From your operating frequency, work out the wave length ( Wl = 300,000,000/F, F in Hz, W in metres). Then you can build your antenna using the Wl you have just calculated. Every Ant has a different charecteristic, basically the more work involved the better it is. The simplest ant. is a vertical piece of wire Wl/4 long. If you put a top horizontal section, this "tunes" the aerial down in frequency (it looks longer then it is) so this is what people used for AM reception because they could not get a 75 meter vertical wire up!!!
Frank