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How do FM Antennas Work

Read on to know how FM Antennas work.
Omkar Phatak Sep 26, 2018
Antennas make telecommunication possible. They can be any piece of metal, that respond to incoming waves, with electrical signal generation and also transmit signals.
If you are the kind of person who likes experimenting and building things on your own, then building an FM antenna will be an enjoyable experience.

Working Explained

An antenna is a type of transducer, which can create electric currents, in response to the reception of electromagnetic waves (receptor function) and create an electromagnetic output, in response to conducting electric current (transmitter function).
Precisely, any antenna is an arrangement of metal conductors. When an alternating current passes through it, 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 how a signal gets transmitted. During reception of an incoming electromagnetic wave, the exact reverse of the above phenomenon 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 the rules that will help you in deciding the dimensions and design of your antenna. Its length must be comparable to the wavelength dimensions of the incoming FM radio signal.
Divide the speed of light (3 x 108 m/s), by the frequency of the wave in Hertz. The resultant, is the wavelength of the wave in meters. Your antenna should have a 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 the 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 2.77 m to 3.4 m. So, its dimensions should be comparable to this size, but they need not be, of the exact same size.
One important parameter in the design is the gain it offers. It is the measure of how strong a signal, an antenna can 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 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 one is a requirement, when the signal strength in your area is not that strong. If the strength is strong enough, then an omnidirectional antenna, with no preferred direction, should do the trick.

Types

An antenna is like a net, which is designed to catch a particular type of 'frequency' 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 antenna to the radio, some insulator rods (wood or plastic), and sticky tape.
The quality of the reception of signal will highly depend on the geometry of the arrangement of the insulated wire, on the insulated rods. There are a number of effective designs for the same.

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 in a central insulator cross. It's omnidirectional but has a low gain.

Multi-Element Arrays

These are antennas with multiple conductor elements, that are mounted parallel on a conductor. An example is the Yagi antenna, often used for television reception. This type of antenna is highly directional and provides a high gain. They are 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.
Try designing the antenna, as designing your own stuff is not only a lot of fun, but you get to learn a lot in the process.