Braid-breaker  A braid-breaker is a filter that prevents television interference (TVI). In many cases of TVI, caused by a high field strength of a nearby high frequency (HF) transmitter, the aerial down lead plugged into the back of the TV acts as a longwire antenna or as a simple vertical element. The radio frequency (RF) current flowing through the tuner of the TV tends to generate harmonics which then spoil the viewing. The braid breaker works by preventing RF signals picked up on the outside (common mode signals) flowing into the TV set, while passing RF inside the coax (non common mode) from the antenna. Designs for diminishing unwanted signals are based on two types of filters: a “choke” filter which blocks signals in the electrical "mode" most interference uses, and filters that selectively admit or impede signals depending on the signal frequency. 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. “Chokes” work by blocking signals traveling via common mode and pass through differential-mode or “balanced” currents unchanged. In the common mode all signals running along all the wires travel in the same direction. When the unwanted signal is only in the common mode then the desired signal can pass through the filter via the differential-mode. One design is a snap-on ferrite choke, fastened over the aerial cable just before it plugs into the TV set. The UHF signal is sent in differential-mode, that has equal and opposite currents running on the interior surfaces of the conductors. Those two opposite currents lie close enough together for their magnetic fields to cancel each other, and so those signal currents don't couple magnetically with the ferrite, and pass through unaffected. However currents that are traveling in the same direction on the two conductors (common mode current) re-enforce each other's magnetic fields, which are channeled through the ferrite. The ferrite in effect enhances the magnetic fields thousands of times, by keeping it concentrated near the surface of the ferrite, rather than allowing the fields to spread out. The common mode alternating currents are then impeded by the magnetic fields they themselves created. So “choke” is a very good name: The ferrite in effect “chokes off” the signal path for interference that generally travels via common mode currents, but leaves unaffected the desired signals which are typically arranged to flow via differential mode current.