> >External loops that affect the meters unequally with
> >power
> would never occur.<
>
> Had one on an oil rig installation in the early '70s.
> Operating at 2MHz (roughly), the SWR varied with the
> linear on and off - that was using both a Bird and SWR
> meter made in house, which was a Bruene type. Suspecting
> harmonics, I tried an extra LPF to no avail. Fixed it in
> the end by ferrite rings on the coax feed to the antenna
> tuner, but I had suspicions we had a rusty bolt effect
> somewhere to explain the non-linearities. I tend to use
> ferrite rings on the tuner feed coax anyway, although my
> tuners are all remote.
Hi Peter,
This can easily be seen on paper.
He had external meters that all agreed. He had an internal
meter that NO meter wiring external to the cabinet that
disagreed. That rules out all external "ground loops" and
other external defects regardless of cause.
The only two ways an external meter can be affected by
ground loops is through voltage offset, anything from RF to
DC, induced into poorly bypassed meter wires or indicator
wiring that is foolishly grounded at multiple points
allowing a dc offset or ac offset on the meter returns.
We cannot induce a "ground loop" into the RF cables that
makes one meter respond differently than another unless
someone builds the RF sampling unit with poor ground
integrity from coax shield to coax shield right on one
sampling unit.
Which brings us to a good hint for external metering of any
type that goes hand-in-hand with the same problems we find
on lower level audio systems like microphone or headset
leads. Don't ground at multiple points. Just use one common
ground point.
I haven't seen a commercial meter to date that violates
these rules unless the user modifies it, in which case the
meter with external wiring has the wiring problem.
73 Tom
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