Reg, G4FGQ wrote:
"Consider the tuner, an impedance-transforming network. It is clear that
the insertion loss of a correctly operating tuner is in fact a "Gain"
and differs considerably in dBs from overall system loss before it was
installed."

It`s early morning in England so Reg is likely snoozing. He probably
wanted to instigate before turning in. I`ll bite. The decibel (dB)
expresses gain, loss, and relative power levels. Its logarithmic
character allows addition and subtraction to keep track of system gains
and losses.

An amplifier can increase power so it can be a +dB insertion in a
system. The amplifier is an active device. A tramsformer can change
volts and amps by transformation of impedance (V/I), or the transformer
can give isolation between circuits.Because the transformer is a passive
device, it can`t be a +dB insertion in a system. It must be a -dB
insertion in a system despite a "voltage gain" in some cases. A good
transformer usually is very efficient, so its power loss is small, but
it`s still a loss. The same is usually true of an antenna tuner.

Best regards, Richard Harrison, KB5WZI