The isolation between the input and the isolated ports may be different from the isolation between the two output
ports. For example, the isolation between ports 1 and 4 can be 30 dB while the isolation between ports 2 and 3 can be a
different value such as 25 dB. If both isolation measurements are not available, they can assumed to be equal. If neither
are available, an estimate of the isolation is the coupling plus return loss (see VSWR section). The isolation should be as
high as possible. In actual couplers the isolated port is never completely isolated. Some RF power will always be present.
Waveguide directional couplers will have the best isolation.