[TenTec] Built in SWR meter bannans

[Jim Brown]

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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