[Elecraft] Balun for KAT2

[Bill Coleman]

On 1/18/02 10:41 AM, George, W5YR at w5yr@att.net wrote:

>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. 

Current baluns was all I was talking about.

>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.

Right. It's essentially the same as the W1JR balun, except that W1JR 
winds the coax on a toroidial core. The only difference is you need fewer 
turns to get sufficient inductance because of the permeability of the 
core.

>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. 

However, you have to account for the fact that the choking impedance (as 
it true in all current balun designs), is shunted across half of the 
antenna element to ground (via the coax sheild). This circuit faces 
potentially half of the antenna voltage (provided the antenna is 
balanced), and so some current will flow through the choke. 

With sufficient inductance, this current ought to be small, but if the 
balun was designed for resistive loads and is suddenly faced with higher 
voltages due to reactive loads, failures can occur.

The coiled-coax balun won't fail easily, which was my point.

>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. 

There are W2DU-type designs that fail easily due to core heating. You can 
do the same with the W1JR-type design, too. The old Hy-gain BN86 is a 
W2DU-type design, no? There's a large track record of failures of this 
type balun at full rated amatuer power. 

The basic problem is one of core heating to the loss tangential point, 
which then proceeds to the Curie temperature.

Since the coiled-coax balun has no ferrite core, such failure modes are 
impossible.

>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. 

Isn't roy forgetting that the balun is also shunted across half the 
antenna element to ground?

>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.

If there's no flux in the choke, according to Lewallen, where's the loss 
in the beads coming from?

The main problem with main of these designs is that the beads are often 
potted and sealed. Since Ferrite doesn't disappate heat well to start 
with, having it wrapped in a plastic blanket makes the problem worse. 
Once they start to heat, the heat increases the losses, which can 
snowball to the Curie point.

>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.

Everything you say is true about the operating of the balun. Except that 
it also acts as a coil shunted across one antenna element to ground.

If we could draw a circuit and separate the inner coax braid carrying the 
differential currents from the outer braid, we'd end up with three coils. 
Two are coupled differentially, and have no net flux. The third, however, 
is from one antenna element to the ground of the coax sheild.

When properly designed, there isn't much current involved here. There are 
unfortunate cases where use exceeds the 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.

Thanks! Perhaps we can both learn something here -- I'm always eager to 
learn something new.


Bill Coleman, AA4LR, PP-ASEL        Mail: aa4lr@arrl.net
Quote: "Not within a thousand years will man ever fly!"
            -- Wilbur Wright, 1901