I wrote in a prior posting:
"The circulator I suggested in an earlier posting is a variation on a
device described in a couple of books I used to have on amateur radio
repeaters."

I still have not come across the repeater books but my 1987 ARRLHandbook
has such a circulator (called a hybrid ring) in Fig. 15 on page14-6, A
short escription is on the previous page. How it works is more
completely described in the repeater books.

Best regards, Richard Harrison, KB5WZI