[TowerTalk] Force 12 B-1/B-1S Baluns

[Ian White]

G3TXQ wrote:
>
>Air-cored coax-wound chokes perform exactly like any high-Q parallel-tuned
>circuit: very high impedance over a very narrow bandwidth; reactive at all
>frequencies other than right at resonance; tuning very susceptible to
>proximity effects. 

All true; and with a well designed ferrite-loaded choke it is exactly the opposite. A common-mode (choking) impedance of several kohms can easily be maintained across a 2:1 frequency ratio. 

The big advantage of ferrite loading is that the impedance is mostly resistive over much of that useful bandwidth. Resistive impedance cannot be cancelled by de-tuning so the performance is much more dependable than any type of air-wound coil,

A good feedline choke can dramatically reduce the common-mode current on the outside of the coax. In this type of balun the ferrite core does not have to handle the main RF power, because that passes through the inside of the coax without magnetizing the core at all. The only RF power being handled by the core is due to whatever low level of CM current remains after the choke has been installed. That would be "I-squared x R", where I is the residual CM current and R is the resistive part of the choking impedance.

The better the choke, the lower that residual CM current is going to be, and the lower the power dissipation in the ferrite will be too. In a bench test into a dummy load, a good choke will easily handle 1.5kW in the ultimate stress test with its output connections reversed (center conductor at the output is tied to ground at the input). This is far more stressful than almost anything the choke is likely to experience on a real antenna. 


>How many times have you seen a coiled-coax choke taped
>to a boom - think about that in terms of a high-Q parallel tuned circuit!
>
Not only that, but taping the choke to the boom is adding extra capacitance between one side of the driven element and the boom. In other words, taping the "choke balun" directly to the boom is making the antenna more UNbalanced!  It's amazing to watch people doing this... 


73 from Ian GM3SEK