K7ITM wrote in
ups.com:

....
If this is in error, I'd really like to know about it, because it
affects something I'm working on.

Interesting findings Tom.

The way I think of these couplers is that you are trying to sample V and
I at a point on the main line, and a longish coupler of that type departs
from that ideal.

The effect I observed, and in several instruments, was obvious and
repeatable. I wonder that if the length of the lines is not the cause, if
it was the untidiness of the way in which the detector circuit was
implemented at each end of the coupler section. Of relevance also, is
that insertion of the instruments also caused significant SWR (1.2 in
the case of one of them) at the extreme uppoer end of their specified
range. IIRC two of the instruments had no equalisation / compensation,
they had a resistor at one end of the coupled line and a cap/diode at the
other end.

I still have one of the things that did this, and I have since nulled it
for 75 ohms, but I will have a play with it when I get home next week.

Owen