So, a 50 ohm attenuator pad does not act as a terminator? I need to figure that out. Wouldn't it be similar to using a pass-thru terminator on the probe input to the scope?
Any pad above 10dB will look pretty much like a 50 ohm load regardless of whats after the pad (> 20dB return loss ).
So, at the expense of sensitivity, adding a pad to the coupled port is one of the best ways to ensure the coupler sees a good load.