[TowerTalk] chokes for antenna testing

[jimlux]

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.