A sleeve antenna is a specific design of radio wave antenna used for both the reception and transmission of radio frequency signals. The two-element construction of the sleeve antenna is what gives the device its name. In its simplest and most common form, a sleeve antenna has a straight metal element, often a rod or wire, placed and centered inside a straight metal tube, which acts as a sleeve over the center element. While a sleeve antenna can be made to work with practically any radio frequency, it gets the most use in high-frequency band applications such as CB radio base station or maritime ship-to-ship, ship-to-shore radio antennas.
An antenna may be classified in many ways, and the sleeve antenna falls into several different category types. First, having two separate elements, a sleeve antenna is classified as a dipole-type antenna, meaning it has two poles. Second, it is also classified as a half-wave antenna, meaning that the antenna is best suited to receive or transmit at a radio wavelength twice the antenna’s physical length. Third, these antennas, which are mounted vertically, can receive and broadcast in all directions simultaneously, making them members of the omnidirectional class of antennas. Finally, because both elements of the antenna share the same centerline, it can be described as a coaxial antenna, meaning that the two poles have a common center axis.
Like all radio antennas, a sleeve antenna operates on the principle of frequency resonance. Frequency resonance dictates that a specific radio frequency transmission will cause a specific length of metal to resonate at an atomic level and generate an AC electrical current. This generated current, identical in structure and modulation to the original radio signal, is then passed through other electronics, like a radio, so the signal can be heard. Likewise, if a radio applies an AC signal to a specific sized piece of metal, that metal will resonate. As it does, it will transmit the signal in the form of radio waves, which in turn, can be received by another antenna.
The range of radio wave frequencies that can cause a given piece of metal, or antenna, to resonate is called the bandwidth. Because sleeve antennas have a wide differential between the inner element and the outer element, they can receive and transmit a relatively broad range of radio frequencies. This attribute makes them useful in many applications, even though the antennas themselves are not as efficient at converting radio waves to electrical current as other types, which may have two or more elements fully exposed to the radio waves.
Despite the diminished efficiency, sleeve antennas do not have the thin and spindly elements needed in many other antenna designs. This makes them physically sturdier than most other antennas. As a result, they are most often used in applications where the antenna must endure severe weather conditions and still operate properly, such as on ships or as permanently mounted outdoor CB antennas.