John, what with the unwanted interference from Richard Clark, you'll
just have to accept the fact that the ferrite material has little or
no effect on transmission through the short length of transmission
line, be it coax or twin-line, wound on the ferrite core.

I can only repeat that, insofar as the normal transmission current is
concerned, the currents in the pair of wires flow in opposite
directions and therefore neutralise each other. No flux is induced in
the ferrite.

The unwanted longitudinal current flows equally and in the same
direction along both wires and therefore comes under the influence of
the choke.

It is all so beautifully simple!
----
Reg.

Reg Edwards wrote:
John, what with the unwanted interference from Richard Clark, you'll
just have to accept the fact that the ferrite material has little or
no effect on transmission through the short length of transmission
line, be it coax or twin-line, wound on the ferrite core.

Maybe it would help to say "... on transmission of differential
signals through the short length of transmission line ...".
--
73, Cecil http://www.qsl.net/w5dxp

A choke is supposed to reduce longitudinal currents on the line to
insignificant amounts.

But very often it doesn't. If you have a choke in the line how much
difference does it make if you remove it? If it makes no difference
you might just as well leave it out.

Insertion of a choke does not affect the manner in which the
longitudinal current is induced to flow in the line in the first
place.

The choke may indeed reduce the magnitude of the current at the
location of the choke. But the current always has standing waves and
the choke may merely shift the maximum current of the wave to another
place along the line. The longitudinal current still exists.

Indeed, if a minimum current should exist at the place where the choke
is inserted then the choke may serve no useful purpose and it can be
removed. It could be re-inserted at a place 1/4-wavelength away at a
current maximum where it MIGHT do some good.

All this requires knowledge of where the standing wave resides. But
one never has such knowledge unless one knows the cause. And if one
knows the cause it can be remedied.

The only way to prevent longitudinal currents on feedlines is to
remedy the cause, or to place chokes at every quarterwave interval all
along the line. Shifting the standing wave maximum, however, might be
sufficient to prevent the trouble it may be causing elsewhere in the
system.

The moral is - don't allow large longitudinal currents to be induced
on the feedlines in the first place. It's poor design.

A little bit of the inevitable longitudinal current on the feedline is
harmless.

If it works - don't fix it!
----
Reg, G4FGQ

- - - - and placing a choke at 1/4-wave intervals all along the
transmission line is not of great help - it is effective only at one
frequency.
----
Reg.

"Reg Edwards" wrote in
:

- - - - and placing a choke at 1/4-wave intervals all along the
transmission line is not of great help - it is effective only at one
frequency.

Actually, it's pretty effective at all frequencies below that one.


--
Dave Oldridge+
ICQ 1800667
VA7CZ