Hi Bill.
Your point about the 25 vs 100 ohm load clears up my misconception that the bird
measures current only and I stand corrected.
I cannot dispute that the current at the end would be small and voltage high
and so
it must sense voltage as well because I see what you see using that instrument
as
you explain.
As I said initially , I didn't understand the small pickup wire and suspected
that
it could be a pick-up as well as the coil itself for capacitive coupling but
would
be working in a summing mode, not in a multiplying mode.
I think my issue is that I see the reflected readings vary as one positions the
meter on the line that indicated that the meter is not a true power reading
instrument.
I think you discussions as well as those of Gary and Tom have been a great
benefit
to me as well as anyone reading the mail.
Thanks for your patience
---
Ron
"wlfuqu00@uky.edu" wrote:
> 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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