Yifan Yin, Ke Wu
IMAGE LICENSED BY INGRAM PUBLISHING
This article reviews and summarizes the developments of endfire circularly polarized (CP) planar antennas. The difficulty in designing such antennas is that there is not a large radiating aperture on a substrate to produce strong enough vertically polarized (VP) radiation to match the horizontally polarized (HP) radiation. Mainly based on the structures of radiation and feeding, endfire CP planar antennas are classified into several groups. The mechanisms of these antennas are identified, their performance is compared with tables, and the relation between performance and structures is explored. It is indicated that the distance between the phase centers of VP and HP radiators is a crucial parameter to affect the axial ratio (AR) bandwidth and beamwidth of an endfire CP antenna. It is addressed that it is essential to have strong VP radiation for a high-gain and wideband endfire CP antenna when substrates are thin.
CP antennas are well known for their usefulness in satellite and mobile communication as well as sensing systems. They manifest certain obvious advantages over their linearly polarized (LP) counterparts, including being less sensitive to antenna axial rotation and multipath interference and having less delay spreading [1], [2]. An endfire planar antenna has its main lobe in parallel with the substrate (toward the edges of the substrate). Endfire CP antennas are essential and promising for low-profile handheld devices and provide additional space coverage in addition to broadside CP antennas for applications such as antennas in packages. Moreover, an endfire planar CP antenna can be easily implemented and integrated with other passive and active components on one substrate, and it provides another degree of freedom for antenna placement in systems, which favors the array arrangement in a very limited space and direct board-level integration. An endfire planar antenna is generally characterized and featured by its planar structure and planar manufacturing process, such as printed circuit board (PCB) processes and low-temperature cofired ceramics techniques.
A CP antenna should generate two orthogonally polarized radiations of equal amplitude and quadrature phase. Compared to a planar broadside LP or CP antenna, it is much more difficult to design an endfire CP planar antenna. For broadside LP and CP radiation, when a substrate is regarded as the reference plane, a planar antenna has a large area to accommodate various radiators with enough equivalent radiation regions, such as trimmed edge microstrip patches and crossed dipole-based broadside CP radiators. For endfire HP radiation, a substrate also has enough space to arrange two metal plates with large spacing in its horizontal dimension of the substrate. For endfire CP radiation, however, a substrate has insufficient space to arrange two metal plates with large spacing in its vertical dimension of the substrate, so there is not a large radiating aperture to produce strong enough VP (perpendicular to the substrate) radiation to match the HP radiation when the substrate is thin. Therefore, the main beam for most planar CP antennas presented to date is broadside; the endfire CP planar antennas in the literature are relatively few.
Here, we use a simple model, as shown in Figure 1, to demonstrate how the configuration of a CP antenna influences the performance of its AR. In the model, the CP antenna consists of a VP radiator and an HP radiator. Without a loss of generality, the phase center of the VP radiator is placed at the origin, whereas the phase center of the HP radiator is at (x, y, z). In the far zone, the phase difference between the two orthogonally polarized electric fields is expressed as follows: \begin{align*}{\Delta}{\varphi} & = {k}_{0}{d} + {\varphi}_{0}, \tag{1} \\ {d} & = {x}{\sin}\,{\theta}{\cos}\,{\phi} + {y}{\sin}\,{\theta}{\sin}\,{\phi} + {z}{\cos}\,{\theta}, \tag{2} \end{align*}
Figure 1. A CP antenna.
where ${k}_{0}$ is the wavenumber in free space, ${k}_{0}{d}$ is the space phase delay, and ${\varphi}_{0}$ is the feeding phase delay of the two orthogonally polarized radiators. For CP radiation, the phase difference should be \[{\Delta}{\varphi} = {\left({n} + {0.5}\right)}{\pi},{\left({n} = {0},{\pm}{1},{\pm}{2},{\ldots}\right)}{.} \tag{3} \]
When the phase centers of the two orthogonally polarized radiators coincide with each other, ${d} = {0}$, and ${\Delta}{\varphi}$ becomes independent of space angles. In this case, the CP antenna is expected to have a wide AR beamwidth. If its feeding phase delay ${\varphi}_{0}$ is not sensitive to frequency, the CP antenna is also expected to have a wide AR bandwidth in all space angles. Therefore, when a CP antenna utilizes only a radiator to produce two orthogonally polarized radiations simultaneously, which means that ${d} = {0}$, the antenna could be expected to have a wide AR beamwidth and bandwidth.
When a CP antenna is composed of two orthogonally polarized radiators with two separate phase centers, ${d}\,{>}\,{0}$, and ${\Delta}{\varphi}$ depends on both the space angle and operating frequency. As a result, it would narrow the AR beamwidth and AR bandwidth. The shorter the distance between the two phase centers, the wider the AR beamwidth and the AR bandwidth of the CP antenna become. On the other hand, when the frequency changes, the phase center of a radiator is usually located across a region rather than at a fixed point. A stable phase center would be beneficial for the CP performance of a CP antenna. In most CP antennas, the feeding phase delay ${\varphi}_{0}$ also depends on the operating frequency. When the feeding phase delay ${\varphi}_{0}$ can offset the space phase delay ${k}_{0}{d}$ when the frequency changes, a CP antenna would be expected to have a wide AR bandwidth. However, this is a challenging task.
To evaluate an endfire CP planar antenna, its performance should be among the top-ranked considerations, but other factors should also be taken into account. The thickness of the substrate is a critical factor of practicability for endfire CP planar antennas. For an easy presentation in this review, we classify endfire CP planar antennas into five groups, namely, planar helical antennas, electric and magnetic complementary dipole composite antennas, substrate integrated waveguide (SIW) horn-based composite antennas, and other endfire CP planar antennas. They are explained and discussed in detail in the following.
Among the endfire CP antennas, the helical antenna, proposed in 1948, is the oldest. A helical antenna becomes an endfire CP antenna when the circumference of the helix is approximately equal to one guided wavelength [3]. An endfire CP helical antenna usually exhibits a wide impedance and AR bandwidth as well as and a wide AR beamwidth because it is made of only a radiator. However, it is rather challenging to devise an endfire CP planar helical antenna in a relatively low-frequency band. When the circumference of a helix is approximately required to be one guided wavelength, the VP radiation is much weaker than its horizontal counterpart, due to the thin substrate. For example, a 2.54-mm-thick substrate is still regarded as thin at 10 GHz because it is less than ${0.1}{\lambda}_{0}$. Only in 2015 was an endfire CP planar helical antenna proposed, in [4]. Since then, diversified endfire CP planar antennas have been proposed, studied, and demonstrated [5], [6], [7], [8], [9], [10].
The endfire CP planar helical antenna in [4] is composed of strips and metallic vias, as depicted in Figure 2. The strips contribute to HP radiation, whereas the vias contribute to VP radiation. An SMA connector mounted vertically on the substrate connects to a grounded coplanar waveguide, and this coplanar waveguide then feeds the helix. The open aperture of the grounded coplanar waveguide could also contribute to the VP radiation. Consequently, the CP antenna can achieve an 8-dB above the gain of an isotrope antenna (dBic) gain at the center frequency on a thin substrate of only ${0.11}{\lambda}_{0}$.
Figure 2. An endfire CP planar helical antenna [4].
The structure of an endfire CP planar plasma helical antenna with a reconfigurable bandwidth reported in [11] is similar to that described in [4]. In the antenna [Figure 3(a)], a set of tubes containing ionized noble gases is used to replace conventional copper to form a multiturn plasma helical [shown in red in Figure 3(a)], which makes the bandwidth reconfigurable. Obviously, endfire CP planar helixes can also be used to construct an array [12]. The element endfire CP planar helical antenna of the array, as illustrated in Figure 3(b), is fed by a planar microstrip line rather than a perpendicular-mounted SMA connector.
Figure 3. An endfire CP planar helical antenna that is (a) SMA fed [11] and (b) fed by a microstrip line [12]. PEC: patch-excited cup.
When it comes to the application of such structures in the millimeter-wave range and above, a thin substrate is no longer an obstacle for the creation of endfire CP radiation. In this case, planar helixes would be available for diverse applications. A 3D, micromachined, silicon substrate, integrated millimeter-wave endfire CP helical antenna was proposed in [13]. Although the structure in [13] is not planar, it could be fabricated with a planar technology.
Chu and Lu et al. devised a type of low-profile endfire CP planar antenna [8], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34]. The conceptual 3D prototype of these antennas can be found in [35] and [36]. In the antenna, a planar magnetic dipole produces VP radiation, and a planar electric/magnetic dipole produces HP radiation. Therefore, this type of antenna is referred to as the electric and magnetic complementary dipole-based endfire CP antenna. The magnetic dipole associated with VP radiation also feeds the HP radiator, either directly [19], [20], [23], [25], [30] or by a phase shift line of one-quarter [8], [15], [16], [17], [18], [22], [24], [26], [27], [28], [29], [31], [32] or three-eighths of a wavelength [14], which is used to achieve a quadrature phase of the two orthogonal modes for the CP generation. A practical and complete electric and magnetic dipole-based endfire CP antenna usually includes four functional parts: the VP radiator, HP radiator, phase-shifting component, and feeding connector. Lu et al. have given a detailed review of this type of endfire CP planar antennas in [18].
A low-profile planar magnetic dipole can produce a large amount of VP radiation, and it is a significant benefit, especially for low-frequency CP antennas. The planar magnetic dipole in these antennas is actually a resonating radiator rather than a traveling-wave radiator because it works in a resonant mode rather than a propagating mode. The planar magnetic dipoles include two types of structures: a rectangular resonating radiator, as in Figure 4(a) and (b), and a semicircular resonating radiator, as in Figure 4(c). Rectangular and semicircular resonating radiators are covered with conducting layers on their top and bottom surfaces. Figure 4(d) describes the conceptual configuration of a planar electric and magnetic complementary dipole-based endfire CP composite antenna [21]. One [Figure 4(b)] or three [Figure 4(a)] of the four rectangular dielectric edges are shorted by metallic via arrays or walls, and at least one edge is an open aperture for radiation [8], [14], [17], [19], [22], [23], [27], [28], [29], [30], [31], [32], [33], [34]. These CP antennas usually have unidirectional endfire patterns on the substrate plane. Similarly, the straight edge of the semicircular dielectric radiator is shorted by metallic walls [Figure 4(c)], and its circular edge is open [15], [16], [18], [20], [21]. These CP antennas typically have a very wide beamwidth on the substrate plane. Due to a larger proportion of open edges, a circular sector dielectric radiator is used to achieve a wide AR beamwidth [20]. On the other hand, a large proportion of the edges also leads to a small Q value and a large bandwidth.
Figure 4. A rectangular resonating radiator with (a) three shorting edges [21], (b) one shorting edge [31], (c) a semicircular resonating radiator [21], and (d) a conceptual configuration of planar electric and magnetic dipole-based endfire CP composite antennas [21].
In the aforementioned antennas, the basic structures of the diverse HP radiators are generally dipoles and loops. The single straight dipoles have an omnidirectional pattern and a low gain on the elevation plane, and thus endfire CP antennas also exhibit a relatively low gain of about 3 dB [14]. The various annular dipoles can provide a wide beam of HP radiation to achieve a wide AR beamwidth on the azimuth plane [15]. The inverted-V dipole has a shape and AR radiation pattern similar to annular dipoles [17]. Because the straight, annular, and inverted V-shape dipoles do not have endfire radiation patterns, the gains of the endfire CP antennas associated with these dipoles are relatively low. Compared to straight dipoles, the V-shape dipole has an endfire pattern and a relatively high gain and therefore is applied to high-gain endfire CP antennas [23].
An endfire CP planar loop antenna that uses a printed quadrangle loop fed by a microstrip line demonstrates very good performance; however, certain important parameters, such as thickness, are missing [10]. The meandered dipole in Figure 5(a) is employed to facilitate the assembly of p-i-n diodes, reduce the lateral dimension, and enable switchable polarized radiation between left-hand circular polarization and right-hand circular polarization [25]. Compared to the aforementioned dipoles that produce HP radiation with an omnidirectional pattern on the elevation plane, the squared loop in [8], as shown in Figure 5(b), is a magnetic dipole that produces HP radiation but has an omnidirectional pattern on the azimuth plane. When a higher gain is desired, magnetic and electric dipole arrays are preferred. Figure 5(c) and (d) give diverse endfire CP antenna arrays that use meandering strip lines to provide a quadrature phase and directors to increase gains [27], [32].
Figure 5. A (a) meandered monopole array [25], (b) square loop [8], [c] dipole cavity Yagi array [27], and (d) dipole Yagi array [32].
These endfire CP planar antennas are all composite structures composed of VP and HP radiators. The two orthogonally polarized radiations should exhibit a quadrature phase, and the strip lines between the two radiators are used to achieve it. Therefore, the strip lines might be meandered ones [26], [27], [31], [32]. The connection between the strip lines and the VP could be achieved by a coupling slot, as in Figure 5(c), which introduces a phase shift [27]. These dipoles and loops of the HP radiator are located on the opposite faces of the substrate so that they not only produce the HP but also VP radiation. Although the substrate is less than 0.05 m 0 in most cases, the HP radiator also presents VP radiation, which effects the CP performance of the antenna.
Table 1 provides a brief comparison of the key performance metrics of these planar electric and magnetic complementary dipole-based endfire CP composite antennas. The complementary dipole-based endfire CP antennas in Table 1 generally work at low frequencies. Because the gain of a single straight electric dipole for HP radiation is low compared to magnetic dipoles, the gains of the complementary dipole-based endfire CP planar antennas are relatively low, at about 2 dBic. To achieve a high gain, a Yagi architecture is preferred [19], [22], [27], [29], [32], [33], [34]. Directors in [19], [29], [32], and [34] are used to enhance the gains of electric dipoles. Because the HP radiator is in the front of the VP radiator and fed by it, it is difficult to add directors to the VP radiator, and a reflector can be added to the VP radiator to increase its gain. In [22], [27], and [33], reflectors and directors are used. In [23] and [25], transverse two-element dipole arrays are used to enhance the gain of HP radiation. The gain of a single complementary dipole-based endfire CP antenna would not be very high, e.g., over 8 dBic.
Table 1. The planar electric and magnetic dipole-based endfire CP antennas.
The structure of a resonating radiator can produce strong enough VP radiation for a CP antenna with very thin substrates, but it is difficult to feed a resonating radiator with a planar structure. All the antennas in Table 1 are fed with SMA connectors perpendicularly mounted on the bottom face of the substrate. Therefore, this type of feeding structure is not fully planar. It would introduce difficulties when forming an endfire CP antenna array, which is necessary and required in most cases to achieve a high-gain beam or a scanning beam.
An SIW horn-based endfire CP planar antenna is feed by an SIW and is a type of traveling-wave antenna because it is excited by a transmission mode. The VP radiation of an SIW horn-based endfire CP planar antenna is usually produced by an SIW horn, whereas the HP radiation could be produced by an antipodal linearly tapered slot antenna (ALTSA) [5], [6], [37], [38], [39], [40], [41], the SIW horn itself [6], or an electric dipole [42]. An ALTSA is a wideband traveling-wave endfire HP radiator, and its gain could be high. An ALTSA fed by an SIW is excited mainly by the longitudinally magnetic field of the ${TE}_{10}$ mode in the SIW. The VP radiation from the SIW horn is excited by the transverse electric field of the ${TE}_{10}$ mode. The transverse electric field and the longitudinally magnetic field of the ${TE}_{10}$ mode in an SIW are quadrature phase in nature. The phase centers of the VP and HP radiations could be in close proximity. Hence, the quadrature phase of the transverse electric field and the longitudinally magnetic field could ensure the phase condition of CP radiation in a wide band and a wide beamwidth. Since an ALTSA is feed by an SIW and its two flaring wings are over different faces of the substrate, it also produces a little VP radiation. The wider the overlap of the two wings, the larger the VP radiation becomes.
To the knowledge of the authors, the oldest endfire CP planar antenna based on SIW techniques was derived from an ALTSA fed by an SIW in 2007 [5]. Although the antenna in [5] is not fully planar due to a dielectric rod, it has paved the way for a fully planar antenna. Still, it was a long time before an SIW horn-based endfire CP planar antenna was proposed, in 2016 [37]. The endfire CP antenna, as described in Figure 6(a), consists of an antipodal Vivaldi antenna and an SIW H-plane horn. Both the Vivaldi antenna and the SIW horn are fed by an SIW 3-dB coupler to achieve equal amplitude and a quadrature phase. The phase centers of the two radiators are obviously separated. The authors of this article have proposed a compact SIW endfire CP planar antenna [7] consisting of an SIW horn and an ALTSA fed by one subhorn of the SIW horn, as detailed in Figure 6(b). The phase centers of the two radiators are relatively closer to each other than in [37]. Also, the phase controlling and power dividing in the antenna are achieved by the arrays of metallic vias inside its horn, which is compact and space saving.
Figure 6. (a) An SIW endfire dual-CP antenna [37]. (b) A compact SIW endfire CP composite antenna [7].
Differing from the VP radiators in [7] and [37] that do not feed HP radiators, the VP radiators in [38], [39], [40], [41] produce VP radiation and feed HP radiators simultaneously. This could make the phase centers of VP and HP radiators close and widen the AR bandwidth and beamwidth of the associated CP antenna. The endfire CP antenna, as presented in Figure 7(a), is actually a composite of an ALTSA and an SIW [38]. The SIW produces VP radiation and feeds the ALTSA. The ALTSA could be simplified to two antipodal notches etched at the two broad wall edges of the SIW, as in Figure 7(b), and a thick, tapered, three-layer dielectric rod is placed in front of the antenna to increase the gain by 5.6 dBic [39]. The authors present an endfire CP ALTSA antenna, as given in Figure 8(a), consisting of a slotted width-tapered SIW and an ALTSA fed by the slotted SIW [40]. Compared to the conventional ALTSA fed by the SIW, the antenna utilizes slots on the spare broad walls of the SIW to form a slotted width-tapered SIW radiator to enhance VP radiation. As opposed to a conventional width-uniform slotted SIW, the slotted width-tapered SIW radiator works with slow waves because there are no space harmonics in it.
Figure 7. A planar ALTSA fed by an SIW (a) without loaded dielectrica [38] and (b) with loaded dielectrics [39].
Figure 8. (a) An ALTSA fed by a slotted SIW [40]. (b) A CP unidirectional dielectric radiator fed by an ALTSA [41].
It is usually expected that an antenna be directly integrated with a radio-frequency (RF) subsystem due to the requirements for miniaturization and low cost. To achieve this, the planar antenna should be fabricated on a thin single-layer substrate or on a multilayer substrate with a thin feeding layer on which the RF signal is feeding the antenna. When a substrate is thick, there will be high-order modes and surface wave modes in it, which could lead to self-sustaining oscillation in the RF subsystem connected with the antenna. Besides, when the feeding substrate is thick, the transition from an antenna to an RF subsystem might produce radiation loss, which would degrade the performance of the antenna. Therefore, the feeding substrate should usually be less than ${0.05}{\lambda}_{0}$ in the millimeter-wave range. The CP antennas in [7] and [37], [38], [39], [40], however, are manufactured on a single-layer substrate that must be thick enough, usually more than ${0.12}{\lambda}_{0}$, to produce sufficient VP radiation.
The authors present a multilayer substrate endfire CP planar antenna, as shown in Figure 8(b), consisting of a unidirectional dielectric radiator (UDR) and an ALTSA surrounded by the UDR [41]. The ALTSA is fed by an SIW on a thin substrate of ${0.05}{\lambda}_{0}$, and the UDR is fed by the ALTSA, which enables the UDR to enhance the original VP radiation rather than the HP radiation of the ALTSA. Due to the polarization selectivity enhancement, an endfire CP antenna is achieved with a thin substrate of ${0.05}{\lambda}_{0}$. Cai and Qian proposed a septum polarizer-based SIW endfire dual-CP antenna, as described in Figure 9(a), in 2016 [6]. It is entirely an SIW horn without any other radiator. It uses a thick two-layer substrate and has two input/output SIWs for left-hand CP and right-hand CP antennas. The septum polarizer in the SIW transforms the ${TE}_{10}$ mode into two orthogonal modes, namely, ${TE}_{10}$ and ${TE}_{01}$, at the open ends of the antenna. The endfire CP antenna in Figure 9(b) is similar to the electric and magnetic dipole-based composite antenna in the “Planar Electric and Magnetic Complementary Dipole-Based Endfire CP Composite Antenna” section, but it is fed from an SIW port rather than a vertically mounted cable, so it is still an SIW horn-based traveling-wave antenna [42].
Figure 9. (a) A septum polarizer-based endfire dual-CP SIW horn antenna [6]. (b) An SIW horn dipole CP composite antenna [42].
Table 2 presents a performance comparison among SIW horn-based endfire CP planar composite antennas. The waveguide transitions used in SIW-based CP antennas in [6], [38], and [39] are not fully planar structures. Although they could be replaced with planar transitions to achieve fully planar structures in principle, this would degrade their performance, due to the radiation loss from thick substrates [7], [37]. Consequently, many SIW-based endfire CP antennas still use nonplanar waveguide transitions for better results.
Table 2 . The SIW horn-based endfire CP planar antennas.
Besides the aforementioned three types, there are diverse kinds of planar endfire CP antennas. A Yagi array [43] and its planar forms [44], [45] are classical endfire antennas. Such an array could be used as a high-gain HP radiator to achieve a high-gain CP antenna. An endfire dual-CP planar antenna, as provided in Figure 10, uses a magnetic dipole Yagi element and an electric dipole Yagi element [46]. The antenna is fed by a hybrid network composed of microstrip lines and parallel strip lines to achieve dual-CP radiation.
Figure 10. A Yagi endfire dual-CP planar antenna [46].
In addition to unidirectional operations, an endfire CP planar antenna can be designed to generate bidirectional [9], [47], [48] and omnidirectional patterns [49]. A single-layer, single-fed bidirectional endfire CP planar antenna, as portrayed in Figure 11(a), is actually a combination of two back-to-back unidirectional endfire CP planar antennas, where the meandered double-sided parallel strip lines provide phase compensation for CP radiation [9]. The CP antenna in Figure 11(b) has a structure similar to that in [9] but with reconfigurable polarization, which is achieved with two pairs of switchable electric dipoles [47]. A bidirectional endfire CP surface wave antenna fed by an SMA connector in [48] is composed of a microstrip radiator for the VP radiation and two hook-shaped strips on the ground plane for the HP radiation.
Figure 11. (a) A single-fed bidirectional endfire CP planar antenna [9]. (b) A reconfigurable polarization bidirectional endfire CP planar antenna [47].
Figure 12 shows a single-layer omnidirectional endfire CP planar antenna fed by a coaxial probe [49]. It is composed of a circular resonating VP radiator, five annular HP dipoles, and five meandered double-sided parallel strip lines, which provide phase compensation for CP radiation. The meandered shape makes HP dipoles close to the VP radiator, which will benefit a wide AR bandwidth. The five radial slots on the top layer of the VP radiator ensure equal powers between the VP and HP radiation. The aforementioned bidirectional and omnidirectional endfire CP planar antennas have a very low profile of ${0.029}{\lambda}_{0}$, which is important for applications in the 5.8-GHz Industry, Science, Medicine band and lower frequencies.
Figure 12. A single-layer omnidirectional endfire CP planar antenna [49], measured in centimeters. (a) The top view. (b) The bottom view.
To achieve flexible and diverse performance with potentially reconfigurable and tunable features, researchers and engineers have heavily focused on the development of antenna arrays. Based on the structures and topologies of the building elements, endfire CP planar antenna arrays can be divided into two groups, namely, dipole-based [50], [51], [52] and septum polarizer element antennas [53], [54], [55], [56]. The antenna in Figure 13(a) is a beam-scanning endfire CP planar antenna array [5]. It consists of eight tapered HP dipoles, and each dipole is loaded with parasite strips. The array is fed by a leaky wave antenna made of a periodic microstrip line, which produces VP radiation. The antenna has a main beam scanning from –5 to +37° in a lateral plane, and it demonstrates a wide bandwidth due to the leaky wave radiation mechanism. The V-band endfire CP antenna array of notched dipoles, as in Figure 13(b), is fed by an SIW Butler matrix to achieve a multibeam with wide beam coverage [51]. The eight-element complementary dipole array antenna in Figure 13(c) uses two metal blacks as reflectors and demonstrates a high peak gain of 15.3 dBic.
Figure 13. An endfire CP planar antenna array: (a) beam scanning, (b) multibeam [51], and (c) fixed beam [52].
Wu and Wang et al. developed and extended an endfire dual-CP SIW horn [6] into a multibeam array, as illustrated in Figure 14(a), for 5G millimeter-wave applications [53]. The antenna in [53] uses four dielectrics in front of the open apertures of SIWs to enhance a peak gain of 12.9 dBic. Cheng and Chen et al. studied and developed a compact endfire CP septum antenna array, as shown in Figure 14(b), and it also uses an SIW-based Butler matrix to achieve a multibeam with a peak gain of 11 dBic [54]. Al-Amoodi et al. devised and demonstrated an endfire CP septum-based antenna array, as depicted in Figure 14(c) and (d). The array in [55] has a fixed beam with a peak gain of 10 dBic, and the array in [56] can achieve a steering beam through different parameters of passive shifters in its feeding network.
Figure 14. Septum polarized-based endfire CP antenna arrays: (a) a multibeam array with loaded dielectrics [53], (b) a multibeam array [54], (c) a fixed beam with loaded dielectrics [55], and (d) a fixed beam [56].
Table 3 compares various endfire CP planar antenna arrays in terms of performance, size, and the structure of the element antennas. Feeding networks in the aforementioned endfire CP antenna arrays feature large sizes, and their losses are also generally high, which limits their usefulness in a number of millimeter-wave applications. Moreover, although the beamwidth on the azimuth plane in an array becomes narrow, the beamwidth on the elevation plane is still wide since the element antennas in an array are transversely stretched over the plane of the substrate. For the application of 77-GHz automotive radars, the desired AR beamwidths are 80º on the azimuth plane and about 15º on the elevation plane, and the desired AR bandwidth is roughly 800 MHz. In 5G and beyond 5G applications, the beamwidth on the azimuth and elevation planes should also be narrowed for high energy efficiency.
Table 3. The different endfire CP planar antenna arrays.
Most endfire CP planar antennas are electric and magnetic complementary dipole based or SIW horn based, and they use VP and HP radiators to achieve CP radiation. It is interesting and useful to explore the relation between their performance and structures. Compared to complementary dipole-based CP antennas, the structure of SIW horn-based CP antennas is favorable for high gains. The VP and HP radiators in an SIW horn-based CP antenna have an endfire pattern, whereas the HP electric dipole in a complementary dipole-based CP antenna has an omnidirectional pattern on the elevation plane. Therefore, the gain of an SIW horn-based CP antenna usually is above 8 dBic and is higher than that of a complementary dipole-based CP antenna, which is about 3 dBic. A low gain, however, might be beneficial to a wide AR beamwidth.
Complementary dipole-based CP antennas could use directors to form a Yagi array to enhance their HP radiator gains and incorporate reflectors to enhance their VP radiator gains, but directors and reflectors usually increase the HP and VP radiator gain by less than 3 dB, whereas ALTSAs could have much higher gains. Therefore, the gains of complementary dipole-based CP antennas with directors and reflectors could increase to 4–8 dBic, which is still lower than the gains of SIW horn-based CP antennas. To design a high-gain and wideband endfire CP antenna, it is essential to have strong VP radiation when substrates are thin. With thin substrates, the VP radiation in both the complementary dipole-based and SIW horn-based CP antennas is usually weak due to small radiating apertures. Using extra VP radiators could enhance the VP radiation. When only a single-layer substrate is available, there is little space to accommodate the extra VP radiator, due to the HP radiator in front of the original VP radiating aperture, so a VP reflector is used in [22], [27], and [33]. The slot array uses the spare broad walls of an SIW to produce VP radiation, and it is a space-saving solution [40]. When a multilayer substrate is available, multilayer dielectrics are employed to enhance VP radiation [41] and CP radiation [39].
To be a wideband and wide beamwidth CP antenna, it is crucial that the phase centers of VP and HP radiators coincide or are in close proximity. Most complementary dipole-based and SIW horn-based CP antennas usually exhibit narrow bands because the phase centers of VP and HP radiators are not in close proximity. Due to close phase centers, an ALTSA fed by an SIW radiating aperture could be a wideband and wide beamwidth CP antenna [38], [39], [41]. Among the cited references, some CP antennas are manufactured partly by planar technology [5], [23]. Some CP antennas are fed by cable probes mounted vertically on substrates or thick substrates. The former is not a fully planar structure and would bring certain difficulties for integration with RF circuits. The latter would also present certain problems for integration.
When the operating frequency approaches 100 GHz or beyond, the thickness of a substrate would not be an obstacle for endfire CP antennas, due to the short wavelength, and the structures of endfire CP planar antennas would become diversified. Traveling-wave radiators, such as SIW horn-based endfire CP planar antennas, might attract researchers’ attention due to their fully planar structures. When a CP antenna has two radiators, their phase centers are usually not at the same location. That would degrade the AR bandwidth and AR beamwidth. An endfire CP planar antenna with a single radiator would be a good option. Therefore, researchers and engineers should pay more attention to helical and dual-mode single-dielectric resonating endfire CP planar antennas.
With regard to anticipated applications, endfire CP planar antennas should be developed toward a conformal topology so that they can be mounted on diverse moving and aerodynamic platforms, such as aircraft and high-speed trains. When an endfire CP planar antenna is mounted on a conducting plane that is much larger than the antenna, both the HP electric dipole in a complementary dipole-based CP antenna and the ALTSA HP radiators in an SIW horn-based CP antenna will not work normally due to the negative image of the dipole or the ALTSA on the conducting plane, and the gain, the AR bandwidth, and the beamwidth will largely degrade. Endfire CP helical antennas also suffer from the problem, but the SIW endfire dual-CP antenna based on a septum polarizer, to a certain extent, is immune to the effect of the conducting plane because the VP and HP radiation is produced through an SIW aperture. On the other hand, the antenna beam would deviate from the endfire direction, and this problem is desirably mitigated for practical implementations.
Yifan Yin (yifan.yin@polymtl.ca) is with the College of Communications and Information Engineering, Jiangsu Key Laboratory of Wireless Communications, Nanjing University of Posts and Telecommunications, Nanjing, 210023, China. His research interests include antennas and periodic structures.
Ke Wu (ke.wu@ieee.org) is the industrial research chair in future wireless technologies and a professor of electrical engineering at Polytechnique Montréal, Montréal, H3T 1J4, Canada, where he is the director of the Poly-Grames Research Center. His research interests include substrate integrated circuits and antennas, array techniques, ultrafast interconnects, guided-wave structures, joint field/circuit modeling, and wireless powering. He is a Fellow of IEEE.
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Digital Object Identifier 10.1109/MAP.2022.3154977