W3ULS wrote:
>With the changes ARRL has made in reporting IMD3 for receivers, all you have
to do is subtract 8-10 dB to get very close to Rob Sherwood's findings. No
biggie.
The actual IMD difference is ~12 dB which is the difference
in noise bandwidth between 3 Hz (spectrum analyzer) and 50 Hz
(approximate BW of the human ear) using [10 log(BW1/BW2)] or 12.2
dB. I'm afraid it IS a biggie if someone looks at PA1HR's
unfootnoted listing and concludes the FT-5000 is head and shoulders
above other rigs. It is not, as can be seen in Sherwood's table. It
simply has the benefit of being tested using *new* methodology versus
other rigs using the *old* methodology, and there is not even any
indication of when the measurement methodology was changed!
One of the major benefits of any published test data is
comparability, and ARRL's older data (I'm not sure of the exact date
of the methodology change) is definitely not directly comparable to
current data. Unfortunately I believe Peter Hart of RSGB's RadComm
is now using the same IMD measurement methodology so his data is also
not comparable over time.
>IMHO, Sherwood and his work are admirable, even irreplaceable. Yet I think
he is overly critical of the ARRL and its lab procedures, given the fact of
the ARRL's large overhead that must be paid for and the good work they do
overall. They beat the FCC in the U.S. Court of Appeals for the District of
Columbia, for example, which is no small accomplishment. So I can live with
a little less rigor in the testing area as long as Rob Sherwood (and Peter
Hart) are around to offer their opinions.
I'm certainly not critical of everything ARRL does for
us. However when published comparisons are made of their data
without so much as a footnote detailing the differences in
measurement methodologies, then criticism may be justified. The
average person reading these comparison listings may be seriously
misled if they simply take them at face value. I'm copying this to
PA1HR so hopefully Hans will consider footnoting the differences in
measurement methodologies, and perhaps Joel W1ZR will tell us exactly
when ARRL's methodology changed.
73, Bill W4ZV
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