Tom Rauch wrote:
> > Tom,
> >
> > Are you saying that because the bird meter measures both voltage and
> > current that it will read right no matter what the line impedance is
> > that it is connected to? This of course subtracting reflected from
> > forward readings.
>
> That is EXACTLY what I am saying, because that is precisely how
> the meter works.
>
> What is particularly disappointing is some of the most vocal critics
> on the RRAA forum (no one present in this forum) don't even have a
> clue how the meter actually works. They oddly think the meter,
> which is a negligible fraction of a wavelength long, has an
> important characteristic "design impedance" that somehow
> causes reflections of waves.
>
> People will stoop at anything to convince themselves (quite
> wrongly) that you can not subtract reflected power from forward
> power and get true power, or that the meter is only accurate in 50-
> ohm lines or with 50-ohm loads. Even though that is an incorrect
> idea.
>
> Just to prove my point, I measured power today in this setup:
>
> Generator----Test Meter-----Tuner-----Reference Meter----50ohm load
>
> I substituted a vector impedance meter for the test meter, and
> adjusted the tuner for the proper test impedances with the
> negligible loss tuner on 7MHz.
>
> This, in effect, puts the test meter into different impedance loads
> while the reference meter always operates into a 1:1 load.
>
> I use a calibrated digital 1% meter for the reference meter, and a
> regular Bird 43 with 50 watt slug for the test meter.
>
> First reading is test meter forward over reflected, second is
> reference meter forward.
>
> With the test meter seeing 50 ohms, I read:
>
> 50/0 // 47.2
> 25/0 // 23.5
>
> This is typical for off-the-shelf Bird 43 meters. It is about a 5% or
> so error.
>
> With a 75 ohm load I read:
>
> 26/1 // 23.5
>
> With a 150 ohm load I read:
>
> 32/8 // 23.5
>
> The 75 ohm load test agrees PERFECTLY with the power
> measured in the 50 ohm system when F-R is used.
>
> The 150 ohm test agrees PERFECTLY with the power measured in
> the 50 ohm system test.
>
> This will happen (I have done this hundreds of times over the past
> 30 years) time and time again, both with complex load impedances
> and resistive loads.
>
> The Bird manual is correct. Walter Maxwell is correct. Forward
> minus reflected equals true power at the output of the meter.
>
> > How does this happen when there is no line connected to the output of
> > the watt meter and no current can flow? When you reverse the element
> > you are only reversing the current pickup phase. If that is true then
> > in the case of no line connected and no current the only thing that
> > there is to measure is voltage which the meter does not discriminate
> > against direction.
>
> The meter reads near infinite SWR, very high power, and nearly
> zero difference between forward and reflected power. That means
> zero net power delivered to the load.
>
> It works every time.
>
> The only errors are normal calibration and component tolerances.
> 73, Tom W8JI
> W8JI@contesting.com
Hi Tom,
I finally drug out an old bird 43 manual from 30 some years ago and read it
again. It pretty much describes the operation as you describe it. But they
do go on to say a bit about operating it at loads other than 50 ohms and
that there are additional errors introduced when doing so. This is in
addition to the fact that the line section itself being 50 ohms causes a
bit of an impedance irregularity but that is only of major concern above
about 200 mc.
Both Bird and a QST article by DeMaw (QST Dec 1969) on inline power meters,
makes mention that the internal resistor in the meter that sets the
proportion of line current to voltage that is sampled is set for a given
operational impedance of the line. It is adjusted so that the current and
voltage that are sampled are equal in amplitude. There is not a lot said
about it but I get the impression that when the line impedance is far from
the 50 ohms where the circuit was calibrated at the ratio of voltage to
current gets too far off and causes additional errors, especially in the
reflected power reading. The Bird manual makes mention that when measuring
a 70 ohm line that this additional error needs to be accounted for.
Something to the effect of increasing the reflected error reading from 3%
to 10%. I think that was at a 2:1 swr.
In your test that you did above I would have thought that you would have
put the test meter where the reference meter was, near the load. That way
when you adjusted the matching network the first meter (this would be the
standard) would always see 50 ohms in and out and the second meter would
always see whatever the load impedance was, both in and out. Maybe it
doesn't make any difference or I didn't fully understand what you did.
I also wonder how the two meters would compare in readings when the load
was way off from what was used. Like in the open circuit load that we had
been discussing.
I still do not see that there is any significant current flowing at the
measuring point with the very short line between the slug and the end of
the meter line. It seems as though in this situation that only voltage
would be present for meter sampling.
Between the possibility of no current in the situation with no line or load
connected and the seemingly possibility that the directional properties of
the meter deteriorate when drastically moving away from 50 ohms I wonder if
the meter is really showing the product of what we think it does. Is it
only reading voltage and not power giving the illusion of power being
present?
73
Gary K4FMX
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