Stefan wrote:
However, when making shield terminations, many would tell you that
it is important to terminate the shield 360 degrees to the chassis
in order to have the best effect. Surely, that also has to be true
for RF frequencies when you choose to raise the near-DC impedance of
the shield-to-chassis connection to combat a hum loop?
I'm just trying to take this to the logical conclusion, which would
be an annular capacitor for the shield connection.
Be careful, taking things to their logical conclusion often sets up a
reductio ad absurdum.
In theory, annular bypassing may be better. In practice, it doesn't
seem to make any difference. Which probably explains why nobody
makes annular shield-bypass capacitors.
Note that there are two phenomena here. We have been talking about
the *conducted RF* case where RF current on the shield (in
particular, RF current from the signal being carried inside the coax
that appears on the outside of the shield due to imbalance) needs to
be bypassed to chassis. In this case, bypassing (or directly
connecting) one point on the shield to the chassis works fine. The
other case is preventing *radiated* RF unrelated to the signal being
carried by the coax from entering the chassis. If there is a gap
between the shield and the chassis, radiated RF will be admitted
according to well-known rules. Annular shield bypassing can help
this case, but again -- that is not what we've been talking
about. In any case, the tiny gaps produced by shield-isolated
connectors will not admit much RF energy at any sane frequency and
field strength.
Best regards,
Charles
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