please explain somebody what it means I am totally confused.
welcome to PF
its perfectly matched where there is no reflected power from the interface of whatever the waveguide is terminating into
this goes for any transmission line
of course a perfect match would be near impossible to obtain and we would always have to put up with some reflected signal
which is why in RF transmission lines, coax or waveguides, we use circulators to terminate reflected power into a dummy load, rather than letting it go back into the transmitter where it will produce all sorts of nasty intermodulation products
if you need a better explanation than that, then maybe ask a more specific question
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