Google is your friend for that. Just a word of caution: As soon as you start messing with outdoor antennas you've got to worry about grounding (lightning!), corrosion, getting the cable into the house while keeping the weather out, and possibly planning permission and that sort of stuff. (Not to mention permission from your significant other...
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Airband is VHF and thus line-of-sight. Due to the curvature of the earth, which limits the Designated Operational Coverage (DOC) of an ATC station, this means that the aircraft you are listening to will rarely be more than 300 km away, and usually much closer than that. The transmit power in any airband radio is such that reception is not a marginal business at those distances. So the difference between an indoor antenna and an outdoor antenna, both more or less tuned to airband frequencies, will not be noticable. Because of that, I don't think it's worth messing about with outdoor antennas for airband reception. (Unless your house is a Faraday cage, in which case you've got no choice.)
The major improvement in reception comes from replacing the tiny whip antenna, which has to be a relatively small form factor in order to be portable, and has to be tunable to all frequencies that your scanner might be able to receive, with something that's specifically tuned for airband.
On the other hand, if you want to experiment with frequencies that are not line-of-sight but follow the curvature of the earth (HF for instance), a good outdoor antenna will make all the difference.