On 7/21/20 1:32 PM, Jim Brown wrote:
On 7/21/2020 10:38 AM, Steve Maki wrote:
From what I've heard, the properties of ferrites can vary wildly,
especially from run to run, so unless you measure the one you make,
you don't know.
OTOH, I'm not sure that it's possible to make a choke using
recommended (by K9YC) method & parts and end up with a *useless* choke.
My recent (2018) Cookbook is the result of measuring several hundred #31
cores, selecting four at opposing limits of their characteristics,
winding chokes on each, and tabulating Z data in a spreadsheet for each
ham band. Recommendations for each band and each cable type are on the
basis of the choke meeting spec on the WORST of those cores as well as
the best.
Of course, the "requirements" for the choke are sort of fuzzy anyway -
you want about X ohms, where X is "big" compared to the impedance at the
point being choked (which may not be 50 ohms..) because the idea is to
keep currents at "much lower" than the currents in the antenna so that
the pattern isn't perturbed.
Much lower is kind of a fuzzy thing too.. Is it 20 dB (1/10th the
current?) 40 dB (1/100th the current?)
The current where? at the feedpoint? at the tips?
So we handwave a bit and say "the current at the feedpoint" vs "the
current on the coax" should be 100:1, which is 40 dB down (a lot..
something at -40dB is truly "negligible" in ham antenna world).
And that leads to a "choking impedance should be around 5k" which is 100x.
But if it's 4k, it's still good. If it's 3k, it's still good.
I think the value of Jim's cookbook is not so much the choking
impedance, per-se, but that he's
a) bounded the core variability (designed for worst observed case
performance of the core)
b) also taken into account side effects like resonance and leakage
capacitance
The latter is MUCH less affected by core properties (I think, Jim can
correct me if I'm wrong).
And one can *always" construct pathological cases..
You have a 2 element beam, there's a choke at the feedpoint, and you
route the coax diagonally from there to the middle of one of the arms of
the driven element, with a dangling loop, so it's 1/4 wavelength long,
and then you loop it to the end of the element, with another 1/4
wavelength. What you've done is essentially drape another resonant
dipole over your antenna.
I suspect that the choke at the feedpoint is almost immaterial in this
scenario.
(lest one think this is bizarre and unrealistic, I've seen similar
things in lab setups at UHF on a bench, where they had test leads in the
wrong place forming parasitic elements, and were wondering why the S11
of their antenna made no sense. Clearly, they didn't draw the boundary
around the antenna properly)
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