On 8/8/01 9:58 AM, Bill Coleman at aa4lr@arrl.net wrote:
>D) 10 turns of coaxial cable on a toroidial core.
These are known as W1JR type baluns. It's not surprising to find them in
the R5 and R7, since Joe designed those antennas.
>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.
W8JI pointed out to me in a private e-mail that this isn't true.
In the BEST case (all impedances matched and no reactive impedance or
common coax currents), these baluns will see HALF of the antenna terminal
voltage. This is because the choking impedance is shunted across one
antenna terminal and the coax sheild ground.
W8JI also suggested a test that puts twice this best case voltage across
the current balun: Hook up the balun backwards, as a 180-degree phase
shift network. Connect a dummy load to the input, with the sheild
connected to a common ground.. Connect the output terminal that
originates with the coax center conductor to a common ground. Connect the
other output terminal to a signal source fed against the common ground.
A good balun should deliver all the power into the dummy load with very
little loss.
In the worst case of complex feedpoint impedances, mismatched impedances,
and complex currents flowing on the coax sheild -- the actual voltage
across the balun could be MANY TIMES the best case.
>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)
According to a QEX article by Sabin, W0IYH, the 500 ohms is right but my
formula is wrong. Sabin suggests at least 4 times the sum of the input
impedance, output impedance and the maximum offset impedance of the
output to ground (typically half the output impedance).
For a 1:1 balun for 50 ohms, that's 4 * ( 50 + 50 + 50/2 ) or 500 ohms.
>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?
I've asked several EE types this question locally. Unfortunately, the
only useful answer I've gotten is: "Build one and measure it."
>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?
W8JI answered this. We can predict the current by estimating the voltage
across the balun. We know it will be at least HALF the antenna voltage,
in the BEST case. At a minimum, it ought to handle the entire antenna
voltage (W8JI's test case). A more practical balun should handle a
variety of mismatches, and the voltage would be substantially higher -- 2
to 4 times the antenna voltage is probably a better design rule.
Bill Coleman, AA4LR, PP-ASEL Mail: aa4lr@arrl.net
Quote: "Not within a thousand years will man ever fly!"
-- Wilbur Wright, 1901
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