Filters

This information is for those wishing to know about different types of filters and how they are used. Unless competent to do so, we would not normally advise householders to specify and fit filters themselves unless the source of the problem has been established beyond doubt, and it is clear which filters are most suitable.

TV/FM filters

The idea of a filter is to prevent unwanted signals from reaching your equipment, whilst allowing the wanted ones to pass through unaffected. They come in several types for use in different situations:

1. Low pass filter
2. High pass filter
3. Bandpass filter
4. Braid-breaker
5. Ferrite rings

filter2

 

Low-pass and high-pass filters

In most cases, filters are effective at preventing transmissions from radio amateurs and CB enthusiasts from causing problems to TV and radio reception. Since many amateur transmissions are lower in frequency than TV and FM signals, a low pass filter is fitted at the transmitter to prevent any unwanted spurious signals escaping. At the same time, high pass filters can be fitted to the receiver to prevent the low frequency amateur signals from reaching the receiver and causing problems. High pass filters are a common component in what are known as ‘TVI' filters, sometimes also called ‘TETRA' filters. In addition, a ferrite ring may need to be fitted to the mains lead if the filter alone is not enough. The diagram below shows where to fit the high pass filter (X) and the ferrite ring (Y). Note that filters must not be fitted between a masthead amplifier and its power supply (unless the filter specifically states it is suitable for this purpose).

tv_filters

Fitting filters to a TV set

Band Pass filters

Band pass filters are usually fitted where the amateur is using frequencies close to FM or TV ones. The filter is normally fitted to the receiver, and allows only the range of required frequencies through, rejecting all others. Band pass filters must be specified for the correct use - to use a TV band pass filter on an FM tuner will prevent reception of FM signals - and vice versa. Band pass filters are often fitted to TV masthead amplifiers, as unfiltered ones are known to be easily upset by strong local transmissions of all kinds - not just radio amateurs. A different kind of band pass filter can be used on the transmitter, again to ensure no spurious signals can escape.

Braid Breakers

Braid-breakers work by preventing low frequency signals from reaching TV or FM equipment down the outside of the co-axial cable. They can be made in various ways, sometimes using components or a transformer, but the easiest way is often to wind several turns of co-axial cable around a suitable ferrite ring. In practice, many commercial braid-breakers often incorporate a high-pass filter in the same box, to provide a ‘belt and braces' approach to interference suppression. As with the simple high-pass filter, this is used in position X in the previous diagram. If you have a VCR or DVD connected to your TV, you may need to fit filters to the inputs to both units, and possibly ferrite rings to both mains leads as shown in the next diagram. Just what is needed is best determined by experiment.

tv_vcr_filters

Fitting filters to TV set plus VCR/DVD

Ferrite rings

Sometimes strong amateur signals can get into audio equipment as well as TV or radio. This usually happens because the loudspeaker wires or mains cables act as ‘aerials' and conduct the signals inside the case. Once inside, it is difficult to overcome problems without modifying the circuitry - a method of last resort! An easier solution can often be to wind a short section of the cable in question around a ferrite ring using a number of turns, and to do so as near to the HiFi/TV/radio as possible. The diagram below shows how to do this.

ferrite_ring

Ferrite ring wound with cable

Ferrite rings work by providing a high impedance to radio-frequency signals, with little effect on mains or audio frequencies.

A range of filters and ferrite rings can be supplied by the Radio Society of Great Britain or from other outlets.

Diagrams reproduced by courtesy of the Radio Society of Great Britain (RSGB).
More detailed information on the use and design of filters is available from the RSGB website.

 

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