Topband: TRANSFORMER QUESTION

[Mr. and Mrs. Magoo]

I recently built and installed a 2:1 matching transformer on a pair of 
phased 750 ft Beverages.  The antenna works extremely well on 160 and 80, 
but I noticed that it is not flat much above 5 mhz, unlike the 9:1 
transformers which I use on all my Beverages.  The transformer design I used 
is the input transformer from W8JI's "magic T" coupler and employs the 
BN202-73 binocular core.  I wound up a few of them and they all exhibit the 
same response.

Anyone tell me what might be going on here?

Bill, VE3CSK