"Tom Horne" wrote in message
...
Is a current balun or line isolator effective in limiting common mode
current from the outside of a coaxial feed line when the antenna is a
two meter & seventy centimeter dual band vertical? What sort of line
isolation would be best practice for keeping current off of the
outside of the coax shield?
--
Tom Horne, W3TDH

Yes, for current balun; not sure exactly what you mean by "line isolator";
it appears to be a choke, maybe just another name for choke balun. Let me
be on that.

You have some easy ways to make your own current balun, which might be
called a choke balun.

(1) Slide a couple dozen Mix 43 ferrite cores over the coax and secure them
to the coax close to the antenna. I say Mix 43 because it's pretty common.
Mix 31, 43, 44 and 46 are suitable for 2m choking. Mix 61 covers 440 but
not 2m. See:
http://www.fair-rite.com/newfair/pdf/CUP%20Paper.pdf for one company's view
of things.

(2) Wind a section of the coax (again, close to the antenna) into a fairly
tight coil of three to six turns on a cylindrical form. There are many
opinions regarding size and number of turns if you search the web.

(3) Some advice would have you wind a few turns on a ferrite toroid but it
would have be with a short section of odd, small diameter coax, since RG-8,
RG-58 can't be bent that tightly.

As to "best practice," that, sir, is far too risky to declare in a
newsgroup. (If I had a few thousand dollars more in test equipment, I might
risk an opinion.)

I will say that if you can manhandle the line -- grasping, waving,
wiggling -- and you see the VSWR changing, it's possible you've not gotten
it right, yet.

"Sal"
(KD6VKW)