I'm building some cookbook choke strings for generalized antenna
testing, from a box of 2.4" 31 cores. The idea is to build up a generic
broadband choke that is used for antenna measurements (doesn't have to
handle any significant power). The antennas are not necessarily matched
(e.g. if you want to measure a dipole cut for 20 meters over the entire
3-30 frequency range at some points, the Z of the antenna is pretty
reactive)
There's a fairly complex tradeoff between number of turns and number of
cores and I'll almost certainly need multiple chokes in series.
It's fairly straight forward to figure out what the series impedance of
the chokes is using K9YC's handy measured data.
I think my real question has to do with "how much impedance is enough"
and "where should that impedance be placed"
I'm not particularly concerned about RFI, for instance, but I am
concerned about coupling to the feedline and any asymmetry in the system
(and the antenna surroundings) perturbing the measurement.
So, from that basis, I would think that you want chokes periodically
along the feedline, so that no piece of the feedline is "significant" in
terms of coupling to the antenna under test.
And, for lower frequencies, the "significant length of a unchoked piece
of wire" in the near field is longer than for higher frequencies. A 5
meter long conductor near a dipole cut for 10 meters is a big deal
because it's a half wavelength, but probably insignificant for 80
meters, where's 1/16th wavelength.
So this would imply that "low frequency chokes" (e.g. 7 or 8 turns
through 5 cores) could be farther apart than "high frequency chokes"
(3-4 turns through 5 cores).
Or, as Jim recommends for a 40-10 meter - 4 turns on 5 cores and 3 turns
on 5 cores = this gets you >5k from 7 to 25MHz (fig 46 in the pdf)
Is there an advantage in stacking cores other than ease of
building/mechanical? I would think that 5 turns on 5 cores is about the
same as the series combination of five separate 5 turn on one core chokes.
I doubt I'll have more than 100 meters of feedline to worry about.
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