On Jan 2, 4:59 pm, Richard Clark wrote:
On Wed, 31 Dec 2008 08:50:28 -0800 (PST), wrote:
On Dec 23, 12:39 pm, Richard Clark wrote:
Nothing astonishes me more than the simple dash-off notes that claim
power measurement is a snap. I can well imagine, Jim, that you don't
do these measurements with traceability to the limits you suggest.

In point of fact, I *do* make measurements like that,

I note you slip loose from the constraint of "traceability." Doing
measurements "like that" is vastly different in outcome and holds
accuracy claims like a sieve holds water.

Traceability only applies when you need absolute measurements, and
there you will need something as a standard, since most RF power
measurements are really more of "transfer" measurements. Since power
is basically an energy measurement, then all manner of calorimetric
approaches will work (or radiometric, if you're working in
microwaves).. then it comes down to how accurately you can measure/
hold temperature. It comes back to what's a reasonable thing for a
ham to have that can serve as a local standard. Time/frequency are
clearly the easiest to get to high precision (1E-10 is straightforward
these days with a GPS disciplined oscillator), voltage is a bit
tougher.. 1ppm would be very, very good for hams, 1e-4 seems plausible
with decent high quality voltage references that cost $100.
Temperature to 0.1 degree should be doable, so that gives you a part
in 3000, roughly.

A ham seriously interested in 0.1 dB measurements will probably be
able to scrounge up something to use as a transfer standard and
scrounge up a way to get it calibrated. For instance, Noise/Com used
to offer a discounted calibration service for sources based on their
noise diodes. Once you've got a standard, if you take care of it, then
you can use it for lots of things.

The original question had to do with accuracy of measurements vs NEC,
and those would be relative, and, I maintain, not too tough to do to
0.1 dB, because you're making comparison measurements with the same
sensor, at pretty much the same level, etc.




moving on to some very telling points offered in rebuttal to obtaining
0.1dB accurate power determinations:... it *is* within the reach of someone at home with a lot of
time and care to substitute for expensive gear and calibrations
(basically, you have to do your own calibration).
And again with:
(back in the 40s, one built one's physics experimental gear and calibrated it yourself)

where both reveal a disastrously circular logic of what could only be
called "self determination" with a very tenuous grasp to accuracy.

All calibrations, whether in a cal lab or your garage start somewhere.
It's how much trouble are you going to go to for that first standard.
Do you do it calorimetrically and use water triple point and boiling
point as references? Do you trust a good calibrated DVM? 0.1dB means
2% in power.. not exactly gnats eyelash precision (e.g. measuring
temperature to 1 degree C out of a change of 100 degrees is 1%)



Standard power measuring head on a Agilent power meter is better than
5% at HF, probably in the range of 1% for one head in comparison
measurements over a short time. The 8902 is sort of a special case,
but can do very accurate relative measurements. FWIW, the 8902
calibrates out the measurement head effects.

I have already cited accuracies and errors that conflict with your
supposition. You are taking characteristics in isolation and citing
them as being representative of the whole scope of determination of
power to a stated accuracy. The single example of your stating: probably in the range of 1% for one head in comparison
measurements over a short time.

Sure.. if you are concerned about 0.1dB, then you're going to need to
calculate for yourself, and not take an offhand assertion. That said,
I still think that 0.1dB is reasonable after you take into account all
the uncertainties (and eliminate things that add to the error.. mate/
demate, temperature changes, equipment changes, etc.).

I am interested in tenth dB accuracy, aren't you? Let's recall where
this began:

On Tue, 23 Dec 2008 10:29:05 -0800, Jim Lux wrote:

At HF and VHF, you should be able to do
power measurements to a tenth of a dB, with moderate care.

This statement has now been dismissed by your inclusion (supporting my
observation) of mismatch error - which you subsequently diminish:

you're going to have to measure the mismatch and account for it. It's
not that hard, just tedious.

Accounting for mismatch does not correct it. The error it contributes
remains. This is not an actuarial gimmick of Enron bookkeeping. The
off-hand hard/tedious baggage appears to be another objection without
substance.

No.. if you KNOW the mismatch, it's not an uncertainty anymore. There
is an uncertainty in the amount of mismatch, but in a relative
measurement (e.g. received power from a probe on an antenna range..
the original question) the mismatch doesn't change from measurement to
measurement, so it doesn't contribute uncertainty to the measurement.
Likewise, if you actually measure the reflected power (e.g. in a VNA)
then you don't have to use the "power uncertainty due to mismatch"
equation which assumes that the reflection coefficient is of unknown
angle. Yes, the reflected power measurement will have an uncertainty,
but that is a smaller contributor to the overall uncertainty than the
"unknown phase of reflection" uncertainty.



The typical power meter head doesn't change it's Z very much,

And yet it still is NOT the Z you would like it to be, except by some
margin of error. Change is not the issue, absolute value is. This
appears to be yet another manufactured objection that points out error
only to dismiss it with a cavalier diminution of "very much."
Metrology doesn't accept adjectives in place of measurables.

For a relative measurement the source and load Zs are constant, and
whatever mismatch there is will be the same for all measurements (e.g.
in an IF substitution measurement, it's the Z looking into the
measurement system's attenuator or amplifier). If you're trying to
measure the output of a source with varying Z, then, of course, the
mismatch will affect the net amount of power crossing the reference
plane into the sensor, and you'll need to do that. But there are lots
and lots of cases where a ham might make measurements to 0.1dB where
the source Z is constant. (say, making noise temperature measurements
of the sun with an antenna, or measuring the received signal from a
distant transmitter.. same antenna, same physical location, same
receiving system.. the measurement system isn't changing)


Let's return to the claim, however. In fact, a typical power meter
head DOES change its Z, and it is by this very physical reality that
it performs power determination. You may be relying on a specific and
rather atypical head to support your argument. As you offer nothing
in the way of your typical head's design to support another off-hand
comment, we will have to wait for that coverage.

The datasheet for the head gives this. If you're looking at the
thermistor/thermocouple mount style head, the Z looking into the head
is basically that of the load resistor, which, if held at constant
temperature (constant = within a few degrees), I doubt it changes more
than a fraction of a percent. A diode head (like the 8481,8487, etc.
for HP/Agilent meters) is also going to be pretty good. Agillent
claims the increase in uncertainty for ALL causes from an extended
temperature (0-50) over the specified 25 +/- 5 is something like
0.9%. Obviously, if want to dot is and cross ts, then you can
actually measure it, but you'd only need to characterize it once
(that's the moderate care thing.. a lot of good metrology is just
record keeping).

There's also a change in Z with frequency (an issue if your transfer
cal standard is at a different frequency than your measurement
frequency; not the case in a relative measurement on an antenna
range), but again, you can either take the worst case in the data
sheet, or measure it yourself. That's sort of the difference beween
modern VNAs and old style measurements. The modern VNA uses a set of
cal standards that has properties determined by its mechanical
construction (e.g. short, open, thru) and does the arithmetic for
you. Even the $1000 N2PK and TAPR VNAs do this. As long as you're
at the same temperature, it should be good.


so once you've measured YOUR head and keep
the data around, you're good to go for the future. (and do your tests
at the same temperature, don't use the head for a door stop, etc.)

No, you do not make traceable measurements. Your statement of
futurity is an illusion only. Within the context of HP's fine
craftsmanship, it is a fairly safe illusion, but not into the
unlimited future. The calibration cycle for an RF head AND its
reference source (two sources of error) is 3 to 6 months where that
calibration data would be amended and changed to follow the natural
variation in characteristics. A skilled bench tech might trust the
instrument out for several years for relative loss measurements, but
absolute power determinations will have long lost their credentials.

I think that reasonable folks could differ on the concept of "required
calibration cycle" and "aging life"... It's not like the thermocouple
or load resistor in a power meter head has an aging process that
causes it to suddenly go out of cal after 6 months. More likely, the
cycle is a good blend of economics and the expected variation from a
typical rough and tumble bench environment. Most of the time, the
calibration cycle is more to make sure that the device hasn't
"broken". If your device is using something like a crystal
oscillator, then there IS an aging thing to worry about.

You have raised an interesting question, though, so I'll have to go
ask the folks at the cal lab to see if we have any long term data on,
say, a 848x head to see what sort of aging or changes there are.




This repetition of the hard/tedious mantra has the odd appearance of
an objection diminishing the importance of established procedures of
power determination. It reduces the profession of precision
electronics to the repetitive motions of a trained monkey.

No, I think you misunderstand "hard" in this context. It doesn't
require any special new thinking to do accurate power measurements.
The methodology is well known, as are the error sources, and the
evaluation of uncertainty. The "hard" part is reducing the
uncertainties (i.e. the equipment design) in the first place or in
choosing a measurement method that tends to cancel errors (e.g. Dicke
switch radiometers use the same sensor for both reference and unknown
measurement, eliminating sensor/sensor uncertainty, at the expense of
the uncertainty due to the switch). The tedious part is in being
careful, doing repeated measurements, controlling the environment, and
then grinding the math.


that crude metric of adjectives. Throwing more gear and procedure
that is freely available (rather than as costly necessary) at the
problem, compounds error outrageously.

I'd say "may" compound error.


This, again, is an appeal to substituting just-plain-hard-work for
accuracy. You fail to show any correlation to standards or their
necessity. The effort you describe may well pay off in superlative
precision; again, investing in resolution without paying for the cost
of accuracy.

Or, perhaps, getting your accuracy from standards you DO have handy,
rather than relying on the instrument's internal transfer standard. An
example might be using thermal hot/cold loads to calibrate a
radiometer rather than a calibrated diode noise source (where the
diode was calibrated somewhere else against a thermal standard). If
you don't have the diode (or the wherewithal to send it to NIST for
calibration on their radiometer), then perhaps hard work and time can
get you a comparison against something you CAN measure accurately
(boiling LN2, boiling water, etc.).


Z is unconstrained. In a typical ham situation, these things probably
aren't the case.

This appears to be yet another objection: "probably aren't the case."

The original post had to do with comparing measured antenna patterns
against NEC models. That IS the case there.




This was your choice of instrumentation; this was your choice of power
determination. I have provided an incontrovertible example of how
your off-hand assertion failed from your own choices. It doesn't get
remarkably better however you decide to amend the conditions and those
amendments certainly won't come close to your original off-hand
observation of power determination of 0.1dB or less error.

I note that a later page in the same presentation shows absolute power
measurement worst case uncertainty at 30 MHz of 0.02dB over a power
range of -10 to -70 dBm (slide 47).. I just picked the 8902
arbitrarily as an example of something that I know can do relative
measurements to this sort of accuracy (as opposed to, say, a Bird 43
watt meter, which cannot) to refute your original statement
(paraphrased) that measurements to better than 0.5 dB were
impractical.

Some may appeal that their Relative Accuracy has the merit of Absolute
Accuracy - until you ask them to measure the actual, absolute value of
their ad-hoc standard. I have built primary standards and measured
them to 7 places. Over several years they migrated in value by 2 of
those least significant digits, but only in comparison to standards
that had "aged" and been calibrated at a national primary standards
laboratory. By themselves, I could have fooled myself (and perhaps
others less sophisticated) that they were absolutely accurate to the
extent of the number of digits my instruments could resolve.

--
So, you measured to 1E-7, and over several years, they changed by
1E-5? That's a whole heck of a lot better than 1E-2 (which is what
0.1dB implies). Those 3 orders of magnitude are why I think it's
reasonable and plausible for hams to make measurements to 0.1dB.

--

http://hdl.handle.net/2014/18497 describes one measurement system I
designed, built and calibrated. That paper was aimed at a more
general audience and doesn't give much of the uncertainty analysis,
but it can be found in the several dissertations based on the
calibration station. There's nothing special in this system that
couldn't be duplicated by a ham for HF or VHF use. Certainly, the NIST
Type IV power meter (used in the system to measure the level of the
reference source used to calibrate the receiver chain) is something
eminently doable for ham use (See Larsen's paper from 1975) and mostly
depends on a "really good" DVM for its accuracy.

Jim