On Sat, 10 Mar 2007 14:38:00 -0800, Roy Lewallen wrote:

Owen Duffy wrote:

Roy, NEC models suggest that lossy chokes (eg suppression sleeves or cores
where Q is very small) don't modify the current distribution much unless
they are of sufficiently large impedance, and that introduction of low Z
chokes just introduces another loss without much impact on the current
distribution or resultant antenna pattern.

The magnitude of Z needed to be effective in forcing a current minimum at a
point might be quite impractical to implement using suppression sleeves, so
the time honoured insulator looks the better solution.

Yes, that's exactly the point I've been trying, apparently
unsuccessfully, to make. It is practical to use ferrite sleeves for
suppression of current at a single point or a couple of points, as Walt
Maxwell pointed out some time ago. Often called the "W2DU balun", it's
done by putting a lot of cores -- typical several tens of cores -- over
the line. But you wouldn't want to do this at a dozen or two points on
guy wires. I personally prefer to use multiple turns on a single core,
because ten turns on one core gives the same impedance a single pass
through 100 cores. But then I don't run so much power that I need to use
RG-8 or larger size cable or go to heroic efforts to insulate the turns
on a single core.

The guy wire requirements would be about the same as for a "current
balun" (common mode choke) -- somewhere around 500 - 1000 ohms is
typically necessary. At that impedance level, it makes no difference
whether the impedance is reactive or resistive from the standpoint of
effectiveness in choking current or in terms of dB loss. But there can
still be enough power dissipated to overheat the cores if they're
resistive and the power level is very high. Then you're stuck with using
ferrites which are more reactive and less resistive (e.g., Fair-Rite 60
series), but they also give you a lot less impedance per core so you
need more cores yet. That makes the ferrite solution even less attractive.

Roy Lewallen, W7EL

Apparently no one on this thread has read my Chapter 21A from my web page at
www.w2du.com, where I showed that placing one #43 bead at every 1/4 wl along a
feed line eliminated the current flowing on it while immersed in an EM field in
the 130 to 150 MHz frequency range. It was the success of this one bead approach
during radiation-pattern measurements of spacecraft antennas that led to the
development of the W2DU balun with several beads at one location.

Walt, W2DU