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Section 6
Electromagnetic Compatibility


Filtering and remedial measures

6D1 Recall that the immunity of most types of equipment can be increased by fitting suitable external chokes and filters in mains or antenna leads.

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.

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

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.

6D4 Recall 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 any connected interface leads and into the mains.

The first test that you can do to check quite easily is to connect your transmitter (transceiver) to a suitable dummy load that can handle the power you wish to dissipate into the dummy load (Dummy Antenna) if then a problem exists with other equipment connected to the mains then you will know that the RF is also being conducted out of your transmitter along power leads and a remedy will need to be found.

Recall that a dummy load is a screened resistor of the correct value and a suitable power rating connected instead of an antenna to allow the transmitter to be operated without radiating a signal

You need to know that a Dummy Load is a screened resistor of the correct value usually 50Ohms and of a suitable power rating (it is suggested that if you are running 10 watts then a resistor (or resistors) need to be able to dissipate (absorb) say 20W of power as heat. Sometimes a dummy load has the resistor in a vegetable oil to help take away the heat.

The screening can be a sealed can with the suitable RF connector on the top and the leads of the resistor (s) need to be as short as possible as the Dummy load needs to be resistive without and inductance.

Here is an example of a QPR Dummy Load note how short the leads are to the CARBON not wirewound resistors.



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