A scanner lets you listen in on the fascinating worlds of police, fire brigade and air traffic control radio systems. Most scanners come with a small antenna attached; adding a larger, homemade antenna can improve your scanner's reception.
An add-on homemade antenna lets you hear radio transmissions from farther away and increases the audio clarity of these transmissions. Building your own antenna is, of course, cheaper than purchasing a commercially manufactured one, and helps you learn how your scanner works.
Different homemade antenna designs have different strengths and weaknesses. A groundplane antenna provides good, omnidirectional reception, but takes up a lot of space. J-pole and dipole antennas are small and easy to build, but have weaker reception. A unidirectional Yagi antenna can focus in on a faraway transmission; however, unless the radio transmitter is stationary and you know where it is, you might miss it.
Calculate the length of your homemade scanner antenna in relation to the frequency of the transmitters you usually listen to on your scanner. If you're putting your antenna up outdoors, keep it away from power lines, trees and any other obstructions. Make sure that the scanner you're using is legal in your area.