[TowerTalk] common mode chokes, baluns and multiband doublets
jim at audiosystemsgroup.com
Tue Dec 12 00:24:32 EST 2017
YES, YES, YES! Let's use the correct word to describe things. FAR less
Several points in this discussion.
Kevin Stover, Wes Stewart, and David Gilbert are entirely correct in
their posts. 2-wire line by itself does NOT make a system balanced.
Current on the two conductors will be equal and opposite ONLY if the
rest of the system is balanced. But many (most?) wire antennas that we
hams CAN rig are unbalanced by their surroundings -- ground slope,
unequal heights of the two ends, surrounding conductors (in buildings,
trees, other antennas, towers that support them). They are also BADLY
unbalanced if the two lengths are unequal (like OCF "dipoles").
Dean Straw, N6BV, retired editor of the ARRL Handbook and Antenna Book,
wrote an excellent applications note that ran in QST about 2 years ago
(April '14 or '15?) in which he observed that badly matched antennas
place very high voltage and current points along the antenna, and either
can fry even the most robust of common mode chokes.
The first solid work I've seen published on 2-wire common mode chokes
was by W2FMI. He reported that the Zo of closely spaced #12 enameled
wire is about 50 ohms, and is in the range of 90 - 100 ohms for #12
THHN. When I first started winding those chokes I confirmed his
observations, and I've recently measured some #12 teflon pairs with Zo
in the range of 105 - 110 ohms. These values will, of course, depend on
details of the dielectric material and it's thickness. We're using #12
THHN chokes on #31 cores to choke high wire dipoles (feedpoint Z in the
range of 75-88 ohms, depending on height and the soil under them) and
#12 enameled copper to feed 50 ohm Yagis. The design of the Force 12
C3SS that we use for portable operation does NOT like the THHN chokes,
but works very well with the enameled wire chokes.
I've done a lot of work studying this, and published it both on my
website and in the ARRL Handbook. k9yc.com/publish.htm On the basis of
that work, I do not believe that it is practical to effectively choke
any badly matched antenna at any power level above 100W, and even at
100W it's easy to fry a choke.
While Wes is a superb engineer, I strongly disagree with him about use
of the word "balun" -- I can think of nearly a dozen very different
parts or products that are called "balun." Indeed, the great confusion
about what they are and how they work (or don't work) is CAUSED by using
the same word to describe very different things.
73, Jim K9YC
On 12/11/2017 8:29 PM, EZ Rhino wrote:
> Why don’t we permanently retire the term “balun” and use either “choke” or "impedance transformer” in its place?
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