In message , Ian Jackson
writes
In message , rickman
writes



When you tune for VSWR of 1:1, the impedance of the TX side of the ATU
will match the TX. You can't say it is 50 ohms unless the TX is 50
ohms, no?

No, no, no.

As I keep saying, the reading on the SWR meter has nothing to do with
the output impedance of the TX feeding RF into it. It is determined by
the reference resistors in the meter's directional coupler circuits and
the impedance of the load attached to its output. If the load is 50
ohms, a 50 ohm SWR meter will read 1:1, regardless of the TX output
impedance.

The purpose of the ATU is not to match the TX output impedance to the
outside world. It is to convert the impedance of the outside world to
50 ohms - which is the impedance the TX is designed to work into.

And just to illustrate the point, consider the following:
You have a TX with a 50 ohm output impedance feeding through a 50 ohm
SWR meter into 50 feet of 50 ohm coax terminated with a 50 ohm resistor.
What does the SWR meter read, and why?

You then replace the TX with one with an output impedance of 25 ohms.
What does the SWR meter read, and why?

Finally, you replace the TX with one with an output impedance of 100
ohms. What does the SWR meter read, and why?
--
Ian