dansawyeror wrote:
Do the measuring ports of a bi-directional coupler accurately represent
or preserve the relative phases of the signal?

Let's look at a typical SWR meter sampling circuit. The current is
sampled by a one turn primary on a ferrite toroid. The voltage is
sampled by a tap on the line close to the point at which the toroid
is mounted. At HF frequencies, a wavelength is so long compared to
that configuration that physical sample point errors are usually
considered to be negligible. That obviously changes at UHF+.

No coupler 100% preserves the relative phases. The question is:
What is the accuracy? For any configuration, a worst-case accuracy
can be specified. At 4 MHz, it's not a problem. At 4 GHz, it's a
big problem. At visible light frequencies, most don't even try.
--
73, Cecil http://www.qsl.net/w5dxp