Topband: Probably-dumb question re current mode chokes on RX antennas

[Jim Brown]

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