My antenna cal lab gives me gain and antenna factor data at 57 points, from 20
MHz through 300 MHz, using 5 MHz intervals, for my Raven biconical antenna. I
normally use this antenna only between 30 MHz and 200 MHz, but it's nice to
see how the antenna performs beyond its normal use range.
Over the 30 MHz to 200 MHz range, the AF change from one data point to the
next rarely varies by more than 1 dB, and often varies only 1/4 dB. I enter
all 57 data points into my acquisition system antenna factor file, and the
HP-85869PC software does a linear interpolation between data points.
I feel that this gives me less than a 1/2 dB uncertainty in all cases, and
likely better than 1/4 dB over most of the frequency range.
Ed Price
[email protected] <blocked::mailto:[email protected]> WB6WSN
NARTE Certified EMC Engineer
Electromagnetic Compatibility Lab
Cubic Defense Applications
San Diego, CA USA
858-505-2780
Military & Avionics EMC Is Our Specialty
________________________________
From: [email protected] [mailto:[email protected]] On Behalf Of Cortland
Richmond
Sent: Sunday, June 07, 2009 11:19 PM
To: [email protected]
Subject: RE: Measurement Accuracy and antenna factors
I favor scanning somewhat past the prescribed scan edge (and either
including
calibration points, or if not provided, extrapolating straight line in the
transducer tables) because often, automated systems fail to display an
emission right on the edge of the scan and extending it makes sure these are
caught.
Many of us may also have noticed that automated systems can produce
misleading printouts unless scan segments are kept short enough to insure all
data is properly represented on paper. That is properly another discussion.
It's already been mentioned that where large excursions are present,
more
data points should be entered in transducer tables. Data points far enough
apart to introduce an additional 1 dB of error are in my opinion too far
apart.
I suspect that in-band computational errors due to reliance on missing
out-of-band data points may be satisfactorily contained by short straight line
extrapolations as above.
Cortland Richmond, KA5S
GE Aviation
Opinions my own, not my employers'!
----- Original Message -----
From: ce-test, qualified testing bv - Gert Gremmen
<mailto:[email protected]>
To: [email protected]
Sent: 6/7/2009 12:49:05 PM
Subject: Measurement Accuracy and antenna factors
A lot of effort has been put into specification of
measurement accuracies in radiated emissions.
CISPR 16-4-2 has a number of uncertainty budgets listed.
One factor that I have not seen in any budget is the
error introduced by interpolation between
antenna factor calibration points by the measuring receiver.
In general the characteristics of a calibrated antenna
are entered into the measuring receiver as a number of
F/AF pairs, more or less randomly selected from
the calibration graph. Then the AF values for frequencies
in between those pairs a quadratic spline function is used
to interpolate. The function requires 4 calibration pairs to
operate
correctly
of which 2 must be lower and 2 must be higher then the
interpolated frequency. Especially near 30 MHz, where modern
antennas have steep AF graphs, a calibration point
below 30 MHz is not always available and I assume
the software duplicates the 30 MHz pair to
say 25 MHz to complete the function’s requirements.
This must introduce interpolation errors near 30 MHz.
I do now know the error that might be introduced by this
Type of function. I know that Taylor series have alternating
sign
In their expansion, and that the values diminish each term,
so the error of approximation remain smaller as the last term
used to interpolate. But Taylor does not suit itself
for approximation of non computable data (such as AF).
My questions for the group are:
What requirements are to be met for the F/AF pairs to
minimize errors?
What are the errors introduced by interpolation?
How do YOU handle this additional uncertainty…?
Gert Gremmen
Ce-test qualified testing bv
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