If memory serves, the BC246 is not milair (255-400 mhz) capable. A better antenna is in order. That does come with a caveat, however. You see a little handheld cannot deal with the amount of signal a large antenna such as a discone or scantenna might deliver. Overloading (which can show up on the air bands as hearing pagers, FM stations, and other junk where it doesn't belong) is a real possibility. There are filters available that can cut this out, but it increases your costs.
You didn't say anything about how you are using the handheld, or where. Are you using it at home? While driving? These will influence the decisions you need to make about the antenna(s) (and coaxial cable, if you're putting something outside) you will need. Height - getting the antenna away from buildings, trees and other obstructions - and feeding it with the right cable are important.
If you are using a scanner at home just for aircraft (VHF/UHF), you would likely be much better off with an older desktop such as the BC780, 785 or 795. Even some of the older RS PRO models were quite good on VHF, and some covered UHF milair as well. Scanners like the BCT15 cover milair as well as civil air, and I've seen some photos recently having them mounted in a car. I'm not certain that the PRO-2055 (the mobile version of the PRO-97) would fit as well. Older handhelds such as the Uniden BC250 and 296 ought to do well, and since they've been recently discontinued, you could probably get them rather inexpensively. Of course, the BR330 was also recently discontinued, and it too will handle both civil and milair.
Our wiki has lots of information on other scanners, antennas and such. Time for a little reading, and you can start here (anything in blue and/or underlined is a link - both here and in our wiki)
http://wiki.radioreference.com/index.php/Milcom_Receiving_Equipment
73 Mike