[TowerTalk] Is it a Baku or is it a choke?

Jim Brown jim at audiosystemsgroup.com
Wed Dec 13 01:16:13 EST 2017

On 12/12/2017 7:05 PM, Shane Youhouse wrote:
> 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.

An excellent explanation, of course. Thanks for sharing it.

Note that it dates to the late '70s - early '80s, before the 
proliferation of noise from digital devices, switch-mode power supplies, 
variable speed motor drive controllers, and broadband radiation/leakage 
from CATV and DSL systems. Where Walt's 20 dB criteria was quite 
sufficient for controlling pattern distortion, greater suppression is 
desirable to prevent that noise coupling onto the transmission line. AND 
-- the one-size fits all math that yields that 20 dB is, IMO, 
oversimplification of the broad range of conditions in the real world to 
which the choke is applied. BTW -- it was W1HIS whose app note on chokes 
first articulated the virtue of a high choking Z for noise suppression, 
but he didn't understand the virtue of resistance nor of resonance, and 
he didn't understand how to measure them. Those were my contributions, 
which I published in 2005.

Note also that Walt's published work on this tested three designs for 
his choke -- good, better, and best. The difference was the number of 
beads in the string, and the only one that made it into commercial 
production was the cheapest one. Also, in a great piece of engineering, 
Walt carefully selected what was probably the only commercially 
available bead material, #73, that had the characteristic of a very low 
Q resonance centered on the HF bands, and the largest one in the 
Fair-Rite catalog was just large enough to fit the miniature coax he 
used to build the chokes. In my research, I learned a better way to 
achieve that low Q resonance at HF, how to control it, and how to get 
much higher choking Z (and thus better noise suppression and power 
handling) and for lower cost.

73, Jim K9YC

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