> >"Richard (Rick) Karlquist (N6RK)" wrote:
> > > The high pass filter will transform any impedance
other than
> > > 50 ohms to a different impedance.
> >
> >Not according to the spec sheet which came with the unit.
And as far
> >as extraneous RF getting into that thing, it is so
heavily shielded
> >that I would say such is near to
> >impossible.
The antenna is the source of troublesome RF, not leakage
through the case of the unit. The case could be plastic
without any RF ingress problems.
As Rick said, almost any lowpass filter will change SWR and
impedance. Any manufacturer that says otherwise is full of
"Bean Soup".
I designed a solution for MFJ that uses a broad band
parallel-tuned network with a series tuned trap for AM BC. A
simple parallel network has only a minimal effect on the
system. Unlike a low-pass or high-pass, it does not
transform impedance or near the resonant frequency so
reading remain accurate. The only problem is to control cost
they use cheap poor "Q" inductors, so it isn't quite as
selective as it could be.
These units all suffer from the same problem. Even my $35K
HP4191A has that problem. The accumulation of RF from
off-frequency signals gets into the detectors and screws up
the readings. True network analyzers (BIG bucks), impedance
meters using selective receivers in the detectors, or high
power RF bridges running at a few watts are the only
solution I know of.
73 Tom
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