High gain VHF/UHF antenna for airband recommendations?

It’s in my post: 30’ AGL (30 feet above ground level) i.e. 2nd story roof + 10 foot fence pole. I’m well aware that “higher is better” (I’ve been listening to scanners for 13 years), but my chimney is over 100 years old and the guy who sold me the mount said with brick chimneys that old, anything over 10 feet will make them topple, so more height is NOT an option. (I also have 2 TV antennas on the same mount).

So which antenna should I get?