Posted by
Jim Brown-10 on
May 24, 2017; 5:18pm
URL: http://elecraft.85.s1.nabble.com/Choking-KPA500-and-testing-a-box-of-unknown-chokes-tp7631010p7631075.html
On Wed,5/24/2017 9:39 AM, Bill Leonard N0CU wrote:
> Two of the
> main trade offs are bandwidth and choking impedance. The highest choking BW
> is usually obtain with an air core choke (coax looped with or without some
> form like PVC). However, this approach is very high Q, which yields
> relatively low R and narrow BW.
Bill,
This simple-minded analysis is WRONG because it fails to realize the
fact that the choke is adding inductance to a part of the antenna (the
transmission line as a common mode conductor). It is, in fact, no
different from adding a loading coil to an antenna to resonate it! At
some frequencies, that transmission line will look capacitive, and at
those frequencies, the added inductance causes the line to resonate,
INCREASING common mode current rather than reducing it.
Think about it -- how is this air core inductor different from a loading
coil that we add to the base of a short vertical to resonate it? Using
your logic, that loading coil ought to block the current, but we all
know that it does not.
THIS fundamental principle is why air core chokes DON'T work in many
systems. That is, they don't choke common mode current. Now, the ANTENNA
works to the extent that it radiates (and may yield a fine SWR, which is
NOT a measure of how well the antenna works), so users think the choke
is fine.
The virtue of chokes wound on lossy ferrite cores is that by using a
suitable number of turns, the choke, with the stray C between turns and
the loss (resistance) coupled from the core forms a parallel resonant
circuit with a very low Q, and with a high value of resistance at
resonance. And the low Q (typically on the order of 0.5) makes that
resonance quite broad so that it can cover multiple ham bands. In
addition, #31 material have a second dimensional resonance below 3 MHz
that broadens the choking Z curve much as did stagger-tuned" IF stages.
That large value resistance reduces common mode current -- it can never
"resonate" with the antenna (transmission line) into which it is inserted.
This, and other concepts associated with common mode chokes are
articulated in a tutorial that is on my website, and was added to the
ARRL Handbook in 2010.
73, Jim K9YC
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