Someone help me out here. I am tired after a long day and may have some typos.
There is both capacitive and inductive coupling to the loop in the slug in
the Bird watt meter.
The voltage to current ratio into 50 ohms is 50 to 1.
The voltage produced on the loop due to capacitive coupling is
equal to that induced on the loop due to inductive coupling
when the voltage to current ratio is 50 to 1 and the load is resistive.
This ratio is determined by a number of factors such as the area
of the loop, its position, number of turns and the value of the resistor
and others. In fact the resistor is a key element for this to work because
it affects only the voltage produced by capacitive coupling.
It does not affect the voltage produced by inductive coupling because it is
solely determined by the rate of the change of the magnetic field that goes
thru the loop and not by the series resistance (one of Maxwell's
equations). While the resistor and capacitor forms a voltage divider which
is really a differentiator that produces a voltage on the loop proportional
to the rate of change in voltage on the line. Xc is much less than R.
The capacitive coupling does not change when you reverse the slug.
However, the inductive coupling does reverse sense. That is that the
voltage induced by inductance in the loop is the same in either direction
but is either in phase (forward measurement) or 180 degrees out of phase
(Reverse measurement) with the voltage generated by capacitive coupling.
So when driving a purely resistive load the two voltages add when the slug
is pointing in the forward direction and subtract when it is rotated to the
reverse direction. If the resistance is not 50 ohms the two voltages do
not null out when the slug is in the reverse direction. And they do when
the resistance is 50 ohms.
Here, prove it to your self that the meter does not only sense
current.
What happens when you do not connect any load to the meter on say 80 meters?
You get a large reverse reading and forward reading. But with no load there
is no current flowing thru the small section of the transmission line in
the meter. You can't say that there is a reflected wave from the output
connector because the slug is a very small fraction of a wave length away
from the open connector. The slug is detecting the voltage at this point
and not current.
And think about it. You get the same reflected power for the same
incident power if the load resistor is 100 ohms or 25 ohms.
Either produces a 2:1 SWR but a 100 ohm resistor would produce 1/4 the
current as a 25 ohm resistor. OHM's law still applies.
The meter does not precisely measure power for all load conditions
but it is real close when the load resistance is close to 50 ohms.
OK on to something else. My sympathy goes to those that don't understand it.
I am going home to get dinner...
73
Bill wa4lav
At 06:04 AM 4/3/02 -0500, you wrote:
>Hi Bill.
>The meter measures current magnitude in one direction only.
>Turn the slug around and it sees current flowing the opposite direction
>The difference is the net current flow.........that's all.
>You and I agree completely.
>---
>Ron
>
>
>"wlfuqu00@uky.edu" wrote:
>
> > The current is oscillating at RF frequency. There in no way to determine
> > if the power is inbound or outbound with out also detecting
> > the oscillating voltage on the feed line as well. If they are in phase the
> > power is moving in the right direction. It they are 180 degrees out of
> > phase the power is moving in the opposite direction.
> > The point is that you can't tell which way the power if flowing
> > with out having both magnetic field and electric field measurements at the
> > same time.
> >
> > 73
> > Bill wa4lav
> >
> > At 07:27 PM 4/1/02 -0500, you wrote:
> > >Tom /Gary.
> > >The bird only picks up voltage or current but not both.
> > >It is not capable of measuring power except that it displays power from
> > >the equation P=I^2R =V^2/R.
> > >The directionality is where it is picking up current flowing back and
> > >forth
> > >into and out of the line. If the coupler is positioned to measure
> > >outbound power , it see the out bound current and ignores the inbound
> > >current and visa versa. Net power through the line is zero if it an
> > >ideal lossless line.
> > >---
> > >Ron
> > >
> > >Gary Schafer wrote:
> > > >
> > > > Tom Rauch wrote:
> > > >
> > > > > > Hi Gary.
> > > > > > That is not the case .
> > > > > > The Bird 43 picks up current directionally. With a standing wave on
> > > > > > the line , current is washing back and forth through the line. It
> > > > > > still picks up signals directionally. --- Ron
> > > > >
> > > > > The Bird 43 element, like every other directional coupler device that
> > > > > measures forward and reflected power at one single point in a
> > > > > transmission line (rather than points separated by an appreciable
> > > > > fraction of a wavelength) samples both line current and line voltage.
> > > > >
> > > > > The null is cause by line voltage and current being in a
> > > > > predetermined ratio, and out-of-phase. When you reverse the
> > > > > element, the phase of the voltage sample always remains the
> > > > > same. Only the phase of the current sample inverts.
> > > > >
> > > > > The voltage and current are directly summed at the operating
> > > > > frequency, and the resulting voltage is detected and fed to the
> > > > > meter.
> > > > >
> > > > > The meter responds to both voltage and current, and that is why
> > > > > even if you totally do not terminate a meter it still registers
> > > > > correctly, with an infinite SWR and the correct apparent power.
> > > > >
> > > > > I can not think of any way to make a non-transmission line type of
> > > > > directional coupler that does otherwise.
> > > > >
> > > > > Of course there are accuracy limits caused by limitations in
> > > > > components, but this is a much better system with far fewer flaws
> > > > > than people here seem to think.
> > > > >
> > > > > 73, Tom W8JI
> > > > > W8JI@contesting.com
> > > >
> > > > 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.
> > > >
> > > > 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.
> > > >
> > > > 73
> > > > Gary K4FMX
> > >_______________________________________________
> > >Amps mailing list
> > >Amps@contesting.com
> > >http://lists.contesting.com/mailman/listinfo/amps
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