Don:
I've been giving the subject a bit of thought as well and a noise
bridge came to mind rather quickly. I have an old MFJ 202 noise bridge I
bought 25 years ago.
However, I believe a broad band detector such as the AD8307 will fail.
The null condition is rather narrow frequency wise, and the noise
generator detector output is broad, so the different in total power at n
AD8307 detector between no null and perfect null will be tiny,
particularly if the bridge is constructed to have good balance, which
is, of course, the objective.
This is a case where one needs a narrow band tuned detector, I believe.
Or, a narrow band source.
Jack K8ZOA
www.cliftonlaboratories.com
Don Wilhelm wrote:
Those who have both an Elecraft N-Gen and a KX1 have everything needed
except the bridge elements to construct and use a relatively compact version
of that Improved Noise Bridge described by John Grebenkemper in that August
1989 QST article.
The N-gen is the noise source and the KX1 can serve as the detector - listen
for the null. Of course, it is easy to construct the noise generator, so
those with only a portable receiver like the KX1 can have an accurate
impedance meter once it is calibrated.
Detectors other than a receiver can also be used - an AD8307 based power
meter should work just fine and fit into a small case.
The 'disadvantage' of the noise bridge is there are two knobs to deal with,
but when a deep null is achieved one can read both the resistive and the
reactive component of the unknown impedance.
73,
Don W3FPR
-----Original Message-----
Dave G3YMC wrote:
I would also make a plug for a noise bridge. I built the one by KI6WX
which appeared in QST in August 1989 (available to download from the
ARRL site). If you have a suitable portable rig to couple to it does
most of what the fancy analysers can do at a fraction of the cost.
It should not be too difficult to design a fairly simple receiver
drawing a
small amount of current to be enclosed with a noise bridge to
make an 'easy
to carry' instrument. Top of head thinking.
73,
Geoff
GM4ESD
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