On Apr 9, 6:34 pm, Owen Duffy wrote:
Roy Lewallen wrote :



Owen Duffy wrote:

If in fact the power delivered by the "100 watt IC706" radio was
indeed 100W, and some directional wattmeter correctly indicated 200W
forward, it must indicate 200W-100W reflected which is indicative of
a VSWR of 5.8, which should have reduced power output from the IC706
markedly.
. . .

Nah, no problem.

Connect your rig through a half wavelength of 250 ohm ladder line to a
50 ohm load. Presto, 200 watts "forward power" and 5:1 SWR on the
line, and the poor ignorant Icom doesn't have any hint that all those
waves of power or energy or whatever are bouncing around on the line,
trying desperately but unsuccessfully to overheat the final or
whatever they're supposed to do.

Of course, it would take a 250 ohm directional wattmeter to read that
"forward power" or SWR. But we don' need no steenkin' meter -- we know
it's there, don't we?

Roy Lewallen, W7EL

Roy, I was assuming that the instrument was a nominal 50 ohm instrument
measuring conditions adjacent to the transmitter.

Your workup is correct enough for the case you describe (though for
Pf/Pref=2, VSWR=5.8).

Of course, if you had a coaxial reflectometer calibrated (nulled) for 8.6
ohms or 290 ohms then you would get the same indications on a 50 ohm
load, you don't actually need the 8.6 ohm or 290 ohm transmission line.

These are just examples that question the reality of these "component
powers" when you can change their magnitude by choosing the reference
impedance for measurement or calculation. They reinforce the view that
whilst Pf-Pr has meaning (irrespective of Z), Pf and Pr each alone have
no meaning.

Owen

Hi Owen,

(Noted your other response, to my posting, about the assumption of 50
ohm line. I obviously didn't make that assumption...)

I've been harping in this forum for YEARS about the need to calibrate
an SWR meter or a return loss bridge to the impedance you're wanting
to use it for. I'm always somewhat amazed that some folk just "don't
get it." Thanks for helping reinforce the need for the proper
calibration. Those bridges are not magic; they can only measure
voltages or currents produced by (hopefully) linear combinations of
transduced line voltages and currents. I think it was about ten years
ago in this group that I posted the derivation of the equations to
determine forward and reflected from measurement of voltage and
current at a point on a TEM line, IF you know the impedance of the
line.

Cheers,
Tom