> My question: Should I bond the EMT behind the rig to the shack
> ground? I'm concerned that I'll be feeding noise into the RF ground.
> I'm also concerned that the service ground is about 65' away (85' if
> you follow the EMT around the building).
You may not realize this, but the grounds are connected to each other
anyway through all your gear, which has line bypasses between the
chassis and the power lines as well a electrical safety grounds on
the third wire used as a safety ground. All you are doing by NOT
tying those grounds together is increasing risk of lightning
following the paths THROUGH your plug-in equipment, rather than
through an external safe ground connection.
Besides that, if you can "feed noise" into a ground system and hear
it, you have a major system problem with feedline radiation.
> After pondering all this for some time, I'm considering not attaching
> the EMT to the RF ground and opt instead for a very small spark gap
> fixture between the two.
A better connection between power line grounds and the shack ground,
and the better the shack ground, the LESS chance that you will have
noise ingress via conduction through power lines into your gear.
It is electrically SAFER to bond the two ground, and is much better
as a general rule for RFI.
The only way we could hear noise conducted through "grounds" is if
the antenna system were improperly designed or constructed, such a
vertical with totally inadequate ground system and no feedline choke.
73, Tom W8JI