AW: [Elecraft] PlugIn Balun for K2/Kat1 ATU

George, W5YR [email protected]
Wed Aug 21 17:04:04 2002


No, not at all, Vic. But, I must confess that I do not follow

"However, if the system impedance is very high, then the additional
impedance to the common-mode current provided by the balun will have less
effect on it -- i.e., poorer performance."

Maybe you could restate that a different way for me in another posting.

I think that maybe the situation is more complex that we would like. And I
am not at all sure what you mean by "system impedance." Are you speaking of
the differential-mode impedance presented by the feedline at the point
where the balun is connected? Or of the antenna/load impedance? I am going
to assume that you refer to the differential-mode load impedance seen by
the balun, although it is unclear to me where this enters into the
question.

The issue is that a high balun load impedance does *not necessarily* equate
to a large common-mode current. The level of CM current is a function of
the CM impedance of the outer braid of the coax from its connection to the
antenna to "ground." Depending upon the surroundings and characteristics of
the antenna and feedline, the CM current can be high, low, or near zero. 

CM current magnitude is determined by three factors:

1. the placement of the outer braid in the field of the antenna; if it lies
within the zero-rf-potential plane, then no current is induced upon the
outer braid.

2. the impedance to ground of the outer-braid as seen at the antenna
connection

3. the division of inner-braid current at the connection of the braid to
one-half of the antenna.

How that current divides depends entirely upon the impedance presented by
the antenna half and the impedance presented by the outer braid back to
"ground."  Ohm's Law at work . . .

So, based upon the above, I can see no reason why the feedline input
differential-mode impedance would have any effect on the magnitude of CM
current at that point or of the effectiveness of the balun in blocking CM
current beyond that point. 

One can argue that a high load impedance at the antenna might actually
result in less current being available to produce CM current in which case
the impedance presented by the balun could be adequate to completely
eliminate CM current beyond that point on the line. This is a matter of
feedline length, Zo and other factors, so it is almost impossible to make
generalizations. 

The key point in my mind is that the 1:1 current balun does its 1:1 job
regardless of the load impedance at the antenna or the line input. There is
little doubt about what the impedance will be at the input knowing the load
impedance, provided that the balun is being operated within its design
limits which are quite broad compared to those of a 4:1 balun.  

Its ability to reduce or limit CM current is determined by the impedance
(largely inductive-reactive but with some real loss) which it presents to
the CM path: the outer braid of coax. This property is independent of the
load or "system" impedance or of the magnitude of the CM current. It is a
function of frequency, of course. Otherwise, it is determined solely by the
construction and design of the balun.
 
Whether or not it is capable of doing the job is another question
altogether. A balun may be required with higher impedance on a given band
or with more intrinsic loss. None is perfect for all applications. 

On the other hand, a 4:1 balun, especially if it is a voltage balun, has a
transformation ratio that is determined not by a turns ratio, as in a
conventional transformer, but rather by the transmission line action within
the balun. In order for the balun to operate at a 4:1 ratio, it must work
into a fairly narrow range of frequencies and load impedances, which is far
from the picture that the usual multi-band antenna presents at the tuner
end of the ladderline over several bands. Thus, given an arbitrary load, it
is not always possible to predict that a 4:1 balun will in fact present an
input impedance that is 1/4th of its load impedance, or that the complex
impedance will remain the same, since the presence of reactance in the load
can affect the apparent transformation ratio.

A further disadvantage is that a voltage balun *is* very much affected by
the differential-mode load impedance which is a major reason that current
baluns have largely replaced voltage baluns in most amateur line isolation
applications. Voltage baluns have a magnetizing winding that carries
current proportional to the signal power level and the load impedance. This
is the origin of the heating and potential saturation problems presented
with such baluns when improper loading, frequency or power level is
involved.

In summary, the 1:1 balun will do what it does regardless of the magnitude
of the CM current or of the impedance of the load or line input. It does
not "see" the differential-load impedance so it is unaffected by it. It is
a linear device, so its characteristics are not current-dependent. It is,
however, frequency dependent, unless made from a coil of coax, due to the
properties of the ferrite devices normally used in its construction.

A 4:1 balun *may* do what it is designed to do, provided that its design
restrictions are met. In the real world of temporary QRP antennas tossed up
in the field, it is not always possible to do that. Thus, the less-troubled
path would be to use a 1:1 balun for isolation purposes and let the tuner
take care of impedance transformation and reactance cancellation.

I highly recommend the article by Roy Lewallen in the first ARRL Antenna
Compendium and the paper by Walt Maxwell in the March 1983 issue of QST.
They are regarded as *the* writings in the amateur literature. Also, the
works of Jerry Sevick are key to understanding both the theory and the
construction and applications of baluns.

73/72/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 I-LINK 11735
Icom IC-756PRO #02121  Kachina 505 DSP  #91900556  Icom IC-765 #02437


Vic Rosenthal wrote:
> 
> "George, W5YR" wrote:
> >
> > This is an advantage of the 1:1 current balun: its performance is
> > independent of the load impedance.
> 
> Hi George,
> 
> I find this hard to believe, for the following reason: imbalance is equivalent
> to common-mode currents on the line.  The balun works by presenting a high
> impedance to these common-mode currents and not interfering with the
> differential-mode (balanced) current.  However, if the system impedance is very
> high, then the additional impedance to the common-mode current provided by the
> balun will have less effect on it -- i.e., poorer performance.
> 
> Am I missing something?
> 
> Vic K2VCO