As a matter of interest, here is a response I received from Holl_ands to a similar question I posed on the AVS Forum:
"It isn't so much the bigger reflector on the CM4228 that extends performance into the upper VHF band,
as it is the center feed structure: http://www.solidsignal.com/prod_dis...zoom=zoom#xview
Look at the individual view angles to see the detailed connection.
For UHF frequencies the "quarter-wavelength" feed structure combines the signals from eight individual
bowtie elements, with impedance matching as best as can be expected for a wideband antenna system.
However, for VHF frequencies the feed structure is only a small fraction of a wavelength,
so the entire antenna structure acts more like one very wide (and fat) center fed dipole
(with a reflector screen that is closer than optimum spacing).
If you simply use an RF Combiner (even a low loss Lindsay Stripline Combiner) to combine two CM4221's,
the VHF performance will be considerably worse than a CM4228"