I've been doing some research on my balun question from earlier.
Consider the following types of 1:1 baluns:
A) 10 trifilar turns on a toroidial core. Windings abc connected as
follows: a(start) to b(end) and feedline sheild; c(start) to feedline
center; b(start) to c(end) and antenna side 1; a(end) to antenna side 2.
B) 10 bifilar turns on a toroidial core. Windings ab connected as
follows: a(start) to feedline sheild, b(start) to feedline center; a(end)
to antenna side 1, b(end) to antenna side 2. (Guanella balun)
C) 50 ferrite beads on a coaxial cable. (W2DU balun)
D) 10 turns of coaxial cable on a toroidial core.
--
Observations
1) Nomenclature - The 1991 Handbook identifies type A as a voltage balun,
and type B and C as a current balun. (Type D is as well, D is just like
C, electrically)
2) The 2000 Handbook makes no mention of type A baluns. However, type B
and C are mentioned.
3) W6SAI Handbook mentions Type D in passing -- discussing three turns of
RG-8 on a extremely large core. No mention of it in ARRL publications I
had on hand.
4) The Wireman has kits for constructing Type A and Type C baluns. The
same kits for Type A could be used to construct Type B baluns. No one I'm
aware of has kits for constructing Type D baluns, although they are
apparently sold commercially. (Force12 sells one, the R7, R7000 (and I
assume the R5 and R8) have this type of balun as part of their matching
network)
--
Discussion
Since the 2000 Handbook doesn't even MENTION voltage baluns, I must
assume they have fallen out of favor with the Handbook editors.
Type B and C are both labelled current baluns, but it seems to me that
Type A and B have more in common. Type A is sort of an auto transformer.
Think of it as a single winding with taps at 1/3 and 2/3 of the winding.
You apply the ground to the 2/3 tap, and excite the start of the winding.
You then connect the antenna to the 1/3 tap and the end. Both the input
and output cover 2/3 of the windings -- hence the 1:1 transformation
ratio. The output is balanced because there's a ground connection in the
middle of the 1/3 tap and end tap.
Type B works differently. Since the windings are separate and in series
with the connections, only differential currents are allowed to flow.
Common-mode currents would be opposed.
However, in both Type A and B, the core is excited by the current in the
feed. Type A fully excites the core with all the energy transmitted to
the antenna. With Type B, the core excitation may only be partial,
depending on the feedpoint impendance and the inductor reactance. In
either case, you'd need to size the core with respect to the feedline
currents.
Type C and D baluns, however, can expect little or no feedpoint current
to excite the core. In either case, common-mode currents flowing on the
sheild are inhibited by the inductive reactance of the core. Differential
currents flowing on the INSIDE of the sheild should not excite the core
at all. This would mean that the core only needs to be sized according to
the expected magnitude of the common-mode currents.
The fundamental operation of Type C and D baluns involves the choking
reactance of the inductor. For Type C, this reactance is proportional to
the number and type of beads used. For lower frequencies, more beads need
to be added.
Type D baluns, however, can increase their reactance by increasing the
number of turns. Since the reactance increases with the square of the
number of turns, a few turns can make a lot of difference. A wide range
of choking reactances could be accomodated with a single core.
Type B and D baluns have upper frequency limits, due to capacitive
coupling of the turns. At some frequency, the inductor becomes
series-resonant. Long before this point, the capacitive coupling tends to
decrease the inductive reactance. More windings lowers the
series-resonant frequency. So, having too many windings can be worse than
having much fewer, depending on the operating frequency.
--
Questions
0) Any fault in the discussion above?
1) Are voltage baluns so bad that the ARRL handbook editors wrote them
out?
2) Given the Type D's ability to balance over wide range of frequencies,
why does no one appear to have a kit for this type of balun? (Or should I
just e-mail Press and see if he'll create one?)
3) The typical rule of thumb for a Type C or D balun is the inductor
should have at least 10x the reactance of the antenna impedance. (eg 500
ohms reactance for a 50 ohm feedpoint impedance) While it is easy enough
to compute the reactance of a wire through a bead, or a few turns on a
core, how does one compute the net capacitance of the windings in a Type
D balun? How can we predict the series-resonant frequency?
4) For Type C or D, how does one select the material and size of the
core? Naturally the material depends on the desired reactance, but also
on the amount of current exciting the core. Core size is also predicated
on practical considerations (eg how tightly can we wind coax), but also
on the magnitude of the current. How do we predict or estimate that
current?
Bill Coleman, AA4LR, PP-ASEL Mail: aa4lr@arrl.net
Quote: "Not within a thousand years will man ever fly!"
-- Wilbur Wright, 1901
List Sponsored by AN Wireless: AN Wireless handles Rohn tower systems,
Trylon Titan towers, coax, hardline and more. Also check out our self
supporting towers up to 96 feet for under $1500!! http://www.anwireless.com
-----
FAQ on WWW: http://www.contesting.com/FAQ/towertalk
Submissions: towertalk@contesting.com
Administrative requests: towertalk-REQUEST@contesting.com
Problems: owner-towertalk@contesting.com
|