A few years ago, a gentleman I considered a friend (M. Walt a maxwell,
W2DU) and I had a talk about chokes, baluns, nomenclature, etc.
I am going to post his reply to me about it. I have snipped a sentence
from. The beginning and end, due to them being personal and not having
anything to do with it.
Argue about it, discuss it, doesn't matter to me. But, this is what the
person who invented the W2DU ferrite choke BALUN has to say about it:
"I understand your semantic problem concerning the balun, because it has
been
presented to me over and over again. In one instance, almost immediately after
my QST article on the W2DU balun was published in March 1983, Lew McCoy
took issue with me in a rather heated personal conversation, in whichhe
refused to agree that my balun was a balun. I tried to persuade him that if
it performed like a balun it is a balun, but he would not agree. So let me
try to explain the issue as I see it.
First, there are many different configurations of baluns that perform
the procedure
required to make a transition from an unbalanced line or circuit to a
balanced line or circuit, and vice versa. The coupling device that makes
that transition in such a way that no significant undesired or extraneous
voltages and currents appear on either the unbalanced or balanced line or
circuit is a balun by definition, no matter what the configuation.
The word 'significant' is operative here, because no balun configuration is
perfect in the real world.
The W2DU bead balun satisfies the definition, so yes, the W2DU
configuration is a balun.
To clarify the issue let's consider the circuit involving the W2DU
configuration.
The center conductor of an unbalanced feedline (the coax) is connected to
one terminal (A) of the antenna, while the outer conductor, or
shield, is connected to the opposite terminal (B) of the antenna. The
two antenna
terminals are considered to be balanced with respect to ground. However,
at the junction of the shield and antenna terminal B, due to skin
effect the current flowing on the inner surface of the shield sees two
paths which it can follow; one path into antenna terminal B and the other
path down the outside surface of the shield. How the current divides
between the two paths is determined by the impedance appearing in each
path. If the impedance appearing at the outside surface of the shield is
fairly low (and
with no balun that impedance will normally be low), currrent will flow on the
outer surface of the shield.
On the other hand, if an impedance placed at the antenna-terminal end of
the shield that is much greater than the impedance appearing at antenna
terminal
B, the current flowing into terminal B will be greater than that flowing onto
the outside surface of the shield. If that impedance placed at the
antenna-terminal
end of the shield is made sufficiently high with respect to
that appearing at terminal B, the current flowing onto the shield can
be reduced
to the value where it is insignificant.
In designing the W2DU balun I compromised cost, physical size, and the amount
of shield current that could be tolerated that would result in
insignificant effect on the radiation pattern of the antenna. I made
the decision
that if the shield current was at least 20 dB less than that flowing onto
terminal B, the eff effect of the shield current on the radiation pattern
would negligible. I considered what I believe is the worst case scenario
seen by the amateur community: An 80m dipole resonant at 3.750 MHz operated
at 3.50 MHz. From extensive measurements I've made on my 80m dipole, 125'
at 40 feet above ground, the impedance at resonance is 64.8 +j0 ohms, while
at 3.50 MHz the impedance is 53.1-j122.7 ohms. The impedance appearing at
each terminal, A or B, with respect to ground is half the impedance
appearing between the two terminals. Thus the impedance appearing
at terminal B is 26.66 - j61.35 ohms, for an impedance Z = 66.9 ohms. In the
graph of impedance vs frequency pertaining to the W2DU balun, the balun
impedance in series with the shield at 3.50 MHz is approximately 1050 ohms.
The ratio of 1050 to 66.9 is 15.7:1, which amounts to 23.9 dB. Thus the
shield current flowing in the worst case is 23.9 dB less than the current
flowing onto terminal B of the antenna. From the standpoint of effect on the
radiation pattern of the antenna, the W2DU balun is performing like a
practical balun, should be considered as a balun and therefore should be
called a balun.
I hope this clarifies the semantic discussion on whether the W2DU balun is
really a balun.
Walt, W2DU"
I'm not trying to open a can of worms, step on anyone's toes, or anything
else.
This is Walt words, about the subject being discussed. Since he's the
inventor, I'd say his wishes ought to be respected.
Anyway.
--Shane
KD6VXI
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|