John Smith wrote:
If the antenna is not 50 ohms, or the balun from 450 to 50 at the antenna is
missing, then part of the power is reflected back into the Tx output.

Just as I drew it, with a 1:1 balun, the system is Z0-matched. No
reflections reach the TX output. Saying a 9:1 balun was a huge mistake.
That's not what I meant. I certainly meant a 1:1 balun.

The optics example is good, as the anti reflective coatings are 1/4 wave
thick, and the coating constant (speed of light in the material) is the
square root of the product of Air and Glass, it is an optical matching
section. Need a coating on both sides of the glass.

Likewise, you can put 1/2WL of thin-film between air and air and still
have a match, i.e. no glare. That's what I did in the following diagram:

100w XMTR--50 ohm feedline--x--1/2WL 450 ohm line--50 ohm load

similar to:

Laser----air-----|---1/2WL thin-film---|---air---
--
73, Cecil, W5DXP