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Section 6
Electromagnetic Compatibility
EMC concepts
6A1 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.
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)
6A2 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.
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.
6A3 Recall that radio transmitters can cause interference to nearby electronic and radio equipment.
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.
6A4 Recall that radio receivers can also suffer from interference from local and other 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.
The origin of some of the text on this page is from the RSGB with additions by the web master
