Richard Harrison wrote:
Cecil, W5DXP wrote:
"The reflection from surface "A" is canceled by an equal magnitude and
opposite phase reflection from surface "B:."

Is this not analogous to what happens on a short-circuited 1/4-wave
stub? The hard short reverses the phase. That, combined with travel to
and from the short, produces a total phase rotation of 360-drgrees.
The result is that the open end of the short-circuited stub, the
incident voltage is in-phase and of the same magnitude (no stub loss) so
that no current flows between the incident and reflected sources.

No net current flows but the forward current and reflected current
are constant RMS values as are the forward and reflected voltage
values. The virtual impedance at the mouth of a lossless 1/4WL
shorted stub is (Vfor+Vref)/(Ifor-Iref) where Vfor/Ifor = Z0
and Vref/Iref = Z0. Since Ifor=Iref for a lossless stub, the
impedance is zero. But the current is quite high at the shorted
end of the stub where it is Ifor+Iref.

You can estimate that current if you measure the voltage at
the mouth of the stub. 0.5*V/Z0 will yield the estimated forward
or reflected current.

It is as if one connects identical battery cells ib parallel. The
impedance is, in effect, infinite between sources of identical voltage.

Optical experts must have siezed upon the opposite of this somehow.
Their quarter-wave must have ben terminated in the equivalent of an
open-circuit. This 1/4 wave would accept 100% of light presented at its
surface, or would it need to present 377 ohms at its surface?

The phase of reflections follows a different convention in optics.
And the index of refraction is inversely proportional to Z0. But
a 1/4WL of thin film is akin to 1/4WL of transmission line used
as a series matching section - not parallel but series. This is
essentially how non-glare glass works.

source---50 ohm coax---+---1/4WL 61 ohm coax---+---75 ohm load

50/50 = 1.00, the index of refraction for air
61/50 = 1.22, a good index of refraction for a thin-film
75/50 = 1.50, a good index of refraction for glass

I am ignorant of optics and find the analogy difficult to understand.

I've learned more about reflections and superposition from "Optics",
by Hecht, than from any other single source. I highly recommend it.
--
73, Cecil http://www.qsl.net/w5dxp

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