Hi Dan,
Seems you are correct in your assumption. Here's something from
someone who doesn't want to appear to "offend" Rich..........enjoy.
Hey Tom....you can post this if you want, but please don't attribute
to
me. I
am interested in technical correctness, but this would come across
as
Rich-bashing which I shall leave to others.
I used to work for NIST. I know about the Naval Calibration Lab
procedure
that Rich refers to for peak RF measurements. In the late 1970s,
we at
NIST
(then NBS) declared that procedure to be so fraught with problems
that
we
dropped official support for it. Without official NBS support, the
military
cal labs can't use a procedure. There's an NBS nomogram on RF
power
measurements that very clearly and repeatedly makes the point
that
oscilloscopes
are bad news for measuring RF power. There are too many ways
to get
errors.
SWR on the test probe cables (how many hams ever thought of
that?), or
when
using a so-called 50 ohm terminating load at the scope input, just
how
good
is the SWR, the fact that most scopes have very poor response to
peak
signals
above about 20% of their 3dB bandwidth, and more, make a scope
a highly
suspicious way to measure power. Even in those cases where
pulse times
are so short that a scope must be used, NIST recommendations
are to
calibrate
the scope against a thermal-responding RF wattmeter, and only
supports
the
practice if a vector-corrected network analyzer is used to verify
impedances
throughout the system.
The fact is, a scope calibrated for DC is not necessarily trustworthy
at
RF.
And you can, indeed, build a peak-reading RF wattmeter fast
enough for
anything
amateurs transmit.
Another lister mentioned that the FCC requires the measurement
to happen
over
an RF cycle. Sigh. Unfortunately, the FCC has never bothered to
consult
with
NIST about whether the measurements they require are actually do-
able by
the people to whom they apply....
73, Tom W8JI
w8ji@contesting.com
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