> of reflected power, because your load on the Rx port gets
hot. It
> doesn't much depend on length of transmission line. The
typical SWR
> bridge works this way, comparing power flow in the two
directions.
Not really.
The typical SWR bridge compares the voltage across the
transmission line to current through the line. Every bridge
we commonly use works that way.
It does this by summing the voltage across the line with a
voltage directly derived from current through the line. The
ratio of sensitivity between those samples is set by the
bridge null. When the null is set properly exactly the same
RF voltage appears from the current sample as appears from
the voltage sample, and since they are 180 out of phase the
result will be zero voltage.
The forward power is from the in-phase sum of the two. Into
a perfect load 9where the null is set) it is double the
voltage produced by line current and line voltage.
When there is reactance the phase between voltage and
current shifts and the voltages are unbalanced and increase,
just as they do when there is a change in resistance from
the null value.
The typical directional power meter or SWR bridge really
just compares the across and through vectors. That's why it
is called a bridge. Directional coupler, bridge, they all
work this way. The exception is a long line where we
measure or sample voltages across the line at various
distances, measuring only the same parameter.
We can make up a big story about forward and reflected
waves, and that model works within limits of the model, but
the circuit itself works the way I described above and we
don't need wave mechanics to see why.
73 Tom
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