Richard Clark wrote:
On Tue, 23 Dec 2008 04:20:26 -0800 (PST), Richard Fry
wrote:

A table
on Page 3 there shows a measurement uncertainty at the NIST test
facilities of ±1/4 to ±1 dB, depending on the DUT and the frequency
range.

Actually, ±1 dB would be the most likely error for instrumentation
error (±¼ dB could never be achieved); matching error would compound
that; the antenna would add another ±1 dB; path would scramble that
further if not performed in an anechoic chamber or on a calibrated
range.

At HF and VHF, you should be able to do power measurements to a tenth of
a dB, with moderate care. (obviously, you'd have to deal with
measuring the mismatch, etc.). A run of the mill power meter should
give you 5% accuracy (0.2 dB) without too much trouble. A 8902
measuring receiver can do substantially better. Even at microwave
frequencies, better than 0.1 dB uncertainty (2 sigma) are possible with
free space measurements (e.g. from an orbiting satellite to a ground
station), with all the uncertainties stacked up (atmospheric, radome
loss, antenna, electronics, etc.), although this is decidedly non-trivial.

As mentioned, site effects or chamber uncertainties might contribute more.

A typical anechoic chamber might have -20dB worst case reflections from
the walls, and -40dB as more typical. A single scattering path will
then contribute an uncertainty (worst case) of 1%, or 0.04 dB, although
modern measurement technique (using multiple probe positions) can
quantify this error and remove it, assuming the UUT and equipment are
stable enough over the measurement period.

The TEM cell is nice because it gives you a way to create a calibrated
field to characterize your probe.


Mac's test system (from fig. 15 he reports in other correspondence)
would accumulate up to the several dB he reported earlier. It would
exhibit a very good relative accuracy, but absolute accuracy would be
several dB error as he has already offered in prior correspondence.
Path problems would have to be hammered out on their own.

73's
Richard Clark, KB7QHC