[Elecraft] Balun for KAT2

[George, W5YR]

Bill Coleman wrote:
 
> Third, none of the discussed baluns (with the exception of the coaxial
> coil) performs well in the presence of reactive loads. Such loads can
> cause core saturation, heating and additional loss, unless accounted for
> in the design of the balun.

Bill, I enjoyed reading your posting re baluns pro and con, but I have a
problem with the statement above, *provided* that you are including current
or choke baluns in the "discussed baluns" category. 

As I understand current or choke baluns used with coax - I use three of
them to transition from ladder line to coax - they operate solely on the
presence of common-mode current on the outer braid of coax. It is their
inductive and resistive-loss effects that account for the reduction or
elimination of this current. Thus, their effectiveness depends only upon
the common-mode impedance of the outer-braid circuit. Differential-mode
operation within the line is concealed by skin effect and in not involved
in the common-mode operation of the line.

This being the case, then, it is the differential-mode operation of the
coax which is where the presence of a reactive load would be involved
and/or the actual terminating impedance of the load would have an effect.
None of the increased differential-mode current or voltage that could
result from reactive loads would impact the common-mode impedance
environment and thus stress the choke balun. 

In fact, one of the several major advantages of the W2DU balun, as well as
the W1JR, for example, is their freedom from heating and saturation effects
due to reactive loads that present impedances outside the design range of
the balun. 

As Roy Lewallen has posted many times, and wrote in his classic article on
baluns, the only current that the bead balun, for example, "sees" is the
common-mode current, and except under extraordinary circumstances, that
current is usually of a magnitude far below that which introduces core loss
and heating problems with ferrites. 

It is true that bead baluns can be damaged at high power levels, especially
with the balun placed at the antenna feedpoint where the common-mode
current is a maximum. Under these QRO conditions, larger beads are
frequently used nearest the antenna connection to deal with the high
current level and attendent loss in the beads.

So, please point out what I am missing or misinterpreting here. To
summarize, it is my understanding that choke or current baluns respond only
to common-mode current which is independent of the differential-mode
impedance presented by the antenna load. Thus, their design and operation
are unaffected by load impedance, even when highly reactive. Only the
common-mode circuit impedance is involved in balun operation and design.

I read all your postings and always find them informative. Rarely do I find
a point, such as this, on which we apparently hold different views.

72/73/oo, George W5YR - the Yellow Rose of Texas         
Fairview, TX 30 mi NE of Dallas in Collin county EM13qe   
Amateur Radio W5YR, in the 56th year and it just keeps getting better!
QRP-L 1373 NETXQRP 6 SOC 262 COG 8 FPQRP 404 TEN-X 11771
Icom IC-756PRO #02121  Kachina #91900556  IC-765 #02437

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