On Mon, Dec 30, 2024 at 01:58 AM, G8HUL wrote:

The Tx does not see an SWR it sees an impedance, it is the transmission line
that has the standing waves and that must be based on the impedance of the
line NOT the load that the Tx 'likes to see'.

Yes it is the transmission line that has the standing waves on it. However as I stated in my previous post the transmitter power amplifier is designed for a specific load which is 50 ohms for ham transmitters. Modern solid state transmitters contain a circuit that calculates SWR based on the load impedance at the output connector on the Tx. The Tx circuitry cuts back on the output power as the SWR rises.

The amount of reflected power will depend on the mismatch between the line impedance (75 ohms) and the
impedance of the load. The design impedance of the Tx is irrelevant.

Yes the mismatch between the line impedance and the load is what determines the VSWR at the load and the reflected power. However the design impedance of the transmitter is relevant. This TX source impedance will determine how much of the reflected power will be reflected back to the load. If it is 50 ohms none will be reflected back but transmitters are not designed with a 50 ohm source impedance.

Leaving the settings at 50ohms may well give you an equivalent SWR of the
impedance that the Tx sees, but tells you nothing about they standing waves on
the 75ohm line. For Example, if a 1:1 SWR was indicated with a 50 ohm setting
you might believe that there were no standing waves on the 75 ohm line, which
would not be correct.

Yes that is true but the transmitter will be able to output its full rated power with the SWR = 1. And if the transmission line used has low loss there will be little power lost in the transmission line.

Roger