On Sat, 02 Apr 2011 16:04:27 -0700, Richard Clark
wrote:

On Sat, 2 Apr 2011 12:06:16 -0700 (PDT), Tom Horne
wrote:

Richard
I didn't ignore choking the feed line but I will readily confess that
I did not choke it twice. Starting immediately below the bottom of
the matching stub I followed the recommendation of the various authors
and wound a multi-turn coax balun with a six inch diameter coils of
coax. They call for ten turns if I recall correctly.

Hi Tom,

This sound like very common advice - so common that it begs
investigation because it is common advice for HF Choking, not 2M, and
certainly not 70cm. However, this common advice acknowledges the need
for choking.

You would be better served using ferrites (W2DU style BalUn/Choke),
or, if you really wish to stick with wound coax, then use a Grid Dip
Meter to test its resonance (which should reveal you can't serve both
bands). Using an antenna analyzer to do this will give you measurable
Z, and that may give you the data to see how well you are doing. You
may wind a lot of chokes to discover that the diameter is
extraordinarily huge (or so the same for turn count - one or the other
or both).

This thread has shown me that I don't know enough about Common Mode
Chokes. I think they might also be referred to as transmission line
chokes.

What I need to know is
1- When to use them. RF in the Shack is pretty obvious
2- When not to use them. There must be some negatives.
3- "Compare and Contrast" the Inductor-capacitor choke with the
Ferrite Choke(W2DU). Are they electrically equivalent?
4- Are either of them a multi band solution?
5- Are there Upper and lower frequency limitations for using them.


As usual, a Google search provides an overwhelming set of responses
that most likely will cover the subject in greater detail than I can
digest.

Early in the list provided by the search I discovered
http://www.audiosystemsgroup.com/RFI-Ham.pdf

That may have all the answers but I have only glanced at the beginning
of the 66 page document.

John Ferrell W8CCW