> >I originally challenged this statement from Jim: "But SWR has NOTHING to
> with dissipation in a common mode choke." I still maintain that statement
> I would go with something like this:
> In SOME cases SWR may be an indicator (but not direct cause) of changes in
> the common mode current that a choke would see. In other cases it has no
> relationship to it.
I agree with that assessment. Also SWR can, and does, have an effect on CM
choke dissipation. A hypothetical situation was recently described by Dean
Straw on the Elecraft mail list [see archive from 12/12/2011].
The issue discussed was whether or not Alpha (RF Concepts) should have
placed a CM choke on the output of their new tuner, or at the input as
designed. Many of us were condemning their use of an input CM choke since
several references in the literature from W9CF, W8JI, and W7EL all presented
theory and evidence that a CM choke at an unbalanced tuner's input (e.g.,
the common C-L-C "T" design) does nothing to enhance line balance and that
balance may be better when the CM choke is placed at the tuner's output. I
was a believer until Dean presented a real possibility.
Consider a 1500-watt system on 10m with a multiband antenna, fed by
open-wire line to a coaxial-wound, CM choke where its termination could be
3000+j0 at the line input, depending on line and radiator length. Adding
three feet of RG-213 (an amount wound on a CM choke) may now present a 3 -
j0 impedance to the choke. In either extreme Z case, total power dissipated
by the CM choke alone is a whopping 120-140 watts in a 1500-watt system, or
about 45 watts per foot of RG-213 coax. The complex Z creates a high SWR
condition at the output of the CM choke. The extremely high SWR combined
with line loss results in high CM choke dissipation.
Sure, these Z examples are extreme but can typically occur when using
multiband antenna systems of varying radiator length and transmission line
length. Keep in mind that what W9CF, W8JI, and W7EL are advocating is still
likely true as a general statement -- that system *balance* may be better
when the CM choke is place at the tuner output, but at the expense of
additional power dissipation under extreme loads at the CM choke. Probably
this statement is also site and configuration specific.
For multiband operating, I'm back in the camp of placing the CM choke at the
input to an *unbalanced* tuner for the reason that Dean Straw cites. But,
I don't use unbalanced tuners. My current tuners are either link-coupled or
a balanced, reversible "L," (i.e., AG6K type) with CM choke at the input
which is the undisputed place for a symmetrical, balanced tuner.
TowerTalk mailing list