On 9/24/2017 1:06 AM, Ian White wrote:
The dielectric of the coax which affects the choke resonance is only
that of the outer jacket.
Right. Thanks for clarifying what I didn't say well enough. :)
Along with the o.d. of the coax and the precise layout of the turns, the
dielectric of the outer jacket will affect the capacitance between
turns, and three of those factors will affect the choke resonance. That
is a very good reason to:
* Use "cookbook" designs from someone like K9YC, G3TXQ or myself, who
has actually measured those resonant frequencies (which requires some
special techniques in addition to owning a VNWA)
Actually, my VNA (the DG8SAQ design built and sold by SDR Kits in the
UK) is NOT the most accurate system for measuring ferrite chokes, and I
still don't know why. My test setup is documented several places in my
application notes and tutorials.
.and.
* Choose a broadband design which will be more tolerant of small
differences in winding layout and the unavoidable stray reactances when
you actually install the choke
.and.
* Copy that cookbook design EXACTLY - including the coax and the layout
of the turns.
I agree with K5GO about normally preferring a cable with a stranded
copper center conductor in preference to silver-plated solid steel, if
available.
I agree, but I'm also pretty happy with solid copper center. IMO, not
enough attention is paid to the shield construction.
But if this is different from the original cable in the
cookbook design, it needs to be a very close alternative with the same
OD and outer jacket, and you should still aim for exactly the same
layout of turns.
The key point is to apply fundamental principles to these chokes,
realizing that control of resonance is key to them working the way we
want them to.
73, Jim K9YC
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