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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