Cecil Moore wrote:
wrote:
Ian White (GM3SEK), Cecil Moore (W5DXP), and probably others have
discused HF chokes made from flat vs. bunched coils of coaxial cable,
but I must have missed some posting(s), for I haven't recognized
anything that looks like a CONCLUSION.


Sorry, my Internet access is down at the moment, so there's no knowing
when this will get out.


Ian and I agree

.... about quite a lot actually :-)


that the ARRL was overly optimistic when
they asserted that a coiled coax choke can cover all three
octaves of HF.

Agreed. That claim is made in the general ARRL Handbook. The ARRL
Antenna Book gives more detail, but notably does not include that claim.

The requirements for a choke feeding an
all-HF-band, ladder-line fed dipole are much higher than
the requirements for a choke feeding a resonant fan dipole.

* Is "bunched" or "flat" better?

"Bunched" and "flat" both mean the same to me - the cable is wound into
a flat coil like a length of rope. The actual choice is between that
shape and a cylindrical solenoid shape.


Bunching disadvantage: Lowers the maximum choking impedance
and decreases the effective bandwidth. (Bunching increases
the stray capacitance.)

Bunching advantage?: Lowers the parallel self-resonant frequency
so not as many turns are required for narrow-band operation.

The bunched/flat shape has more self-capacitance between turns, so for
the same total length of cable it has a lower self-resonant frequency.
The bandwidth of either shape is wide enough to cover any amateur band
with a very high impedance - way higher than you could get with a
ferrite choke - but you do need to get the resonant frequency right.



* Are they basically single-band or wide-band or somewhere in-between?

Depends upon the minimum required choking impedance.

A resonant coiled-cable choke is unbeatable on a single band, but you
may not actually *need* a choking impedance of several kohms.

If you only need an impedance of a few hundred ohms (though more is
always OK of course) then I'd agree with Cecil:
..
A 2:1 to
3:1 frequency range might be a good rule of thumb, e.g. one
choke can cover a tribander's 2:1 frequency range.

That seems realistic for either the bunched/flat or the solenoid shape.


* Are there "rules of thumb" to make them for hams with
* No test equipment?
* A grid-dip meter?
* A noise bridge?
* An antenna analyzer?

How about: Using a 2L pop bottle as the coil form, don't use
more turns than the number of meters in a wavelength, e.g. no
more than 10 turns on 10 meters? That would probably work for
20m-6m as a rule of thumb.

The MFJ-259B will measure impedance up to 650 ohms. One could
define a "bandwidth" based on that 650 ohm value and knowledge
of a typical impedance waveform between those two points. For
instance, my 8 turn, 5.35" diameter choke falls below 650 ohms
at 4.63 MHz and 28.1 MHz. It might be useful over a 3:1 frequency
range from 40m-15m.

You could certainly try Cecil's suggestions, though the resonant
frequency can be quite critical if you want to spread the performance
across more than one band.

A GDO should be OK for measuring the resonant frequency, with the two
ends of the choke shorted together.

However, maybe we're all trying to measure the wrong thing. The reason
why we're using these chokes is to reduce the unwanted common-mode
current, so why not cut to the chase, and measure the current itself?

Whoever you are, if you don't already have a snap-on RF current meter,
then make it your next homebrew project or ham radio purchase. It is THE
most useful single tool for RFI investigations, so it's always good to
have one in the shack.

In this particular application, you could measure the common-mode
current on the TX side of the choke - the answer is there "in a snap".
Then you can either declare the problem solved, or follow the meter
readings as you adjust the choke to reduce the current even further.

There are details of HB meters on my website, or the MFJ-854 is a good
commercial alternative (but definitely not the MFJ-805).



--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek