Topband: Common Mode Isolation

Tom Rauch w8ji at
Thu Dec 29 08:34:38 EST 2005

> I am feeding a Ewe with a matching transformer whose
primary and secondary
> are isolated, and the coax is not grounded at the antenna.
Would placing an
> isolation transformer in the coax, near the receiver, and
grounding the coax
> between the two transformers be useful in reducing the
common mode problem?
> Is there something better?

I doubt you need something near the receiver. If you have
ingress at that end, it has to be a connection problem in
the shack.

That might be Garry's problem with his monitor. It doesn't
take much of a bad shield ground to allow very strong stuff
into the cabling. There are two approaches to the problem.
One is to use feedthrough connectors or grounding blocks and
make sure everything is grounded to a common point to avoid
ground loops, the other is to isolate things. For lightning
protection I ground everything *including the power line
bypassing* to a common point at the cable entrance.

By the way, you can disconnect the antenna from the feedline
to test for an ingress problem in the shack, since it is a
different coupling point than through the antenna.

73 Tom

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