These ferrite chokes can be used on just about every wire in your house. Make certain that you choose a ferrite mix that will function on the frequency you are trying to suppress. In a cable coaxial antenna the desired signal is restricted to the center conductor and a large toroid can be placed over the coaxial antenna, thus suppressing any common-mode current flowing down the shield without affecting the desired VHF signal inside the cable. Test have shown that 300 ohm twin lead wire can be protected in the same manner, as long as you run both leads through the same toroid. If you put a toroid on each lead, the primary VHF signal will be attenuated also. Obviously this would not be desired.
Since the object is to interrupt the common-mode rf current path, thus breaking that RFI circuit, sometimes it is much easier to wind your A/C power cord through the toroid, rather than trying to wind the stiff coaxial wire through it.
Let's say that you take your A/C power cord and run it through a toroidal core or wrap it around a ferrite rod. This will attenuate the given frequency for that ferrite mix (X) amount. If you wrap it two times it will attenuate it, (X) times 2. Three times, (X) times 3, and so on. The usual way to use these devices is to take your cable coax or A/C power cord that is leading into your TV and wrap it around a ferrite 14-15 turns, as close to the set as possible. Now, if you don't have enough coax, just make up a section out of new coax, wrap it, put end connectors and a barrel connector on it and connect it to your cable connector. (Ed Hare (KA1CV) likes to bring a few of these with him when he does a troubleshooting "service call".) With the A/C power line, you can use an extension cord to wind the toroid if you don't have enough free line.
These toroids can be found in many locations. There are advertisements in most amateur magazines, you can find them in electronic supply stores.
In a pinch, you can even use discarded TV reflection yokes. After the wiring has been removed from these yokes, just tape the two ferrite sections together to form a large toroid. This MAY work, but keep in mind that the ferrite material was designed for 15 kHz. Who knows what it may do at VHF!
There are also other types of cures that we can try, like A/C line filters and tuned stubs at the television antenna lead. First let's try to find out how the RFI is getting in.
(Continued in Part 6)
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