[TowerTalk] Analyzer and AIM comments to Jim Lux

[Jay Terleski]

Hi Jim,  

I ran your comments by Bob Clunn, the designer of the AIM and Power Aim
Analyzers and he has responded below.

Jay

 

Bob Clunn's comments:

 

It's interesting to see a detailed discussion of the issues concerning
calibration
and measurement. 

 

Statements like "works great" or "doesn't work" are subjective and depend on
what the
user is trying to accomplish and what kind of performance he expects. On
eham.com reviews can be found ranging from 1 to 5 for the same product, the
perception of quality depends on the circumstances.     

 

"It's one thing to calibrate out a few degrees of phase or a few tenths of a
dB shift to compensate for the non-ideal directivity in a probe or the
loss/mismatch in a connector or test port cable.  It's entirely another to
calibrate out the response of a reactive device with S11 and S21 varying by
orders of magnitude over the passband."

 

The AIM uses a procedure that compensates for non-ideal parameters over a
dynamic range of about 60+ dB. 
This is illustrated in the manual in the section about Custom Calibration
starting on page 29. Several graphs
show the effectiveness of the cal procedure in canceling the effect of a
complex transmission network.

 

"Basically, if you divide a not very accurate measurement by another not
very accurate measurement, the result isn't very accurate."

 

Agreed.  Another serious pitfall is to subtract two large numbers, their
difference can have a large percentage variance. 

 

"This kind of uncertainty analysis is what separates typical experimental
gear and bench experiments from "lab-grade" instruments like a VNA from
Agilent.  If I make a measurement on an unknown device with a professional
VNA, not only do I get an estimate of what the unknown device's
characteristics are, but I also get an estimate of the uncertainty of that
estimate."

 

Knowledge of the uncertainty is nice to have. Some people, like Larry Benko,
W0QE, have compared the
AIM to HP  equipment (HP8753B, 3 GHz. two port vector network analyzer, with
HP85046A S-parameter test set) and found it to be comparable over it's
frequency range. I don't have equipment like this myself, but I will be glad
to hear from anyone who does a comparison study. The AIM compares quite
favorably to other
antenna analyzers in it's price range, according to data obtained by the
ARRL lab.
These comparisons can be seen on the w5big.com website. 


"So, can you point us to an analysis of the accuracy of the calibration
method? How much (in numbers) does the filter degrade the accuracy of the
measurement (or, more correctly.. what's the uncertainty of the calibrated
measurement after applying the calibration)."

 

The effectivness of the calibration procedure in several situations is
illustrated in the manual. For a
particular situation, you can check the calibration with known loads to see
if it's sufficient for your 
requirements. Depending on the degree of compensation required, the accuracy
when measuring through a
complex network, should be within 2 or 3 percent. Ideally it would depend
only on the accuracy of the loads
used for calibration, but there is some uncertainty in the raw data, so the
compensation is not perfect.


"In fact, the Ap Note describing the 160m antenna measurements is quite
suspicious, from a calibration standpoint, because you made use of
"smoothing"..Those fluctuations with respect to frequency in the calibrated
response before smoothing are often indicative of the calibration constants
being derived from a noisy measurement of the standard.  (the whole dividing
a small noisy number by another small noisy number problem...)."

 

Calibration constants for any instrument are derived from measurements that
are noisy.

During the calibration procedure, the AIM averages 16 readings at each
frequency point. This a perfectly valid mathematical technique.  Averaging
is done in the hardware. Smoothing
uses software to do a running average to further reduce the effect of
measurement noise.  

 

Comment added by Jay, The 160m Antenna in the application note performed
exactly as the VSWR curve on the graph indicates as verified by using a
transceiver, and our NIST traceable VSWR/ power meter.

The intent of the application note was not to show .1% accuracy, but to
demonstrate the usefulness of the hardware and software tools which we think
are way beyond what any $500 instrument provides, I hope you agree with this
statement. The average amateur can solve some typical amateur problems
without resorting to high dollar equipment and tune up a 160m vertical.

 

"It's just that Rick's comment about poor accuracy could be the result of
his recognition of the limitations of the measurement and calibration
techniques being used."

 

The AIM does have its limitations. Whether that is good or bad depends on
what you're looking for.

There's always room for improvement, so I welcome new ideas to improve the
AIM's performance.  

 

Bob Clunn, W5BIG

 

 

 

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