Quarter - Wavelength Antennas

　　Quarter - wavelength antennas are another important type of antenna with distinct characteristics and applications.

　　Structure and Working Principle

　　A quarter - wavelength antenna typically consists of a single conductive element with a length approximately equal to one - quarter of the wavelength (\(\lambda/4\)) of the operating frequency. It is often used in combination with a ground plane. The ground plane can be a physical conducting surface, such as a metal plate, or it can be an effective ground plane created by the electrical characteristics of the surrounding environment. When an alternating current is applied to the antenna, the conductive element acts as an open - ended transmission line. The quarter - wavelength length is chosen because at this length, the impedance at the feed point of the antenna can be matched to the impedance of the transmission line connecting it to the transmitter or receiver. The current distribution along the antenna element is non - uniform, with the maximum current at the base (where it is connected to the feed) and the minimum current at the open end. This current distribution results in the generation of an electromagnetic field that radiates into the surrounding space.

　　Radiation Pattern

　　The radiation pattern of a quarter - wavelength antenna is typically unidirectional. In the far - field, it radiates most effectively in a direction perpendicular to the axis of the antenna and the ground plane. The radiation intensity decreases as the angle from this perpendicular direction increases. The ground plane plays a crucial role in shaping the radiation pattern. It reflects the electromagnetic waves radiated by the antenna element, effectively doubling the radiation in the desired direction. The radiation pattern is often a hemispherical - like shape in the direction away from the ground plane, with a relatively sharp null in the direction towards the ground plane.

　　Applications

　　Quarter - wavelength antennas are widely used in mobile communication devices. For example, in mobile phones, the internal antennas are often designed based on the quarter - wavelength principle. They are also used in some wireless sensors, where a compact and efficient antenna solution is required. In outdoor applications, quarter - wavelength antennas can be found in wireless access points for providing wireless coverage in a specific area. They are also used in some radio - frequency identification (RFID) systems, where the unidirectional radiation pattern can be used to read or write data to RFID tags in a specific direction.

　　Advantages and Limitations

　　The main advantage of quarter - wavelength antennas is their compact size. Since the length of the antenna element is only one - quarter of the wavelength, it is much shorter than a half - wavelength antenna for the same operating frequency. This makes it suitable for applications where space is limited, such as in mobile devices. Another advantage is the unidirectional radiation pattern, which can be useful in applications where a focused radiation in a particular direction is needed. However, quarter - wavelength antennas are more sensitive to the presence of the ground plane. If the ground plane is not properly designed or if there are obstacles near the ground plane, it can significantly affect the performance of the antenna. Also, the radiation efficiency of a quarter - wavelength antenna may be lower compared to some other antenna designs in certain conditions.