3" or so should be the resonant frequency. You may try several to see what works best in your specific situation though. The higher in frequency you go the shorter the antenna should generally be. For example, for 150 MHz the length would be about 18", for 450 MHz it would be about 6", and for 800 MHz, as indicated, it should be about 3". These sizes are based on the standard 1/4 wave antenna. Other designs will use different lengths and have different signal patterns.
Last edited by n5ims; 11-16-2012 at 1:43 PM..