On 4/22/2013 12:15 AM, Rick - DJ0IP / NJ0IP wrote:
With openwire you can build a CMC choke just as easily as with coax.
It's not clear to me what sort of choke you are imagining, but it is
trivially easy to build a VERY effective common mode choke by first
taping together a pair of insulated conductors to form a parallel wire
transmission line, then winding enough turns of that pair around a #31
or #43 toroid to place the high resistive impedance that results from
resonance where it is needed. 16 turns (x2) of such a line made from
#12 THHN (ordinary house wire) on a #31 core is a VERY effective choke
from 1 MHz to about 15 MHz. 12 turns on the same core makes a fine choke
for 3-30 MHz.
Such a choke is a short length of transmission line, with Zo on the
order of 90 ohms, Vf on the order of 0.66, and VERY low loss below 30
MHz (the loss is all copper, and #12 is bigger than most coax. THHN
insulation starts introducing dielectric loss above that range. I've
measured all of these parameters with real chokes. It's difficult to get
much precision, but I trust the data to about 25%, which is certainly
good enough for our purposes. The short length of line (2.5 ft - 3 ft)
introduces some small mismatch, and the result can easily be modeled in
software like Sim Smith, which runs in Java and is free. To do the
model, you will, of course, need to measure and import the antenna Z or
provide comparable data from an NEC model. I've done both.
For all practical purposes, the mismatch doesn't matter -- the length is
too small as a fraction of a wavelength, and it's at the load end.
73, Jim K9YC
.
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