A folded dipole antenna is a common antenna design that has two elements, properly called poles, which are usually made of a rigid material. The two poles have one end near each other, then extend away from each other, and then are turned back, or folded, toward each other until they nearly touch. The folded dipole antenna design is a full-wavelength type of antenna used most commonly for FM radio reception.
Dipole antennas are designed to receive radio frequency transmissions broadcast by a different antenna some distance away. When an antenna broadcasts, an electrical signal travels into the feed of the antenna, which is one end of each of the antenna’s poles. The electrical signal causes the poles of the antenna to resonate at a frequency identical to the electrical signal fed to the antenna. As the antenna resonates, it creates an electromagnetic field that also matches the frequency of the electrical signal.
When the electromagnetic field created by one antenna passes over a second antenna, it causes the poles in that antenna to resonate. This resonance causes an electrical signal to emerge at the feed of the antenna that is equal in frequency to the electromagnetic field causing the resonance. This electrical signal then continues on to various circuits to isolate and amplify the signal so that a radio can finally generate an audible sound, which matches the original signal transmission from the other antenna.
The frequencies that an antenna can receive, or resonate at, are a direct function of the antenna’s length. Ideally, an antenna would have a length equal to the length of the radio waves it is to receive; however, as a mid-range FM radio wave is approximately 10 feet long, such an antenna would be very large and cumbersome. A dipole antenna helps with this problem by using two poles that are one-quarter of the radio wave's length, typically arranged in a straight line or parallel to one another.
By using two poles in this arrangement, called a half-wave dipole, the dipole antenna is capable of resonating to radio waves that are twice the combined length of the two poles. While this does improve the portability of an FM radio antenna, the electrical length of the antenna is still only one-half of the full FM radio wave. As a result, there are sacrifices in terms of reception efficiency.
The design of a folded dipole antenna begins with the length of the two poles, which are each one-half the FM radio wave's length. The feeds, one end of each of the two poles, start close together so that they do not touch. The poles then extend away from one another in a straight line. At a point less than about one-third of the way down its length, each of the two poles is bent into a large, sweeping radius until it is traveling toward the other and stops just before meeting.
In this arrangement, the two poles of the folded dipole antenna, each one-half of the FM radio wave's length, equal the full electrical length of the FM radio wave. As a result, the antenna experiences none of the loss in efficiency of a standard dipole antenna. Thus, a folded dipole antenna can offer performance superior to a standard dipole antenna but at a physical length comparable to a standard dipole antenna.