Braid-breaker
A braid-breaker is a filter that prevents
The braid breaker works by preventing RF signals picked up on the outside flowing into the TV set, while passing RF inside the coax from the antenna.
Designs
Designs for diminishing unwanted signals are based on two types of filters: a “
Further, carefully chosen combinations of filters of either one type or both types multiply each other's effects, so that even if only slightly different, two filters are more effective than a single filter, or either filter alone.
Ferrite choke
The device is called a "choke" because the ferrite in effect "chokes off" the signal path for interference.
High-pass filter
The other type of filters used are based on frequency: Below their operating frequency limit, inductors (coils) impede signals at higher frequencies more, and admit low frequencies, whereas capacitors do the opposite: capacitors admit high frequencies but impede low frequencies. These can be played-off against each other to impede or admit signals based on frequency.
A simple design for a
As an extra precaution, a 1.5
The impedance of the capacitors is very large for
- At and below HF (below about 50 MHz) the coils short-circuit the core to the braid for both input and output, while the capacitors impede signals passing straight through. Hence isolating the input from the output for HF signals.
- At UHF, the impedance of the coils blocks the short-circuit between the core wire and the braid for both input and output, whereas the capacitors pass through the signals almost unchanged. Hence the UHF signals are connected as normal to the respective terminals on the other side of the network.
Combination
An even better option is to use both a choke and a high-pass filter, since the filter described above may not be as effective for common-mode currents, which the choke will selectively remove.
See also
References
- The radio amateur's guide to EMC, Robin Page-Jones (G3JWI), published by the Radio Society of Great Britain, Potters Bar (Hertfordshire), 1992.
- How to Improve Television and Radio Reception - a booklet published by the British Department of Trade and Industry. This booklet is out of print but a copy can be seen at [1]