On Fri, 27 Oct 2006 23:52:03 GMT, Cecil Moore wrote:

Owen Duffy wrote:
Those models also reveal the standing wave nature of the common mode
feedline current, and the futility of taking a current probe
measurement at a single location to infer any more than the current at
that specific location (if that was important).

In fact, the common-mode currents on the feedline
turn the feedline into a standing-wave antenna.
I suspect that's how an Isotron antenna works.

For what it's worth, the following is the method I used in developing the
W2DU current balun in 1981, that was published in QST, March 1983.

I wanted the balun to cover 80 thru 20m. I considered the worst case
situation would be on 80 m with the dipole cut to resonate at mid band, 3.75
MHz. Measured impedance of my dipole centered at 3.75 MHz yielded a terminal
impedance of 53 - j122 ohms at 3.50 MHz, for an impedance magnitude of 133 ohms
at 66.5 degrees.

We must consider that the center conductor of the coax feed line connects
to one half of the dipole, and the outer conductor to the other half of the
dipole. Therefore, the outer conductor sees only one half of the total terminal
impedance of the dipole, 66.5 ohms. I then considered that the choking impedance
of the balun should be no less that 10 times the half-dipole impedance over the
entire frequency range from 80 through 20m.

Fifty No. 73 beads satisfied that requirement with lots of margin to spare,
right down to the top end of the 160m band at 2.0 MHz. Reference to Fig 21-3,
Chapter 21 in Reflections shows the impedance to common mode current on the
coaxial feed line throughout the designated portion of the spectrum. Fig 21-3
can be downloaded from my web page at www.w2du.com by selecting Chapter 21 from
the menu.

Walt, W2DU