[TenTec] Built in SWR meter bannans

Tue Apr 23 17:54:09 EDT 2013

THHN is that stuff they have at Home Depot or Lowe's for house wiring. It is in spools of single wire, many colors and usually 12 GA, though many guages are available. Simply put, it's single strand house wire 

73 

Dave 

WA3F 

----- Original Message -----

From: "Rick - DJ0IP / NJ0IP" <Rick at DJ0IP.de> 
To: "Discussion of Ten-Tec Equipment" <tentec at contesting.com> 
Sent: Tuesday, April 23, 2013 4:47:36 PM 
Subject: Re: [TenTec] Built in SWR meter bannans 

Jim, 

Thanks for the details.  I have noted your suggestion for future use. 
I'm not sure what THHN wire is, but I will google it.  I'm sure we have it 
here, but under a different name. 
You said ordinary house wire, but I don't know what ordinary (American) 
house wire is.  Is that like Zip Cord? 

I had so much trouble burning up baluns back in the 80s that I have switched 
to using only Teflon insulated wire for my chokes and baluns. What I 
typically use is a Teflon-coated twin-wire sourced locally with 0,75mm 
diameter.  This would be about AWG-21. You don't even have to tape it 
together.  For more power, then I use thicker Teflon insulated wire and tape 
it together, which is work I totally detest.  That's the hardest part of 
making a choke or balun.  And the special tape I bought must be gold-plated; 
I paid 25 EURO ($30) for a tiny little roll. (hi) 

The balun I referred to earlier is quite like the W2DU balun except I 
slipped the beads over two thick insulated wires instead of coax. Then I fed 
openwire into one end and the other end was connected to a matchbox.  Why 
did I build it that way?  Because I read about it somewhere and I like to 
try things I read about.  That was about 20 years ago.  I wouldn't do it 
that way today, I would use toroids.   

The only reason I mentioned it is, somebody asked how you do a CMC choke for 
open wire.  My comment was "just as easy as for coax".  Unfortunately I have 
not found a source for -31 toroids here, so I am still using -43. After 
reading you long paper on chokes yesterday, I'm going to intensify my search 
for -31.  I'm sure they're available here somewhere. 

The one place I clearly prefer the W2DU approach over using a toroid is when 
the choke is mounted high on a lightweight telescoping fiberglass pole.  You 
have a much greater chance breaking the pole if you use a toroid.  The W2DU 
distributes the weight a little better and has far less wind load than a 
toroid, especially if you place the toroid in an enclosure. 

73 
Rick, DJ0IP 

-----Original Message----- 
From: TenTec [mailto:tentec-bounces at contesting.com] On Behalf Of Jim Brown 
Sent: Tuesday, April 23, 2013 6:55 PM 
To: tentec at contesting.com 
Subject: Re: [TenTec] Built in SWR meter bannans 

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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