Topband: Beverage resistor triming

Tom Rauch w8ji at
Wed Dec 28 12:58:21 EST 2005

> It seems I could check to see if this is happening by
removing the
> beverage wire connection to the transformer box and seeing
if noise
> is present. Comments?

Won't work. The great difficulty with common mode problems
is we can't change the system.

In this system the antenna "pumps" the ground rod connection
up and down with voltage, since the ground rod is not a
perfect zero resistance ground. This effect is shared by the
coax shield, which if connected to the same ground rod would
also "pump" the ground voltage up and down the same way.

The ground resistance is the common coupling point, and can
be thought of as the center resistor in a T-attenuator.  The
common mode antenna and feedline impedances form the outer
two resistors in this attenuator. When you remove the
antenna you take one resistance to infinity, and common mode
coupling would totally stop.

There almost never is a situation where we can remove an
antenna connection and determine the presence of common mode

73 Tom

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