Not too surprising, the people who designed the radio knew what they were doing when they spec'd the antenna.
This antenna does a good job on VHF through 900mhz, and anything you get that has gain in a SPECIFIC range is going to do WORSE in other ranges. Just use the one that is on it. Save your money. I have found that the 800 mhz antennas don't do nearly as well in the VHF air band, which I enjoy listening to. Some of the most interesting stuff is said between the police helicopters around here, which use this band, and talk quite openly when on these frequencies. I even did a bit of VHF Low listening in California recently, and you can forget that range with an 800 mhz antenna.