COMMON MODE CURRENT EXPLAINED
[ Simple Theory ]
"Simple" is perhaps exagerated. There is no real simple explanation for this, but I would like to point you towards two outstanding resources for understanding common mode current and learning how to deal with it.
I strongly suggest you view/read the articles in the order I list them.
1. GM3SEK's Web PowerPoint presentation: "Cost-effective Ferrite Chokes and Baluns"
DOWNLOAD INSTRUCTIONS: RIGHT-CLICK the link below and save it to your desktop or any convenient location. After the new link is saved to your computer, double-click the new link to start the presentation.
NOTE: It takes a while to click through this presentatin, but it is an outstanding representation of what is going on, what the problem is, and where it comes from.
2. W7EL's paper entitled: "BALUNS: What they do and how they do it."
In fact it is also a lot about what they don't do and highlights that one type of balun that we have been using for many year is not actually doing the job.
DOWNLOAD HERE: http://www.eznec.com/Amateur/Articles/Baluns.pdf
NOTE: This also explains the theory of what causes CMC, but it does not include the color drawings that Ian's presentation (above) include. However it does go into a bit more detail of the differences of the kinds of chokes and recommends what we should use.
[ Intermediate Theory ]
Before going on to the next page on Advaned Theory, it is a good idea to study the following presentation by Jim Brown, K9YC, entitled "Coaxial Transmitting Chokes".
DOWNLOAD HERE: http://audiosystemsgroup.com/NCDXACoaxChokesPPT.pdf
[ Practical CMC Chokes ]
The solutions suggested by Jim, K9YC (above) are technically the best, but can be expensive to implement. Sometimes you don't need the very best.
For a reasonable compromise of cost and effectiveness, I suggest you consider one or more of the chokes described by Ian White, GM3SEK. (Download below.)
If your problems continue even with these chokes, then you need to study the next page on Advanced CMC Theory.
In this article, Ian describes in detail how (and why) to build 3 RF Chokes: High Bands, Mid Bands, Low bands.
This design was first suggested by W2VJN in The ARRL Handbook 2010.
PDF-Dokument [2.9 MB]
[ BEST QUICK REFERENCE GUIDE ]
Once you have a good understanding of all this and get into building your own baluns and chokes, there is an OUTSTANDING reference guide to which toroid (mix) and how many turns to use.
Steve Hunt, G3TXQ has a chart on his web site:
RF Chokes: http://www.karinya.net/g3txq/chokes/