A Little Antenna Modeling Help Needed

[KB3DKS]

 I am trying to extend the coverage of my inside dipole to 160 meters and have been having somewhat favorible results.
 To explain for those that I have not talked with on the air, my 75 mtr. ant is a 1/2 wave 120 ft. dipole strung across the windows inside the fourth floor of an old brick mill building that is in a commercial location and without any surrounding objects to string up a wire outside. The inside measurements are 100 ft X 120 ft with the 75 mtr wire running the 100 ft broadside N-S plus 10 ft of dropped wire on the ends. This has been working very well, all things cosidered.
  Now I added another 120 fot of wire, 2 X 60 ft. at right angles so the entire ant is now a horizontal U shape of 70 X 100 X 70 with the coax feed point in the center of the 100ft section.
  It has been somewhat sucessfull and loads up with flat SWR on 160.
So far so good.
  BUt I have been receiving requests for more modulation. My off air monitor, a 12'' loop connected to my O-Scope, shows good 100 % mod and base line hits with sybilance.
  What I am suspecting is that due to the parallel 70 foot wires that there is some phase cancellation going on with signals received broadside of that direction.
  So, could someone model a horizontal dipole configured 70 X 100 X 70 and send or post it for me?
Please.
Bill, KB3DKS in 1 Land