"Walter Maxwell" wrote in message
...
On Fri, 09 Mar 2007 12:56:12 -0800, Roy Lewallen wrote:

Ralph Mowery wrote:
Has anyone thought of trying to use some of the ferrite beads to isolate
the
guy wires of a tower for RF so the tower could be shunt fed ? Along the
same
lines could the beads be used to electrically brake up the wires into
non
resonate lengths ?

It would require multiple beads at each point, and at multiple points.
While it could be made to work, it would be heavy and expensive. That's
why egg insulators or non-conductive guys are used instead.

Roy Lewallen, W7EL

Yes Ralph, beads could be used to achieve what you want. I refer you to a
paper
that will appear in Reflections 3 as Chapter 21A, which is a continuation
of
Chapter 21 in Reflections 1 and 2. Chapter 21 tells of the need for a
balun, and
how the beads perform as a balun.

However, Chapter 21A describes how I developed the idea for using beads
for the
W2DU balun. The idea came as a spin-off from a method I used during
radiation-pattern measurements for the antennas that flew on TIROS weather
satellites. The downlead for the signal received from the antennas on the
satellite was reradiating EM energy, and distorting the radiation
patterns. I
reasoned that placing a bead every quarterwavelength along the downlead
would
breakup the current on the lead, which it did. I then transformed the idea
into
the W2DU balun, which is described in all editions of the ARRL Handbook
since
around 1985. Chapter 21 is a repeat of my article in QST for March 1983.

Chapters 21 and 21A can be found for downloading on my web page at
www.w2du.com.
Click separately on 'View Chapters from Reflections 2' for Chapter 21, and
click
on 'Preview Chapters from Reflections 3' for Chapter 21A.

Hope this helps.

Walt, W2DU

Walt I have seen some of your work in the handbooks and it makes for good
reading. I do use a current balun on a triband beam I have up.

I just have not seen any beads used on guy wires and did not know if it had
been tried. I agree it probably would be beter to just use the insulators
on the guy wires, but not sure how the price of them and the clamps would
compair to the beads. With the guy wires already in place it may just be
easier to place some clamp on beads on the guy wires if it would work.

73 de KU4PT

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

On Sun, 11 Mar 2007 03:48:28 GMT, Walter Maxwell
wrote:

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.

Hi Walt,

A prophet is not recognized in his own country. If Art could write as
clearly as you (and to some practical purpose) perhaps his cries would
have merit.

However, as to the content of your Chapter 21A. It seems to me I had
come across this treatment some time ago. It inspired me to use your
W2DU balun specification, and spread the beads along a 20 feet length
of cable for exactly the reasons that initially motivated your first
use of them. If every quarterwave can be snubbed by one bead, then
certainly every 40th of a wave can be snubbed even more by the same
resistance. As I saw it, it was the same investment in beads, and the
same bulk resistance even if Kirchoff would point out that their
spread injected significant wavelength into this to invalidate his
lumped circuit analysis of total resistance. Be that as it may,
conceptually, the shorter element's smaller radiation resistance in
relation to the single bead sees a significantly higher port isolation
through the bead.

As for the practicality of a retro-fit, now that's another question.

73's
Richard Clark, KB7QHC

On Sat, 10 Mar 2007 22:51:48 -0800, Richard Clark
wrote:

the shorter element's smaller radiation resistance in
relation to the single bead sees a significantly higher port isolation
through the bead.

As Reggie might have observed, the distributed resistance would
conform to the analogy of distributed inductance and capacitance whose
total contribution would be an extremely lossy transmission line.

73's
Richard Clark, KB7QHC

On Sat, 10 Mar 2007 23:45:35 -0800, Richard Clark wrote:

On Sat, 10 Mar 2007 22:51:48 -0800, Richard Clark
wrote:

the shorter element's smaller radiation resistance in
relation to the single bead sees a significantly higher port isolation
through the bead.

As Reggie might have observed, the distributed resistance would
conform to the analogy of distributed inductance and capacitance whose
total contribution would be an extremely lossy transmission line.

73's
Richard Clark, KB7QHC


Exactly!

Walt, W2DU