Hi Joe
You seems need an S21 Logmag measurement , Knowing that dynamic range of H4 on the UHF are limited to 50db you should not expect more then -45db values for the isolation measurements between the Two bands, obviously you need a 401 sweep points , and many complete 401 point calibrations for each bands (with isolation and Throught) , each calibration should be focused on the desired slice of the band under test, you need to reset the old calibration for each new calibration to be sure to avoid interpolation errors ,
Good Luck.
73's Nizar
