Measuring balanced 150Ohm network with Vec. Net. Analyzer?

Anyone know the best approach to measuring a balanced network (150 Ohm) on a standard Vector Network analyzer (50 Ohm) in the 400 to 900MHz range. My "best" results so far used .047" coax choked around a torroid (#61 mat) and terminated with a minimum loss pad (50 to 150 Ohm).

There is still some response change (S21) when I touch the outside of the coax after the choke. I could only get 4 turns of about .2" dia thru the torroid.

http://www.highfrequencyelectronics....103_Henkes.pdf