> 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
|