Dear Ian:
Most interesting. Thanks.
I have not used the AEA-CIA for R-X measurements. I always assumed
that firmware was used to guess at the sign of X, and it is not too
surprising (though disappointing) that the chap who wrote the software
might blank out small values of X altogether. [In a spherical
navigation program that I wrote long ago for a slow computer, I flagged
results that would have been in the noise because of round-off error.
It took some effort to know when the results were bad. Others ignored
error analysis, being enamored of many random digits, with disastrous
results. Sometimes, with computers, no answer is the best one can do.
As an example: Cramer's rule just does not work to solve some systems of
linear equations using a computer! Even the HP48 uses an iterative
technique, which computers are real good at and people are not.]
The most interesting use that I made of the AEA-CIA, which would
have been difficult to do with a GR bridge, was measuring the apparent
surge impedance of a split boom to be used with a LPDA (prior to the
elements being attached). Leaving out the appropriate lectu I
terminated the split-boom with a series of resistors 'till the variation
in SWR (seen at the other end of the split-boom) with frequency was
minimal. [This is another example of SWR on a line depending on which
way one is looking.]
The strong suit of the AEA-CIA is in giving one repeatable data over
a wide frequency range. Most of the antennas that I deal with have
functional bandwidths of at least an octave. The value of the AEA-CIA
is much reduced if one is only interested in what is going on in a
narrow bandwidth.
Thanks very much for sharing your experience. We have once again
benefited from your experience and Roy's experience. 73 Mac N8TT
--
J. Mc Laughlin - Michigan USA
Home:
"Ian White, G3SEK" wrote in message
...
snip
A few years ago, I was evaluating an AEA-CIA for a possible magazine
review. One of the tests involved a load consisting of some metres of
50
ohm coax terminated in three paralleled 50 ohm chip resistors. When
the
frequency is swept, this load walks around the SWR=3 circle on a Smith
chart, giving |Z| values ranging between 16 ohms and 150 ohms with a
progressively rotating phase; or equivalent results in terms of
(R+/-jX). In other words, the test involves only moderately high or
low
impedances with no nasty surprises.
The AEA-CIA gave good results as a frequency-sweeping SWR meter, and
the
graphical display is unique in this price range; but unfortunately but
it did not give sensible results in the R-X mode (the mode that gives
the "Complex Impedance Analyser" its name). In a frequency range where
the true value of X was falling progressively through zero, the
indicated value came down correctly to about 30 ohms - and then
suddenly
jumped to 0.0. The R readings continued to change with frequency
exactly
as expected, but the X reading stayed 'stuck' at precisely 0.0 until
the
sweep reached the frequency at which X changed sign, whereupon the X
readings started to make sense again. This behaviour was totally
reproducible. Also, the AEA-CIA is also supposed to be able to resolve
the sign of the complex impedance (which it presumably does by
changing
frequency and noting what happens to X), but perhaps not surprisingly
this didn't work reliably either. At a constant frequency where the
value of X was nowhere close to zero, the instrument was often unable
to
make up its mind about the correct sign.
All these symptoms looked like firmware problems to me. Since R and X
are both computed from the same analog voltage readings, and R was
correct while X was not, the problem had to be in the computation. The
AEA management at the time were quick to respond through the UK
dealer.
They sent me schematics, and analog-type mods to try, and even
replaced
the entire instrument... which behaved exactly like the one before.
However, they didn't seem to understand what I just wrote above, and
didn't want to go anywhere near the firmware.
In the end, I abandoned the effort and the UK dealer didn't import the
instrument. The magazine decided we should review the MFJ-269
instead -
which handled the same test load with good accuracy.
Sorry, I don't recall what specific firmware versions gave these
problems with the AEA-CIA, and have no information whether they have
been fixed in later versions. As Roy said:
the problem might have been fixed, but it's something to look
for, particularly on an older used unit.
--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek