NY -- Standard&Poor's CreditWire 11/21/97 -- Standard&Poor's today assigned its double-'A'-minus rating to Portage, Mich.'s G.O. bonds series 1997, totaling $2.2 million. Portage's outstanding double-'A'-minus rating was affirmed. The rating reflects the following strengths: -- A diverse economic base, -- Above-average income levels, and -- A solid financial position. Offsetting factors include a limited tax pledge, some economic concentration, and high carrying charges. Portage is located midway between Chicago and Detroit. The importance to the local economy of Pharmacia&Upjohn Inc., a large pharmaceutical manufacturer, has been underscored by the recent announcement of the transfer of 600 headquarters jobs out of the area. Upjohn is by far the largest employer in the city, with more