On Mon, 18 May 2009 16:12:22 -0400, Pete Smith wrote:
>But what about the case of a different termination at the antenna
>end, such as a preamp or an active antenna amplifier unit? Does the
>same still apply, or does the termination in the output of that
>amplifier attenuate the common mode currents sufficiently to make the
>common mode choke unnecessary?
Good question, Pete. Not dumb at all. The best answer I can think of is
to note that 1) the common mode current is flowing on the coax shield,
2) the earth is far from an ideal conductor, so the connection to earth
at the preamp is unlikely to divert ALL of the shield current, so that
shield current could certainly get passed through to the antenna and/or
transformer; 3) transformers are not ideal, and do have some capacitive
coupling between windings; and 4) a serious common mode choke is
trivially easy (7-10 turns through one of the "big clamp-ons) and
inexpensive, and does no damage to Beverage performance. Is it
NECESSARY? Will it help? I dunno. Can it hurt? Only $12 for the ferrite
and a buck for the extra length of coax.
73,
Jim K9YC
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