Unwanted RF in the shack due to Common Mode Current (CMC) on the coax can be nasty. Besides burning your fingers on the key or burning your lips on the mic, it can also cause some devices to fault. Worst case it can cause TVI or BCI, etc.

To help reduce CMC on the coax, an RF choke (or CMC Choke) can be placed at the antenna feedpoint. In this case we call it a "Balun" or "Choke-Balun".

It is also important to place a second Choke just before the coax enters the house, to help prevent pickup of noise by the feedline, which negitively affects your receiver.

There are several ways to accomplish this;

some good, some better, some not so good.

I will NOT try to explain all of the theory here - just enough theory that you understand what is going on.  

THIS  SECTION  CONTENTS:

• START HERE  . . . . . . . . . . . things to understand before you read about chokes
• Simple Basic Theory  . . . . . Explains how chokes help - in Q&A form.
• Important Background Info . . (not yet written)
• Different Kinds of Chokes  . Shows 5 different types of RF Chokes
• Things to Watch Out For  . . Warnings and Explaination of Differences
• Where to Use Chokes   . . . . Comments by Jim Brown, K9YC)
• Which Choke - Where   . . . . Guidelines for what to deploy, where and when)

This is where I learned about this topic before beginning my own research:

LINKS to more Theory:

If you have more severe problems, then it is  helpful to study the information published by the real experts on this topic.  

These are CLICKABLE-LNKS:

W1HIS    -   W7EL   -   GM3SEK    -    G3TXQ    -    K9YC-1    -    K9YC-2

I can't say which is better - they are all excellent!