Re: susceptibility of the Autek and MFJ to erroneous readings when there
are nearby high power stations.
If you think about the nature of either instrument, you will understand
that the problem is less in the instrument than in what we are asking them
to do.
We want a very low RF source (to not radiate out of band signals at
detectable levels) and hence a higly sensitive very broad band detection
system. Since any nearby signal will induce currents into a significant
antenna structure, the instrument will read these currents along with
those produced by the generator inside. The level of induced currents
will vary with the nearness of the station, its frequency and power,
and--of course--the antenna structure (which has in all likelihood not
been tested for microwatt sensitivities at all frequencies from sub-AM to
supra-FM/TV). You can design an SWR analyzer with prefilters or other
methods to eliminate these disrupting energies, but few of us can afford
the resulting product.
Although it may sound too basic to need repeating, always use a secondary
system to check the readings obtained from a primary instrument--in this
case, the station rig and SWR meter as a cross check on the SWR analyzer.
Also, just as we do when we use the station equipment, always determine
the reliability of the power source before committing the equipment to use
(a fancy way of saying "Check the battery of any test instrument before
using it").
Guilt quiz: When is the last time you checked the battery in your DVM or
the ohm's battery in your old VTVM?
-73-
LB, W4RNL
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