On Fri, 30 Dec 2005 15:31:59 +1100, "Allan Jeal"
wrote:

....
Is it likely to damage an Icom AH4 auto antenna tuner if the "antenna"
terminal is attached to one leg of a V (resonant at 14Mhz) and the earth
terminal is connected to the other leg of the V (resonant at 7Mhz) without
any additional grounding. Is there any reason to swap the legs around.

What you intend doing will connect the "ground" rail of you tuner to
one side of the antenna, and unless you take other steps, the "ground"
rail of the tuner will be connected to the radio chassis via the coax
outer and the control cable. Perhaps the radio will be connected to
ground directly or indirectly.

This is not a lot different to feeding a dipole without a balun
(whether by coax or open wire line).

You might be concerned about whether the likely RF common mode
currents will disrupt or damage the control interface at the radio or
tuner.

There have been lots of reports of people doing what you propose, some
choke common mode currents below the tuner, sometimes with separate
chokes on the coax and control cable, some choke common mode currents
above the tuner, sometimes providing a local ground to the tuner, and
some do nothing.

I favour a choke above the ground mounted tuner, and a local ground on
the tuner to help to reduce the rf currents flowing on the coax and
control cables into the shack. I have done this with open wire feed
above the tuner to the dipole feedpoint and it worked fine without
noticeable "RF in the shack" problems.

I note that SGC is now producing an auto-tuner with balanced output,
but have no experience of its performance.

The main issue that I see with these tuners is the difficulty of
diagnosing faults and identifying faulty components for replacement,
and it makes me think twice about investing in them.

Owen
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