[TenTec] Built in SWR meter bannans

[Rick - DJ0IP / NJ0IP]

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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