On 10/6/2015 3:26 AM, Ian Jackson wrote:
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.

So there *can* be reflected power back into the TX by the ATU if the
impedances don't match. I guess this is not so much an issue in that
the TX is never supposed to drive any other impedance. Rather than talk
about power reflected back into the TX, the power delivered into a 50
ohm load is considered the point of reference. But if a resistive load
of less than 50 ohms is connected to the output, it would dissipate
*more* power than the 50 ohm load would. If the TX output were also 50
ohms, max power would only be delivered into a 50 ohm load.

--

Rick