On 12/14/2013 2:24 PM, Jim Lux wrote:
No, they use some sort of transformer SWR bridge, with diode detectors
(and some use a phase detector too(SGC), to know which direction to
tune). It's not a directional coupler in the sense that there's a
sufficiently long line in terms of wavelengths and you couple off the
traveling wave like you do in microwaves.
Yes, I know. I sometimes get sloppy with my terminology (mea culpa). In
this case, it would be incidental to the function of the auto tuner
whether you use an SWR bridge (Bruene circuit) or a true traveling wave
directional coupler to determine VSWR. The results would be the same.
More like a clever transformer scheme as seen in a raft of SWR meters.
These things measure forward and reverse power, but don't really have
a characteristic impedance in and of themselves. What's 50 ohms about
them is that the "calibration" is correct for 50 ohms.
Yes, the calibration Zo is determined by the ratio of the capacitive
coupling (voltage sample) to the inductive coupling (current sample).
Think about it.. how are you going to build a real directional coupler
that's 1" long for 160 meters? What you can do is build a form of
hybrid which has two outputs which are related to the forward and
reverse power.
That depends on what you consider a "real" directional coupler. You can
build very small "directional couplers" that work across a broad
frequency range with ferrite transformers. If I gave you the
S-parameters for one of those and S-parameters for a parallel
transmission line coupler, you would be hard pressed to tell which one
was "real" and which one was "fake".
Sort of.. They have a routine that takes the ADC values for Fwd and
Rev and turns that into a number that is similar to VSWR, and they
minimize that. I don't have the listing handy, but it's soemthing like
(Vfwd-Vreverse)/fwd, and drive it to zero. The scaling from Vfwd to
actual forward power may change, but optimizing this will get you to a
match.
Since they never need to display actual VSWR, they don't bother to
normalize it or anything. They just have a known threshold for when
to stop adjusting.
Yes, but it is normalized to a specific impedance in the sense that the
VSWR bridge is calibrated for a certain Zo. If I put 100 ohm
transmission lines on either side of a VSWR bridge calibrated for 50
ohms and I terminate the load side 100 ohm transmission line with 100
ohms and drive the source side with a 100 ohm source, the bridge will
still read 2:1 VSWR even though both the input and output lines are
matched (i.e. no standing wave on the line). A travelling wave 50 ohm
coupler would do the same (i.e. show a ~10% power reflection on the
reverse port) if I installed it in place of the VSWR bridge in the 100
ohm matched system.
If the auto tuner were part of the amplifier, then it would be possible
for the tuner to using additional information (like drive power and DC
current) to account for the changing amplifier output impedance.
73, Mike W4EF............
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