[TowerTalk] Baluns - Primer and Questions

[Bill Coleman]

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


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