On 12/14/13 3:33 PM, Michael Tope wrote:
On 12/14/2013 2:24 PM, Jim Lux wrote:
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".
Yes, but I don't think the things in the tuners are that sophisticated.
They're typically a single transformer, and some resistors and diodes,
with a couple pots.
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).
Would it... I'm not sure about that with these bridges. I'd like to try
the experiment (or maybe this is a good reason to get MacSpice working)
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.
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