Tom Rauch wrote:
> > That is what I said from the very start, "there can't be any power if
> > there is no current in the line". Yes I understand subtracting
> > reflected from forward power readings and agree with that. What I am
> > questioning is how to determine reflected power in the line.
> >
> > I just tried another test with a :
> >
> > transmitter-- watt meter-- tuner-- drake watt meter-- capacitor-- 50
> > ohm load.
> >
> > By adjusting the capacitor in series with the load I can get a complex
> > impedance seen by the drake watt meter. I then adjust the tuner so the
> > first watt meter shows zero swr. With that setup with a particular
> > capacitor setting I read 30 watts into the tuner and on the drake
> > meter I read 100 watts forward and about 65 to 70 reflected.
> > Subtracting reflected from forward does give the approximate forward
> > power. (30 watts)
>
> OK, just as it should. You have proven the meter is reliable for
> reading true power when reflected is subtracted from forward power.
>
>
> > But the question is how much reflected power is there really. There
> > can't be 70 watts because the transmitter is only putting out 30
> > watts.
>
> If you prefer circuit theory rather than wave mechanics, consider it
> VAR (volt-amperes-reactive) power. VAR power is simply current
> times voltage or apparent power.
>
> The reactive part causes heating and loss, and does no useful work.
>
> > I am guessing that as the line impedance gets nearer to the watt meter
> > design impedance that the directional coupler part of it starts to
> > have enough isolation to give a valid reading, but where does that
> > start?
>
> At any impedance, assuming the coupler is designed and
> calibrated perfectly.
>
> In measurements I have made, a Bird meter works pretty well even
> with a 300-ohm load.
> 73, Tom W8JI
> W8JI@contesting.com
I don't think that is right. I did another test with the same setup but this
time I used only a 1000 ohm resistor for a load and no capacitor.
Transmitter--watt meter--tuner--drake watt meter-- 1000 ohm load resistor.
By adjusting the tuner for zero reflected power on the first watt meter that
leaves the drake watt meter and a 1000 ohm resistive load that should have
no reflected power.
With 20 watts indicated on the first watt meter the drake watt meter showed
about 100 watts forward and about 80 watts reflected. The difference in
forward and reflected readings is the power delivered to the load. But there
should be no reactive component in the load in this setup. There should not
be any reflected power from the load. There should be no "apparent power".
The reading of high forward and high reflected power on the meter has to be
meter error when not operating at the design impedance.
This brings us back to the question of how do we determine how much
reflected power we have, if any, when looking at a watt meter?
In this case with the 1000 ohm resistor there should be no reflected power
in the circuit. This indicates a gross error in the meter reading.
73
Gary K4FMX
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