> However, do not see their display results as gospel. If
> you need
> to know the actual power, use a digital RF Power Meter
> like
> the kind that is discussed herein made by Array Solutions
> (PowerMaster). That firm prides itself on calibration, and
> the
> spin-off to the Amateur market of milspec-quality
> instruments
> leaves the Bird meters far behind.
We have to be careful to not read all the hyperbole and leap
to conclusions that are a stretch. The powermaster is a good
meter, and the advertised accuracy is certainly comparable
to some of Bird's meters.
But let's not pretend it is far beyond a Bird. It really all
come back to calibration and how we maintain and more
important use the instrument. They say "typically" +-3%. The
Bird 43 is a minium of +-5%. Also when we calibrate an
instrument from another instrument that is 5% or worse
accuracy, we certainly can't expect it to do better than 5%
(which they clearly state is against other instruments that
are tracable to NIST, so all the tolerances in that chain
factor in). The Bird on the other hand goes right against a
caloric standard.
If we were all real honest about it we'd just all admit even
+-5% is pretty darned tough to maintain.
Someone else posted:
> [snip]
> All slugs will give 30uA at indicated power.>>
Which is, as Hal pointed out, not true. More important it is
the wrong concept of what the meter measures. The meter
movement and slug has to be looked at as voltage and
resistance, current and resistance, voltage and current, or
even by power and resistance. You have to know and calibrate
with TWO known parameters, not one.
We can read 30uA and the meter still be off (which would be
the way to build a meter to read voltages much higher than
the meter movement FS voltage), or we can read 42 mV and be
off (which would be the normal way to build a current meter
that has a current sensitivity many times less tthan the
current measured in the shunt).
The slug is designed and calibrated into a certain
resistance and current, or a certain voltage and and
current, or certain voltage and resistance. So when we
check the slug ONLY for current we are not really verifying
anything except current. The slug is not an infinite series
impedance current source that would output the same current
regardless of load resistance.
The normal aging in a meter is in the magnet, although
things can stick. Hairsprings are not generally a time
problem, but they can be a temperature problem.
All that said, one of the most difficult things to measure
(other than Q) is RF power. The most accurate way to
measure it is with heat. Fortunately knowing the absolute
power is one of the least necessary things.
73 Tom
> The slugs have a small variable potmeter that can be
> adjusted for
> calibration. Then you need a good reference and gain
> access to the pot.
> I check Bird meters as you explained with a 12V source
> connected to the
> Slug connection via a 470k pot and a multimeter reading
> microamps.
> I have many slugs where most are purchased seconhand. It
> is not unusual
> that readings are off by as much as +-10%, comparing slugs
> or measuring
> with a Narda coupler and HP435 powermeter. With a meter
> that reads full
> scale with 30uA and new slugs, power is accurate within
> +-5%. The Bird
> 43 is a nice instrument that last for decades if it's
> cared for.
> [snip]
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