> Any idea of what degree of inaccuracy might be introduced
by the
> parasitic asymetric capacitance in the HF range if one
were to connect
> ladder line to an analyzer's SO-239?
Actually you would never want to use a balun with the small
hand-held analyzers, unlwess it was a very compact low power
device with known characteristics.
What you want to do, and what the manual says to do, is what
you suggested above. You simply "float" the unit by running
it on internal power, and connect the ladder line directly
to the unbalanced output. You have to be sure the case is
not held in your hand, and the unit has to be isolated from
ground. By doing that it will have minimal balance problems
all the way up into lower VHF.
> With respect to the "extremes of resistance and reactance"
mentioned
> in my note, I was particularly interested in how these
devices perform
> when analyzing dipoles which are electrically short. Using
EZNEC and
> TLW, I find instances when the resistive component is in
the 5-10 ohm
> range and the reactance is in the 1000 ohm range. I was
wondering if the
> the units in the market place are all about the same or if
one stands
> out from the herd with respect to measuring these
"extremes"?
The MFJ 259B, like most of these small cheap units, is
limited by the number of bits from the A/D conversion. Since
a lot of squares are involved in the conversion from bits
(detector voltages) into impedance numbers, even one bit
makes a big change in impedance under some conditions. The
259B is 256 bits, or about .4% step size change. One bit can
be as much as 50 ohms change in some cases, although it is
more typically 10 ohms or so around 50 ohms.
The 269 uses a better A/D conversion, and is much better
when reading impedances far from 50 j0.
There are reviews and comparisons of popular analyzers
published. You might look at them and read fine print
carefully.
My own 269, properly calibrated, tracks closely from maybe 5
ohms up to 500 ohms or more. I have one custom unit
converted to a 500 ohm bridge that works great between 50
ohms and 5000 ohms. 10:1 SWR from the normalized impedance
is reasonable.
It is possible to build a high-quality small transformer and
adapt a 50 ohm analyzer to 450 ohms.
73 Tom
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