Jeff wrote in
:

As a matter of interest why are you looking at 75ohm cable, when most
people and equipment use 50ohm.

On the basis of that logic, what possible use could there be for 400 ohm
transmission line, and why then is it so very popular?

In the case of the OP's questions, IIRC they relate to a RO application.

To enlighten you, high performance noise optimised ham receivers for
microwave bands are oftenm if not usually designed for a specific input
impedance that is quite different to 50 ohms... yet we use them with 50 ohm
transmission linees.

If you think the choice of 50 ohm line is a no-brainer, you are probably
right.

Owen