihood of interference.

AM and SSB Transmissions are MORE likely to cause interference problems as the affected receiving and audio equipment sometimes is able to extract the audio signal and it is heard from the loud speaker.

As you are likely to be using SSB it is essential that you know that you could easily cause interference to others.

FM transmissions are LEAST likely followed by CW PSK31 to cause interference problems because relatively low power is used to maintain contact and even over long distances with CW and PSK31.


7c Immunity of radio receiving and other devices and filtering techniques.

7c.1 Recall that the ability of any piece of electronic or radio equipment to function correctly in the presence of strong RF signals is known as "immunity".

Recall that the immunity of most types of equipment can be increased by fitting suitable external chokes and filters in mains or TV antenna leads.

Recall that the filters should be fitted as close to the affected device as possible.

If you are operating you transmitter and all the other equipment in your house is operating correctly then that other equipment is said to have "IMMUNITY" to the strong RF signals that you are radiating.

If however some of the equipment is not operating properly then by the use of external chokes and filters particularly on electric mains leads and TV and radio antenna downleads and also by using a LOW PASS FILTER on your transmitter feeder as close to the transmitter as possible the problem of the RF entering the equipment can under many circumstances be prevented, and the Low pass filter on the Rig limits the frequencies transmitted to those below the cut off point of the filter..

All cables that enter equipment can act as antennas and as such received RF signals. When fitting filters they must be fitted as close to the equipment as possible thereby reducing the amount of cable left between the filter and the equipment that can act as an aerial.


7c.2 Recall that anything fitted to the mains wiring must be properly made for the purpose.

Understand that home-made items (other than ferrite rings) are potentially dangerous.

Recall that information about the purchasing, making and fitting of chokes and filters is readily available from several sources.

It is dangerous to put home made devices in the mains but you could fit a ferrite ring if the equipment does not have a moulded plug on it.

Why is this the case. Because the Foundation course is all about you operating safely and thus reducing risks of injury to you. If you are going to fit anything into a mains cable it must be "Commercially" and so properly made for the purpose. Any items which is home made and associated with mains electrical wiring does have a danger of death .

If you are going to fit anything to a mains cable you will have to know how a mains plug is wired and as fitting such a plug is not part of this course but comes into the Intermediate Course and such interference you do at your own risk. With much equipment now supplied with moulded plugs the fitting of even the humble ferrite ring to mains cable is not possible without removal of the mains plug. An alternative is the split ring that is available and simply clips around the cable.

Information about the purchasing, making and fitting of chokes and filters is readily available from several sources especially amateur radio stores.

Photograph showing a Ferrite ring with coaxial cable wrapped around it photograph showing a Ferrite ring compared in size to a pound coin and that the ferrite ring is almost larger enough to fit over the pound coin

The picture above shows how a ferrite ring can be wound with cable. The cable could be a mains cable, microphone cable, computer lead, speaker leads. It is possible to use one or more ferrite rings and the cable wound to leave 1/3 of the ring exposed ,and the ends kept apart.

On an antenna downlead the ferrite ring and the cable is called a "braid breaker" and on a mains lead a "choke".


7c.3 Recall that the function of the RF earth connection in an HF amateur station is to provide a path to ground to minimise RF currents entering the mains earth system and causing interference to other electronic equipment.

You need to know that the function of the RF earth connection in an HF amateur station is to provide a path to ground to minimise RF currents entering the mains earth system and causing interference to other electronic equipment.

Because of developments in the safety of electric mains supply in the home providing an RF earth to your shack is a difficult situation as it may negate the electricity supply company's PME Protective Multiple Earthing.

Thus the PME mains supply and earthing practice can cause problems for radio amateurs.

However you have to know for the written assessment that in your shack there are two basic earths:-

The Electrical Earth

The earth in the mains supply which is connected to your equipment through the main plug.

The RF Earth

The RF earth which is connected to your rig and ATU (if there is such a provision) normally via a wing nut on a bolt to a long ground steak which you will have had to drive into the ground (avoiding electric mains cable, water mains and water waste pipe work that might exist under ground).

You have to know that the only purpose of the RF earth is to provide a direct route to ground for RF currents and thus minimise such current entering the mains earth system and causing interference to other electronic equipment.

With regard to the written assessment, a question about an earthing stake or RF earth the likely answer is - for passing any RF currents to earth.

NOTE: When setting up your station and you intend to use an RF earth check with an NICEIC registered contractor and explain what you propose to do and take the advice given.


7d Social issues of Interference.

7d.1 Recall that EMC problems have the potential for causing neighbour disputes.

Understand the need for diplomacy, the sources of advice available and the role of the local office of Ofcom.

When you operate you Transmitter think that there is the possibility that you could cause interference. You can reduce the likelihood of neighbourly disputes if you have any doubt that may be you could cause interference by :-

  1. using low power

  2. not operating at prime TV time

  3. ensuring that your own home entertainment systems TV RADIO CD etc are not affected by your radio transmissions.

When dealing with a neighbour never admit to causing the interference but refer to it as an EMC problem.

Photograph showing RF interference on a TV second photograph showing RF interference to a TV

The picture above shows what happens if a hand held is positioned near to an ANALOGUE TV whilst the PTT is pressed. This is the sort of interference that your neighbour might see on their TV right in the middle of "Easter Enders" and would almost certainly result in a defiant hammering on your front door

If your neighbour's receiving equipment lacks immunity from strong RF signal that you transmit then it is necessary to carry out a few tests to locate the source of the problem - it might not be you !!!!

If your neighbour is agreeable suggest that you go and transmit and that a friend of yours with a hand held to observe the interference in your neighbours equipment. Assuming that the interference is "real" and not imaginary then suggest you call in the help of your local club to try to resolve the problem with a few filters.

If there is not interference seen then do not just say "Oh well its gone away" but suggest that you neighbour keeps a log of when the interference takes place and what its the effect. You will need the date and time and duration. After that you can get together again and compare your log book and their log and it will be obvious whether it is you or not.

You must be aware that if you cannot resolve the matter that the local office of the Radiocommunication Agency will need to be involved to help resolve the problem. This service is not free and will almost certainly lead to your station being closed for a period of time to let matters cool down. Then when the situation is calmer there will be a better chance to sort things out for if the interference continues you would not be the culprit and would then try to sort it out from a stronger position.


The origin of some of the text on this page is from the RSGB with additions by the web master

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