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

"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)

Tom Horne wrote in news:abad3160-672b-43cc-81b9-
:

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,

I published a design for a common mode choke which had moderate choking
impedance from 50Mhz to 150MHz, see http://vk1od.net/balun/G1-1-FT140-
61/ . It uses four turns of coax on a 1.4" #61 toroid, and common mode
impedance data is shown.

Most manufactured products and most ham designs do not include predicted
or measured choking impedance data which is most often the key parameter
in selecting a device. If you don't have that, you are guessing.

The design mentioned above might be adapted for 130 - 480MHz by taking a
turn off, but I would be guessing. Without modelling and measuring, it
is just a guess.

Whilst I have explored the W2DU style baluns on HF, I don't have any
designs that are intended for your application. I am sure there would be
viable designs for low power. A string of suitable clipons might be
viable (if more expensive).

Owen