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
