One thing that seems to help with this problem is to use a "bead balun" at
the input to the Autek and MFJ units. This at least seems to prevent
induced currents on the outside of the coax shield from making it into the
circuitry, I haven't tested this in a really strong RF field, though, like
a repeater site.
Regarding erroneous readings with the Autek at low battery voltages, I
have found that on my RF analyzer a good indication of battery
condition can be found by switching it to the highest frequency range and
observing the LCD readout for several moments. If the LCD display appears
to be fading, it's time for a fresh battery.
For what it's worth,
Steve Zettel KJ7CH
Libby, MT USA
On Sun, 27 Jul 1997, L. B. Cebik wrote:
> 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?
> LB, W4RNL
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