This addresses a question I thought I saw on this group not long ago. I
apologize for not being able to post this in a timely way. I also apologize
if this information has already been posted.

The question was whether it was necessary to provide an RF ground for the
transmitter in addition to the RF ground provided at the ATU. I believe the
answer is generally no.

First, whether there is any direct connection between the transmitter case
(i.e., the outer shield of the coax connecting the transmitter to the ATU)
and the ground terminal at the output of the ATU is entirely coincidental.
Well, not really coincidental. It would be better to say that such a direct
connection is not required. A good many ATU designs utilize inductive
coupling to the transmitter, or to the antenna, and a direct connection in
those cases is purely optional.

The common marine ATUs, however, rely on a form of L-matching circuit in
which there IS a direct connection between input and output "grounds". And
so the transmitter case winds up being connected directly to the ATU ground
terminal and, therefore, to the vessel's RF ground system, whatever that
might be.

We need to keep in mind that the proper functioning of an antenna, tuner,
transmission line, and transmitter, whether on a boat or off, does not
require as a matter of theory that the transmitter case be connected to RF
ground.

As a practical matter though, the transmitter and the transmission line to
the tuner are both in the very near field of the antenna. This is especially
true on a boat. RF from the antenna can travel along the outer shield of the
coax, along microphone, speaker, and power cables and get back inside the
transmitter with difficult-to-predict consequences!

Getting to the real point, now. So sometimes, we need to do things like run
a copper foil (low impedance) RF ground connection from the transmitter case
to the boat's RF ground system. Sometimes it will help, sometimes it can
make matters worse. Sorry. Also, the system is likely to behave differently
at different frequencies. There are ways to test for these currents and
often ferrites can be used in the lines to choke problem currents.

Finally, caution should be exercised in relying on copper foil to provide
lightning protection. It would vaporize like an old-fashioned fuse with any
significant current flow. It would be fine, however, for draining off charge
accumulations from masts and wire rigging.

Hope this helps.


Chuck