> 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
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