At 11:56 AM 3/16/2005, Rick Karlquist wrote:
>I believe someone else said they used a step attenuator.
>Secondly, they were doing differential measurements between
>a dipole and a SteppIR, not absolute measurements of power.
>
>In any event, the 141 series can be quite accurate if you know
>what you are doing and let the instrument fully warm up. There are
>several adjustments (eg gain and offset) that need to be made. If these
>are properly made they are really quite accurate over a narrow amplitude
>range (much better than a couple of dB). I have adjusted many of them and
>checked them with calibrated signal generators which in turn had been
>verified with power meters. If you read the specs on the 141 series, it
>may seem like they are not so accurate. Having worked for HP/Agilent for
>25 years, I can assure you that may considerations go into setting
>those specs, and in many cases they are very conservative.
>
>Rick N6RK
I agree with Rick here. They were doing a classic substitution
measurement, and the required dynamic range was around 10dB (i.e. the
difference in gain between the reference dipole and the AUT), at least for
the max gain. Given that the F/B is probably no better than 20dB (in
situ), the total measurement dynamic range was really in the 30dB range,
and almost any spectrum analyzer will be good to better than a dB, in the
top 30 dB of the scale at a fixed frequency, if you don't change any of the
settings. You're really looking at the accuracy of the log detector, and
that's fairly good in most cases. Nobody should be trusting F/B numbers
to an accuracy of better than a couple dB anyway(except for very low gain
antennas).
If they had a step attenuator with calibrated <0.1dB steps (no easy feat to
come by one of those, by the way), and used the "match levels" approach
(which would remove calibration of the spectrum analyzer as a source of
error), the other errors in the range measurement would dominate.
One can get pretty good absolute gain measurements by using the "three
antenna" substitution technique, assuming you correct for the other range
effects.
If I had to guess, without doing detailed analysis, the two big sources of
error that they would have are:
1) Measuring at ground level, with only one probe height, so you can't
quantify the effects of the ground reflection, and convert to a free space
number. The textbook by Krauss (and other books as well) have a good
explanation of how to do range measurements on a ground reflection
range. (this is the "null at the horizon" problem Tom mentioned)
2) Multipath effects on the reference antenna (not the probe, but the one
being used to replace the AUT), because the directivity of the reference
antenna is quite different than the AUT.
Other, more minor problems would be things like polarization and weather
variability which is usually an issue on outdoor ranges: humidity and
temperature affect the RF properties of the soil and/or vegetation, which
in turn affects near field loss and multipath and polarization
effects. Something else which may not have been well controlled is the
transmitted power, which could be affected by a whole host of factors
(power supply voltage, rig temperature, etc.)
Making good antenna measurements, other than main beam gain and 3dB
beamwidth, is fairly challenging, especially when you're more than 20dB
down from the main beam. Getting the multipath sources below 40 dB is very
challenging. To get 1 dB accuracy at -20dBc, you need to make sure that
the interference source is no more than -26 dBc. To get 0.1 dB accuracy at
-20 dBc, the reflections need to be -36 dBc. It's left as an exercise for
the reader to determine just how big a chunk of metal you need on the range
to scatter -36 dBc (it's not real big).
In decent anechoic chambers, you typically can get the wall reflections
down to -40 dB or so. This is good enough to get measurement uncertainties
around 1dB when you're down 30dB from boresight.
The summary is that range effects will likely be the dominant source of
error, even if you don't have a very sophisticated measurement receiver.
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