Roy Lewallen wrote in
:

....
A choke at a high-impedance point doesn't do much, since there's no
substantial current at that location to block. You should put the
choke about a quarter wavelength from there where the current is
nominally high. The presence of the choke can move the locations of
high and low voltage and current points, so you'll need a choke at
least about every quarter wavelength to make sure there's no point
where substantial current can occur.

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.

Owen

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