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
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