Most radios have a 50 Ohm antenna input. At VHF freq. a 1/4 wave antenna matches this. A 1/2 wave antenna would need a matching coil added to bring it to a 50 ohm impedance that the radio needs. The better the match, the more effective the antenna. Although not important on a receiver, if you transmit on a mismatch antenna, you risk damage to the radio. As far as horizontally or vertically, ideally it should match the transmitting antenna. Most mobile radio antennas are vert. polarized. Antenna height above ground is far more important for reception at VHF freq. than lenght of the antenna. Also use a good feedline to feed the antenna. http://www.cebik.com/radio.html Has some antenna information Robert
Josh, If this VHF set is only a reciever, you can pretty much use what you like on it. However, something that is resonant at the frequency you want would be good. I can't remember if Aeronautical signals are Vertically or Horizontally-Polarised, however, there is a 20dB loss in signal if you have the wrong polarisation. I have an idea that it's Horizontal (ie: with the elements flat, as opposed to perpendicular to the ground). If you are going to use any sort of dipole antenna, you'll need a balun to match the antenna to your coaxial cable, without one you'll lose signal. Hope this helps.
Back in my RF Engineer days I used to be a rubber necker ( a jerk with a scanner listening in on phone calls or what ever). One good set up is a YAGI antenna with two rotors. One rotor that turns a full 360% to cover the compass for diretional pointing, and another 90% rotor for polarization between vert-horz.
But that is a bit elaborate, what you can do on the fairly cheap is buy an omni directional circular polarization antenna. No rotor needed and works for either vert or horz polarization. You could even make one out of copper tubing. ARRL has lots of designs