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Crossed Dipole Antennas Son Xuat Ta, Ikmo Park, and Richard W. Ziolkowski Crossed Dipole Antennas A review. rossed dipole antennas have been THE HISTORY OF CROSSED widely developed for current and DIPOLE ANTENNAS future wireless communication sys- The crossed dipole is a common type of mod- tems. They can generate isotropic, ern antenna with an radio frequency (RF)- Comnidirectional, dual-polarized (DP), and to millimeter-wave frequency range. The circularly polarized (CP) radiation. More- crossed dipole antenna has a fairly rich and over, by incorporating a variety of primary interesting history that started in the 1930s. radiation elements, they are suitable for The first crossed dipole antenna was devel- single-band, multiband, and wideband oper- oped under the name “turnstile antenna” ations. This article presents a review of the by Brown [1]. In the 1940s, “superturnstile” designs, characteristics, and applications antennas [2]–[4] were developed for a broader of crossed dipole antennas along with the impedance bandwidth in comparison with recent developments of single-feed CP con- the original design. In 1961, a new type of figurations. The considerations of profile crossed dipole antenna, which used a single miniaturization, radiation pattern control, feed, was developed for CP radiation [52]. bandwidth enhancement, and multiband In the 2000s, the crossed dipoles began to operation are emphasized. be fed by two separate ports with the requi- site 90° phase difference between them to generate DP radiation [30]–[51]. Today, this antenna type is used in many wireless com- Digital Object Identifier 10.1109/MAP.2015.2470680 Date of publication: 6 October 2015 munication systems, including broadcasting IEEE ANTENNAS & PROpaGATION MAGAZINE OCtoBER 2015 1045-9243/15©2015IEEE 107 The crossed dipole is a common type of modern antenna with an RF- to millimeter-wave frequency range. services [1]–[14], satellite communications [26]–[29], mobile com- dipole antennas. However, there is still no review article or book munications [30]–[51], global navigation satellite systems (GNSS) that discusses all types of these antennas. In this article, the [17], [68]–[71], [74]–[79], RF identification (RFID) [65], [66], [73], design, characteristics, and applications of crossed dipole anten- wireless power transmission [15], wireless local area networks nas are reviewed. The theoretical developments for crossed (WLANs) [80], wireless personal area networks [14], and world- dipole antennas have been essentially completed; consequently, wide interoperability for microwave access (WiMAX) [81]. this review will emphasize antenna application-oriented contri- The first crossed dipole antenna was developed with an butions. The crossed dipole antenna types are roughly divided emphasis as a new ultrahigh frequency (UHF) radiating sys- into four different classes based on their feeding structures and tem that economized energy by concentrating it in the hori- radiation characteristics. zontal plane equally in all directions [1]. A single element of this turnstile antenna is the basic design of the crossed dipole, ISOTROPIC AND OMnidirectional which consists of a set of two half-wavelength dipoles aligned CROSSED DIPOLE ANTENNAS at right angles to each other with currents of equal magnitude Broadcasting most frequently refers to the transmission of infor- that are in-phase quadrature. Since then, alternative modi- mation and entertainment programming from various sources fications of the traditional turnstile antenna have achieved a to the general public. Commercial broadcasting began in the broader impedance bandwidth, profile miniaturization, and 1920s for audio services and in the 1950s for television. Anten- ease of manufacturing [2]–[15]. Along with the development of nas for broadcasting transmitters are usually located in the heart wireless communications, many applications require antennas of a city and require radiating an equal signal in all directions that radiate a unidirectional pattern with a significant front-to- in the horizontal plane. Among the antenna designs for broad- back ratio to ensure high security and efficiency in the propa- cast purposes, the “turnstile” antenna was one of the first [1]. gation channels. Accordingly, the crossed dipole is generally With the development of broadcasting services, different types equipped with a reflector to generate a desired unidirectional of broadcasting antennas have been developed for broadening pattern with circular polarization [16]–[29] or dual polariza- bandwidth, higher gain, and polarization control, as well as ease tion [30]–[51]. Most of the above-mentioned antennas require of installation. These designs are reviewed in this section. a dual-feed structure that generally complicates the antenna A UHF antenna was developed in [1] with the purpose to design and fabrication. Bolster [52] demonstrated theoretically embody the features of circularly symmetrical radiation with and experimentally that single-feed crossed dipoles connected increased horizontal directivity into a sturdy structure, possibly in parallel could generate CP radiation, if the lengths of the supported by a single mast. The turnstile antenna was finally dipoles were such that the real parts of their input admittances constructed and fulfilled these conditions. Figure 1(a) shows the were equal and the phase angles of their input admittances basic geometry of a single turnstile antenna. It consists of a set of differed by 90º. Based on these conditions, numerous single- two half-wavelength dipoles aligned at right angles with respect feed CP crossed dipole antennas [53]–[81] have been reported. to each other; the currents on the dipoles have equal magnitude With the rapid development of today’s wireless communication and are in-phase quadrature. Consequently, the turnstile antenna markets, higher demands have been raised for the antenna is often referred to as a crossed dipole antenna. It has a near- design, including compact sizes; high efficiencies; broad band- isotropic radiation pattern [Figure 1(b)], i.e., it is linearly polarized widths; multiple bands; specific radiation profiles; ease of fab- with a circularly symmetrical profile in the horizontal plane ( xy– rication and integration; and low costs. A number of alternative plane) and CP radiation in the !z directions; the front side radi- engineering approaches have been used to tailor the crossed ates left-hand circular polarization (LHCP), while the back side dipole antenna design to address these issues. radiates right-hand circular polarization (RHCP). A double turn- In the eight decades since the first crossed dipole was pro- stile in free space with a vertical separation of 02.,5 m as shown posed, there has been a vast amount of literature on crossed in Figure 2(a), generates an isotropic radiation pattern [16]. This 108 OCTOBER 2015 IEEE ANTENNAS & PROpaGATION MAGAZINE behavior is attributed to the two dipoles of each turnstile realiz- port mast form a two-line slot fed by a metal jumper located at ing an omnidirectional pattern in the horizontal plane, while the the center of each wing. The matching condition is obtained by two turnstiles being separated by a quarter-wavelength leads to changing the shape of the jumper and by changing the distance uniform radiation in the vertical plane. As shown in Figure 2(b), between the support mast and the antenna elements. To produce the double turnstile provides a broadened radiation pattern with an omnidirectional pattern in the horizontal plane, a feed power respect to that of the single turnstile [Figure 1(b)]. Moreover, a divider located inside the mast properly phases the inputs for CP 0-dBi gain is maintained over almost all directions in the entire performance (0º, 90º, 180º, and 270º). A single batwing antenna three-dimensional (3-D) space. To further increase the horizontal normally provides a fractional bandwidth of approximately 50% signal strength and maintain the shape of the horizontal pattern, and is ideally suited when low to medium gain is required. With more elements are placed in a vertical array [1]. a proper arrangement of their radiators, surperturnstile antennas Based on the conventional turnstile design, several modi- can be used for either horizontal [4] or vertical polarization [5]. fications, such as the pretuned turnstile [2], cloverleaf [3], and An omnidirectional gain up to 10 dBi can be achieved with batwing [4] shapes, have been developed to produce wider a multiple bay antenna constructed on a single modular mast impedance bandwidths. In particular, the batwing antennas [7]. For instance, as shown in Figure 3(b), a superturnstile tele- yielded better performance in comparison with the others and vision broadcasting antenna contains four bays mounted on a had a broader bandwidth, lower profile, easier adjustment of single mast. An opposing pair of wings functions as a dipole, matching conditions, and easier polarization control. Due to these with each pair of crossed dipoles fed 90º out of phase to create features, batwing radiators, which are often called superturnstile an omnidirectional radiation pattern. The vertical collinear stack antennas, have been installed in most broadcasting systems [4]– of batwings increases the gain of the antenna, concentrating the [9], while the other designs have been almost entirely phased out. radiation in the horizontal direction, so that only a little power is Figure 3(a) shows the normal configuration of the batwing radiated toward the sky or the ground. Two or more elements, radiator with four wings placed around a central metal mast. each covering a different frequency band, have been mounted on Each wing connects to the mast at its top and bottom with a a single mast to effectively span a much wider range of frequen- metal-to-metal connector. The inner vertical rod and the sup- cies. By combining the horizontally polarized and the vertically Dipole 2 m/4 1/4 Wave Phasing Line Dipole 1 z x y (a) (dB ) LHCP (dB ) LHCP ic 0 ic 0 5 RHCP 5 RHCP 330 30 330 30 0 0 -5 300 60 -5 300 60 -10 -10 -15 270 90 -15 270 90 -10 -10 -5 240 120 -5 240 120 0 0 210 150 210 150 5 5 Vertical Plane at z = 0° Horizontal Plane at i = 45° (b) (c) FIGURE 1.
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