[Amps] bi-directional coupler question

[Dan Sawyer]

All,

This journey started several months ago with the purchase on an 8405a to 
measure an antenna. One of the items listed at that time was a 
bi-directional coupler. Finally I have found one and am beginning to 
experiment. The first readings seem to show that the coupler may be 
supplying readings that are out of phase? Direct input to the coupler 
terminated in a 50 ohm dummy load produce readings that appear to be 90 
degrees out of phase. Is this correct? Is it a coupler design issue? I 
am a novice in this area so I am not sure what is normal and what is not.

Is this one of those cases where the 8505 is biased to read 0 phase 
angle into a known termination and then read the difference in the 
circuit under test?

Thanks in advance - Dan