Cecil Moore wrote:
Ian White GM3SEK wrote:
Across the whole 1-30MHz band, the bunched choke behaves as an almost
perfect L-C circuit, free from any unwanted resonances.
One additional point. If the above were true, as the
frequency is increased, the phase angle of the coiled
choke impedance would drop from ~90 degrees to zero at
the self-resonant frequency, and then rise back to ~-90
degrees and stay there. But that's not what happens.
In every single case, the phase angle rises toward -90
degrees *and then decreases* as the 1/2WL self-resonance
point is approached. That is a clear indication of transmission
line effects. A lumped circuit simply doesn't act that way.
The spreadsheet at
http://www.ifwtech.co.uk/g3sek/misc/chokes.xls
now includes graphs of the phase information for all the chokes.
[ Thank you, Owen - the phase axis label is now fixed .
For anyone interested who doesn't have Excel, Microsoft's free Excel
file viewer is at
http://tinyurl.com/cup85 ]
Coming all the way back to the original question, the data confirms that
even though it may look like something held together with duct tape, a
coiled coax choke can be an excellent single-band solution. At its
parallel self-resonant frequency, it will have a much higher common-mode
impedance than a generic string of ferrite beads. It can probably
outperform or at least equal a ferrite choke over two or possibly three
adjacent HF bands; but being a resonant device, it cannot deliver
extreme broadband performance.
In the chokes we're looking at, the low-impedance series resonances of
which Cecil complains do not occur below 30MHz. Those resonances exist,
but not on the HF frequencies where the chokes would actually be used.
Within the practical working frequency range of all of these coiled-coax
chokes, the performance can be accurately described as that of a simple
parallel tuned LC circuit, which displays no transmission-line behaviour
whatever. Cecil complains that
One might say it is misbehaving and is a very poor design.
That sounds to me like the complaint of someone who has a pet theory to
hammer, and is disappointed when he can't find a nail.
I think that's it, really. The graphs themselves say the rest.
--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek