Hi Rich,
> ? We are measuring power into a 50-ohm termination, Tom.
You are, I am not. Some of my antennas are real close to 50
ohms, some are not.
> >You read reflected power near full scale using the proper slug, and
> >subtract reflected power from forward. The result is the actual power
> >keeping in mind the directivity error of the slug and the calibration
> >errors.
> >
> ? I don't believe that its all that simple.
>
> >> >There is absolutely nothing wrong with using a good meter. A few
> >> >people have wrongly assumed a storage system can't respond fast
> >> >enough, but that is certainly not correct. The storage system has to
> >> >track the envelope change rate that follows audio, not the RF cycle
> >> >change rate. A meter with directional coupler also remains accurate
> >> >over reasonable load impedance errors. You simply deduct reflected
> >> >power from forward power.
> >> >
> >> ? Can you explain why this is true?
You can look at the problem two ways. As a circuit problem, or as
reflected waves. Either way works.
As a circuit, voltage difference between the center conductor and
ground and a voltage induced by the current flowing along the
center conductor are summed in the detector at radio frequencies,
so even the phase is detected. In the voltages derived from each of
these sources, if you flip the slug around, phase of the voltage
stays the same but phase of current reverses. One way adds and
one subtracts. The ratios of those voltages are carefully made
equal when the directional coupler is properly terminated by nulling
the detected voltage when the reflected power is measured. When
you read forward, they exactly add.
When you read forward power, you read the VAR (volt-amperes-
reactive) power.
When you reverse the slug, you read the out of phase VAR.
If you subtract the two, you are left with the real power.
The other way of looking at the problem looks at it in wave theory.
The directional coupler samples outgoing power in one direction,
and reflected power in the other.
Outgoing power readings include both forward and reflected power
that was "bounced" off the PA output port. That's why directional
coupler wattmeters read higher when the load has a high SWR and
the transmitter is matched to the new feedline impedance. Reverse
power reads only the reflected power, so you can simply subtract
the reflected power out from the forward reading and you have true
forward power.
Either way works, both models are OK. Virtually all textbooks
explain it more accurately and in better detail, but that is roughly
what goes on.
If I'm using my Bird 4391 to read peak power, I just read peak
reflected power stored in memory and subtract it from peak forward
power stored in memory. The result is true power, within the
accuracy of the instrument.
Some wattmeters do this automatically.
73, Tom W8JI
w8ji@contesting.com
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