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