Antennas are what make telecommunication possible. An antenna is any piece of metal that responds to incoming electromagnetic signals and generates electrical signals in response and can also transmit electromagnetic signals. If you are the kind of person who likes experimenting and building things on your own, then building an FM antenna will certainly be an enjoyable experience.

How do these Antennas Work?
  Before we go into design considerations of FM radio antenna, let us understand the basics of how does an antenna work. As said before,an antenna is a type of transducer which can create electric currents in response to reception of electromagnetic waves (receptor function) and create an electromagnetic output in response to conducting electric current.

  More specifically, any antenna is an arrangement of metal conductors. When an alternating current is passed through such an antenna, the electric field associated with it, changes. This changing electric field creates a magnetic field, and the changing magnetic field creates an electric field back again. This is how an electromagnetic wave is created and transmitted by an antenna. This is called the transmission of a signal. During reception of an incoming electromagnetic wave, exact reverse of the phenomenon above occurs. The incoming electromagnetic wave creates an alternating electric current inside the antenna in response. This is exactly how an FM antenna works. It creates an alternating current output in response to an incoming frequency modulated (FM) electromagnetic signal. However, you cannot simply take any random piece of metal of any length and create such an antenna. There are certain design considerations and rules that need to be taken care of.

Design Considerations
  There are certain fundamental physical principles on which an antenna design is based. Following are the rules that will help you in designing the dimensions and design of your antenna. The length of the antenna must be comparable to the wavelength dimensions of the incoming FM radio signal. The length is calculated in the following way. Divide the speed of light (3 x 108 m/s) by the frequency of the wave in Hertz. The value you get is the wavelength of the wave in meters. Your antenna should have length comparable to it. So, decide on the FM station frequencies that you intend to tune into and calculate the length of the antenna in accordance to the wavelength of tuning frequency. The FM band extends from 88 MHz to 108 MHz in North America. So, the length range for your antenna theoretically extends from 2.77m to 3.4m. So, the antenna dimensions should be comparable to this size but they need not be of exactly same size.

  One important parameter in antenna design is the gain it offers. The gain is the measure of how strong a signal can an antenna generate in response to a received signal. FM signals are in the VHF (Very High Frequency Range) and therefore, they can only be received if the antenna is within line of sight, which is about 30 miles from the broadcasting point. This brings in one more antenna design aspect, that should be taken into consideration. It is that of directionality. Your antenna needs to be designed in such a way that it can catch FM signals from a particular direction more strongly. A directional antenna is a requirement when the signal strength in your area is not that strong. If the signal strength is strong enough, then an omnidirectional antenna, with no preferred direction, should do the trick.

Types of Antennas

An antenna is something like a net which you must design to catch a particular type of fish. It has to be the right size and laid out right. All you need to build an FM antenna is insulated wire, 300 ohm ribbon feeder cable to connect your antenna to the radio, some insulator rods (wood or plastic) and sticky tape. Depending on the geometry of arrangement of the insulated wire on the insulated rods, the reception quality of your antenna will vary. Here is a list of the most common and effective designs used for antennas:

*Unidirectional Dipole: When FM signal is directed from one general direction, a unidirectional dipole antenna design is the best. It consists of two wire elements placed collinear to each other.
*Crossed Dipole: This consists of a square shaped conducting wire that is arranged on a central insulator cross. It's omnidirectional but has a low gain.

* Multi-Element Arrays: These are antennas with multiple conductor elements mounted parallel on a conductor. An example is the Yagi antenna often used as a TV antenna.This type of antenna is highly directional and gives a high gain. Well suited when the FM sources are far away from your home.

*Half Wave Vertical: This consists of a single piece of vertical wire that is about half the wavelength of the FM signal to be tuned.

Using any of the designs above, you can create your own homemade antenna. Designing your own stuff like this is not only a lot of fun, but you get to learn a lot in the process!

Read more information about FM antenna,http://www.fmuser.net/list/?118_1.html

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