[TenTec] Built in SWR meter bannans

[Al Gulseth]

Jim,

TNX for the info. Does it matter whether the THHN is solid or stranded? What 
core size do you recommend (I'm guessing 2" or so)? Also, (purely from 
a "dumb questions dept." perspective) would a ferrite rod of the correct mix 
be suitable, or does it definitely need to be a toroid? 

73, Al

On Tue April 23 2013 11:54:58 am Jim Brown wrote:
> 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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