On Fri, 27 Oct 2006 13:31:57 +0100, Ian White GM3SEK
wrote:

In that discussion, there was a
largely unspoken agreement that, to merit being called "effective", a
choke should have an impedance of at least 10 times the cable Zo, ie at
least 500 ohms.

Hi Ian,

I haven't read to the depth you have, but if your synopsis is
accurate, they are wrong. The generality should be 10 times the
source Z.

If 500 ohms is all you need, a coiled-cable choke of either the
"bunched" or the "solenoid" type certainly can cover at least two
amateur bands an octave apart in frequency... but a wide range of
ferrite chokes can do the same, and these have the advantage of being
much more broadband so they need no tuning.

And if the antennas exhibit a near-match, 500 Ohms is sufficient as
those antennas will be presenting a low (~50 Ohm) source Z.

a 500-ohm choke may completely kill the common-mode problems at one
station (or on one band) and have almost no effect in a different
situation.

This would the consequence of a very high source Z, such as a half
wave end fed design.

On balance, it might be better to say that 500 ohms should be considered
the *minimum* value of impedance that can offer some promise of being
useful in a wide variety of different situations.

Minimum, optimal, best, standard.... All such terms demand a criteria
that is trying to be met. No choke is 100% effective. The two most
compelling of those criteria are
1. RF shock hazard;
2. Antenna null filling.

The shock hazard is often ignored if it is not perceived (can't feel
it, and yet instrumentation might be pegged).

Null filling, or poorer F/B, or F/S performance may only become a
nuisance or a real concern depending upon the individual.

My own standard is 5 times the source Z, or at least 3 times the
source Z. 5 times is suitable for reasonable measurement accuracy,
but filling some other criteria throws us back into the subjective
quagmire.

73's
Richard Clark, KB7QHC