On Mon, 09 Aug 2010 13:29:44 -0500, Stuart Rohre wrote:
>His antennas were in the clear, which can affect RF on the shield,
RF on the shield IS common mode current. Common mode current is the
result of antenna imbalance, mostly with respect to ground and grounded
conductors. For example, a tree, wiring in a house, the ground slopes
under the antenna, the antenna slopes, ground conductivity varies under
the antenna. Using coax is only PART of that imbalance. A common mode
choke simply kills that current.
The virtue of coax is that it is EASY TO PUT AN EFFECTIVE COMMON MODE
CHOKE ON IT, whereas with parallel wire line it is not.
The choke does not BALANCE the antenna, but it DOES reduce the common
mode current to near zero, so all the antenna current flows in the
antenna. That DOES tend to force the current to be equal on either side
of the coax connector. The antenna may still behave asymetrically
because of those imbalances noted, but the coax is no longer carrying
common mode current, either on RX or TX. In other words, the choke
effectively causes the coax to "disappear" -- as far as the antenna is
concerned, the only effect of the feedline is to connect the antenna to
the transmitter and add a bit of loss. The choke does not add loss
(unless it's a lousy choke), it's only the loss in the coax.
73, Jim K9YC
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