"Reg Edwards" wrote
"Cecil Moore" wrote
Of course, depending upon the impedance seen by the choke, the
choking function may be virtually non-existent. For instance,
5000 ohms is not an unusual impedance in a system of this sort.
The choke would need about 25,000 ohms of choking impedance. I
don't know where to obtain such a choke.
==================================

Cec, if the circuit impedance at the location of the choke is so high that
the extra impedance of the choke is ineffective, then the current must be
so
small a choke is not needed anyway. So what are you worried about?
---
Reg.

===================================
Dear Cec,

Just another comment about your oft-described high-impedance choke problem.

In all these discussions the LOCATION of the choke along the transmission
line is always neglected. Yet its location is as just as critical as its
value. If critical it is!

If a two-wire, unbalanced-to-balanced choke is deemed necessary then it
should be located at a lower impedance point in the system where it is
effective.

On the other hand, because very high impedances occur only on sharply
resonant lines, problems may occur, if they ever do, only at rare sharply
defined locations which can usually be ignored even if a resonant frequency
should occur in an amateur band. An operator would very likely be unaware
of it.

What would be the symptoms? Increased RF in the shack?

The solution of such a rare problem, if it occurs, is just to change the
length of the transmission line by a small amount. But you are accustomed
to doing this anyway to save yourself the cost of a tuner. ;o)

Changing the line length will shift an offending high impedance point away
from the choke location.

This very moment it has just come to my wandering mind that you are in the
habit of deliberately changing line length to bring the impedance down to a
value between 25 and 100 ohms at which a choke is certainly fully effective.

So your worries about obtaining an exraordinary high inductance choke really
don't exist.

( Just to tidy up, the length of wire on the choke should ideally be less
than a quarter-wavelength otherwise funny effects occur. And this places a
limit on its inductance. But that's just another factor which contributors
to the argument seem to be unaware of.)
----
Yours, Reg. G4FGQ