At 08:07 PM 4/12/2011, Alistair Bell wrote:
>So now I have determine if that kind of grounding through antenna is
>normal, or a fault.

I don't actually know for certain, but I'd be fairly surprised if the
shield on the antenna lead weren't connected to the mount.  It's
certainly not *meant* to be a power ground, and it might shield the
lead as well or better if the mount were isolated - on a transmitting
antenna this would be a crucial difference but not as much on a short
receiving whip.  One of the VHF hams would probably have a good
opinion about how badly it's expecting to see a ground plane on FM;
it's so far out of resonance on AM that in my ignorance I doubt it
makes any difference.  Back when people cared about AM car radios
used to have trimming capacitors for antenna tuning (and they had a
strong peak, too) but I haven't seen one since the '70s.

You said you moved the supply/ground but didn't make entirely clear
whether you've got a big fat ground connection directly between the
two units.  I don't think we got to the scribbling stage in that
conversation, and there are a number of interrelated issues (and I'm
not an expert by any means).  I'm enclosing a diagram showing how one
boutique manufacturer (Milbert Amps) thinks about grounding.  It
won't show up on the list and I don't see it on their site to point
people to it but I'll send it to people who ask.

As a practical matter though, I think the isolation transformer (aka
ground loop isolator) we talked about in the signal grounds should
take care of you. http://www.radioshack.com/product/index.jsp?productId=2062214

:)
d