You can get commercial single band antennas and combine them with a diplexer or triplexer.
For example, you could use a 6dBD gain 4-bay dipole array to cover the entire public service VHF band, a dipole array or Stationmaster for UHF and there are plenty of surplus Stationmaster type antennas around for 800MHz with 10dBD gain. Combine all these with low loss using a Comet or Diamond Triplexer and your scanner will probably overload but you will have lots of gain on all scanner bands.
prcguy