Ubaid Ullah, Mohd Fadzil Ain, Zainal Arifin Ahmad

1 School of Electrical and Electronic Engineering, Universiti Sains Malaysia, Nibong Tebal, 14300 Pulau Pinang, Malaysia

2 School of Material and Mineral Resource Engineering, Universiti Sains Malaysia, Nibong Tebal, 14300 Pulau Pinang, Malaysia

I. INTRODUCTION

Antenna polarization is one of the most essential features of an antenna in modern telecommunication systems. Polarization of an antenna can be of vertical, horizontal or circular type.An antenna typically converts the input radio frequency electric current to electromagnetic waves and then radiates them into space. Polarization of an antenna is usually determined from the electric field orientation, it will be either linearly or circularly polarized. Linearly polarized antennas can radiate only one plane(vertical or horizontal) in the direction of wave propagation. For example, a vertically polarized antenna can efficiently transmit and receive only a vertically polarized field and vice versa. Due to the reciprocity property of the antenna, the transmission and reception of the antenna is always similar. If a receiving antenna is vertically polarized and a transmitting antenna is horizontally polarized, then it’s called cross polarization which incurs huge signal loss. Whereas, in circular polarization the antenna emits electromagnetic energy in a circular spiral pattern which covers horizontal,vertical and all the planes in-between them.With circular polarization, the orientation of the antenna is not important as the circular pattern of the transmitting and/or receiving antenna will always matches the incoming signal. Another important advantage of CP antennas is multipath rejection, which leads to signal interference in linear polarization.

In this article, the focus is on dielectric resonator antenna (DRA) which is one of the promising antennas for high frequency applications. DRA has many advantages such as,high radiation efficiency, zero metallic losses,versatile geometry, low cost, low profile and ease of availability of different permittivity materials (1-1000) [1-8]. As the polarization of the antenna is expressed by the orientation of electric field vector of the electromagnetic wave, therefore polarization can be altered by placing the excitation source at an appropriate location of the resonator or by placing the resonator in a special arrangement as an array antenna. As a general guideline for CP antenna design, the field excited in the antenna must have orthogonal components of equal magnitude and should be 90oout of phase [9-10].

Several techniques have been applied effectively to the DRA for generating a circular polarization in a dielectric resonator (DR). To achieve circular polarization in an individual DRA various feeding configurations, such as, single feed point and multiple feed points have been proposed. The simplicity of the single feed point circularly polarized antenna makes it an interesting choice for antenna engineers, but its narrow axial ratio bandwidth limits its application in the real world. On the other hand, wide axial ratio bandwidth can be attained from the DRA by appropriately exciting the DR with a multi-point feed configuration. The complexity associated with the multi-point-feed structure puts a reasonable limitation on its use. A sequential rotation of the feed structure and/or the antenna elements in an array configuration has also been adopted for the DRA to operate with a wide axial ratio bandwidth. In the following sections of this paper a comprehensive analysis of each technique used for CP DRA will be presented.A general overview of CP DRA will be given first, and then attention will be drawn specifically toward the wideband circularly polarized dielectric resonator antenna.

The purpose of this article has been to provide technical review of the circularly polarized dielectric resonator antenna. A general overview of the circular polarization, its advantages over linear polarization and its importance especially in satellite communication are highlighted.

II. CIRCULARLY POLARIZED DIELECTRIC RESONATOR ANTENNA

To improve system performance in communication and radar applications circularly polarized signals are preferred for effective communication. Circularly polarized signals are used for satellite communication to have control over polarization rotation effects due to atmospheric changes. In satellite communications circular polarization of the signal is very important because the radio path is predominately line of sight with few reflected paths. Hence circular polarization is preferred in order to make the received power independent of angular orientation. As mentioned in the previous section, a CP field can be generated in the antenna by exciting equal magnitudes fields that are 90oout of phase in the resonator.The purity of the circular polarization depends on the relation between the magnitude and phase of the two linearly polarized components. The axial ratio in a circularly polarized field is more sensitive to the difference in phase than the magnitude of the two linearly polarized fields excited in the antenna. Hence,for achieving the desired axial ratio bandwidth throughout the required operating band of a circularly polarized antenna, amplitude and phase error should be maintained within the maximum allowable error range [11-13].

Dielectric resonator antennas are capable of radiating a circularly polarized field by using a single-point-feed configuration. A single-point excitation achieves circular polarization by exciting two quasi-degenerate modes in the dielectric resonator which are typically in phase quadrature and spatially orthogonal to each other. Single-point-feeding technique for exciting circularly polarized field is a wellknown method and has been applied effectively in microstrip antennas [14-24].

A single-point-feeding technique can be effectively applied to DRA for generating circular polarization. The axial ratio bandwidth can be improved by slight modification in the shape of the DR (cylindrical, rectangular,hemispherical), loading a metal strip on DR,and adjusting the feed structure. Typical 3 dB axial ratio bandwidth value of 1% to 15% has been achieved with a single-point-feed [2]. Table 1 summarizes some selected designs of the published work on single-point-feed circularly polarized DRA. The purpose of the table is, tohighlight performance of a single-point-feed CP DRA in terms of axial ratio bandwidth,shape of the DRA, and type of feeding mechanism implemented by different researchers.Further, in this article the emphasis is on wideband circularly polarized DRA, all the major breakthroughs made in wideband CP DRA are addressed comprehensively.

Table I A summary of single-point-feed circularly polarized DRA

Fig. 1 Two points fed CP DRA [2]

III. MULTI-POINT-FEED CIRCULARLY POLARIZED DIELECTRIC RESONATOR ANTENNA

Dielectric Resonator antenna can be made to radiate circularly polarized fields in wider bandwidth by using multi-point-feeding configuration. Exciting DRA with Single-point-feeding mostly relies on the shape of the resonator to generate orthogonal modes and operates in narrow band with circular polarization. While, in the multi-point-feeding technique the spatially orthogonal modes are generated by the feeding network itself, therefore, CP radiation from the DRA is independent of the shape of dielectric resonator. Figure 1 shows a general example of two-pointsfed circularly polarized dielectric resonator antenna.

For the DRA to radiate circularly polarized fields, feed points P1 and P2 are energized in phase quadrature with equal amplitude signal.Microwave power divider circuits, such as wilkinson, or hybrid coupler are used to divide the power in equal magnitude and introduce phase delay to the input signal. By maintaining the required amplitude and phase delay (for circular polarization) of the input signal over a wide bandwidth will help in translating it to a wider axial ratio bandwidth. To give readers a basic idea of implementing two-points-feed techniques for circular polarization, some selected designs are discussed below.

In [48] the authors reported a relatively wide axial ratio bandwidth DRA (in comparison with Single-point-feeding) energized by two-points-feeding configuration.

In Figure 2 (a) the author reported a cylindrical DRA fed by conformal strips to excite two degenerate TM10in phase quadrature. An integrated microstrip 90ohybrid coupler was used to generate the signals. With this configuration of the feeding network, 20% axial ratio bandwidth has been achieved. Similarly in Figure 2 (b) a ring shaped DRA is shown which was energized by using two probe-feeds placed at different locations of the ring resonator. In this case two mutually orthogonal degenerate hybrid HE11modes were excited by using a 3dB quadrature coupler configuration which resulted in approximately 11% axial ratio bandwidth.

In [49] another wideband circularly polarized cylindrical dielectric resonator antenna fed with quadruple strip was reported. Figure 3 shows the configuration of the 90ohybrid coupler, which is further connected to four orthogonally oriented vertical strips for feeding the cylindrical resonator. To make the resonator radiates circularly polarized fields,each of the vertical strips was provided equal amplitude signals with four different (0o, 90o,180o, and 270o) excitation phases. A conventional 180obalun feed network was cascaded with a pair of 90oconventional hybrid coupler.To achieve a balance power distribution and constant phase difference between each vertical strip, the 180obalun delivers to the 90ohybrid coupler pair. For changing or shifting the phase of the signal, length d1and d2of the microstrip line branches exhibited in Figure 3 were changed.

With this technique the dielectric resonator was made to operate in wider impedance bandwidth (S11＜ -10) of 34.5% and axial ratio bandwidth (AR ＜3) of approximately 26%.This shows that using quadruple strip feed technique enhanced the axial ratio bandwidth of the cylindrical DRA effectively in comparison with conformal strips and two point probe feed.

Fig. 2 Two point fed wideband CP DRA [30, 48]

Fig. 3 Configuration of the circularly polarized quadruple strip feed cylindrical DRA utilizing the 90o hybrid coupler pair [49]

Further, on the strip feed alike configuration, the authors in [50] reported a rectangular shape dielectric resonator antenna excited with vertical strips in combination with a phase quadrature. Multiple circularly polarized modes (TE111and TE113) were generated simultaneously in the rectangular DR for wider axial ratio bandwidth operation. Though the theoretical resonant frequencies of the fundamental mode TE111and the higher order mode TE113are different, however, typically the resonance of these two modes occurs close to each other. In this design, both modes were generated in thexandydirections by placing equal size vertical stripes on the two side walls of rectangular DRA. Figure 4 show the geometry of the vertical strip feed rectangular DRA. Strip_1 excited and modes while strip_2 excited and modes. The author stated that by combining these multiple circularly polarized modes the axial ratio bandwidth of the antenna was increased. With this configuration of the feeding structure, the reported impedance bandwidth (S11＜ – 10 dB) of the antenna was 32.8%. The axial ratio bandwidth of the antenna was measured below 4 dB throughout the operating impedance bandwidth.

Fig. 4 Vertical strip feed CP rectangular DRA [50]

Fig. 5 Configuration for the two point feed broadband DRA [51]

Another interesting technique used for broadband circularly polarized antenna was recently reported in [51]. In this paper a twopoint-feed is used in combination with a switched line coupler for exciting a rectangular dielectric resonator to radiate circularly polarized fields. The configuration of the proposed two-point-feed structure with a wideband switched line coupler (WSLC) is shown in Figure 5.

In this geometry, a square shape dielectric resonator was placed in the center of a grounded substrate. The resonator was excited by a 3 dB Wilkinson power divider and a wideband 90° phase shifter comprised in WSLC. The switched line coupler circuit is printed on front side of the substrate, while the DR is placed on the ground plane. Schematic design of the WSLC is also shown in Figure 5. Port 2 and 3 represent a pair of vertical probes at the open end of WSLC which are attached to the two different side walls of the square shaped DRA.The Wilkinson power divider serves the purpose of equal power division between different feeding arms and transforming impedance between input and output ports. A couple of quarter wave transformers were used to split the signal from input port into two different paths and then transmit it along the path to generate a 90° phase difference at the output ports. An important component of the wideband switched line coupler is the wideband 90° phase shifter which helped in the broadband circularly polarized DRA. As a result of using this method for generating circular polarization, the reported axial ratio bandwidth is approximately 48%. It was stated that this CP DRA has shown stable main beam radiation properties in the broadside direction throughout the operating axial ratio bandwidth. Some more interesting articles published on special cases of exciting DRAs with circular polarization can be found in [52- 58].

IV. CIRCULARLY POLARIZED DIELCTRIC RESONATOR ANTENNA USING SEQUENTIAL ROTATION

With two-points-feed configuration a wideband circular polarization from the DRA can be achieved. The rare drawback of the twopoints-feed structure is the requirement of additional circuit area for designing the power divider network. Also, implementing twopoints feeding techniques for circularly polarized array DRA is not suitable, as this will make the antenna cumbersome. An alternative technique that can be used effectively for array structure is sequential rotation. In this feeding mechanism, either the feeding network or the antenna elements are rotated sequentially.Using sequential rotation technique maintains the simplicity of the structure and uses singlepoint-feed configuration. It merits mentioning here that sequential rotation technique can be used to generate either linear polarization or circular polarization [59-60]. The feeding network for sequential rotation is designed in a way that, the antenna elements in an array configuration are energized in a progressive 90° phase shift at each point. In some designs,the relative location of the feed points and/or positions of the antenna elements are rotated sequentially while maintaining 90° phase difference. The literature shows that by utilizing this feeding configuration for circular polarization, a wideband axial ratio bandwidth can be achieved.

Several design examples of sequentially rotated network are discussed in this section. In[61] the authors reported three different types of sequentially rotated fed DRA. The most commonly used configuration for sequentially rotated parallel feeding network is shown in Figure 6. The illustration of the feeder shows an input port labeled as port 1 which is a 50Ω microstrip line. Some quarter wave impedance transformers are used for dividing the input power into four output ports labeled as port 2,3, 4 and 5.

The output points are arranged in such a way that, each point is located at a distance where mutual coupling between the resonating elements has no influence on the radiation properties. The corresponding output points are excited with a 90° phase shift signal in an anti-clock wise sequence. The corners of the microstrip line are chamfered for a smooth transformation of the power at the discontinuity junction points of the feeding network. The output of the parallel feed network was coupled to the antenna using probe and slot configuration. The axial ratio bandwidth achieved using this feeding arrangement is 26% and 23% for probe and slot respectively.

Another technique of sequentially rotated series feeding network is shown in Figure 7. In this configuration the output port is connected in series with the input and the impedance transformation is done in a similar way as in parallel feeding network i.e. by using quarter wavelength long microstrip line impedance transformer. Due to the limitation of low characteristic impedance of the series feed network, physical size of the width exceeds the length of the microstrip line and it becomes impractical to implement this configuration in thick substrate. Another shortcoming of the series feeding network is; it can be effectively implemented for circularly polarized DRA with slot coupling only. By using sequentially rotated series with slot feeding, maximum axial ratio bandwidth reported is 24.4%. An obvious increase of 1.4% in the CP bandwidth compared to 23%, which was achieved with sequentially rotated parallel network with slot.

Fig. 6 Sequentially rotated parallel microstrip line feeding network [61]

Fig. 7 Sequentially rotated series microstrip line feeding network [61]

Fig. 8 Sequentially hybrid ring microstrip line feeding network [61]

Another interesting feeding structure is a hybrid ring feeding network shown in Figure 8. In this configuration, the power divider(used in the previously discussed feeding configuration) has been replaced with a hybrid ring. Replacing the power diver with a hybrid ring does not change the phase and amplitude of the signal between the two output ports.The hybrid ring illustrated in Figure 8 is customized to a rectangular shape for easy tuning of the printing circuit. The microstrip line impedance for hybrid ring was kept to 50Ω for the thin substrate.

For a thick substrate, 50Ω line impedance is not recommended, because it will increase coupling in the center of the circuit which will lead to radiation losses in the feeding network.The axial ratio bandwidth accumulated with this feeding mechanism is 26.1%. If a comparison is made between the hybrid ring and power divider parallel feeding network (vertical probe type excitation) the axial ratio bandwidth achieved with both is the same. Some more examples on the circularly polarized DRA with sequential rotation are illustrated in Figure 9 [32, 38, 39, 62].

V. SELECTED CIRCULARLY POLARIZED DIELECTRIC RESONATOR ANTENNA

In this section, some selected design and techniques used for exciting DRA to radiate circularly polarized field will be addressed. In [63],the author investigated a hybrid rectangular circularly polarized DRA for satellite communications. Geometry of the antenna design with the hybrid configuration is shown in Figure 10.

In this antenna, a combination of slots was used with 90° phase shift to energize the rectangular dielectric resonator for circularly polarization. The hybrid configuration was proposed to enhance the axial ratio bandwidth of the rectangular DRA by combining the radiation of feed network and DRA. Size and location of the slots were optimized for generating two orthogonal TEδ11and TE1δ1degenerate modes with maximum electromagnetic coupling between the slots and DR. The authors claim that, by using this hybrid approach for circular polarization, a wideband axial ratio bandwidth was accumulated.

Fig. 9 Examples of sequentially rotated CP DRA [32, 38, 39, 62]

In [64], another interesting configuration of the feed line extension was investigated. The structure of the proposed antenna is shown in Figure 11. As illustrated in the figure, the antenna is comprised of a rectangular DR and a lumped resistively loaded monofilar-spiral-slot. The microstrip feed line is printed on top of the substrate, while monofilar-spiral-slot is on the ground plane. The microstrip line attached to the monofilar-spiral-slot is connected to the ground slightly away from the open end of the slot. A lumped resistance was introduced in the feeding network for enhancing impedance matching. The current distribution in the slot is highlighted in Figure 11(b) at different time steps. The varying electric field strength in the slot at different time intervals generated circular polarization in the dielectric resonator antenna. The fundamental TE111mode was excited inxandydirections of the rectangular DR. To compensate for the decrease in the resonance frequency due to spiral slot, modes in the DR were generated at slightly higher frequency. It was reported that,by utilizing this feeding technique, the axial ratio bandwidth of approximately 19% can be achieved.

Fig. 10 Hybrid rectangular CP DRA [63]

Authors in [65] studied another interesting technique for wideband circular polarization of dielectric resonator antenna. The geometry of the antenna is illustrated in Figure 12. As can be seen, a hollow rectangular shape DR is parasitically coupled to a conducting loop which is connected to the ground plane. Primarily the rectangular hollow DR was excited in linear polarization by proximity coupling technique using a straight microstrip line. For circular polarization, orthogonal fields were generated by the printed loop on the ground plane. The quadratic phase required in the rectangular DR for CP was also introduced by the printed loop which is parasitically coupled to the strip-feed line. To further increase the axial ratio bandwidth and gain of the antenna,an F-shape additional strip was connected to the rectangular strip printed on the ground plane. By using this feeding technique, for exciting a hollow rectangular shape DR, as wide as 51% axial ratio bandwidth with reasonable gain properties was achieved.

Fig. 11 Configuration and electric field distribution of monofilar-spiral-slot CP DRA [64]

As mentioned in the introduction, configuration of the feeding circuit is not the only factor that controls the polarization of the antenna; it can also be altered by the shape of the DR. Example of a special shape DR can be found in [66]. The authors reported a C-shaped dielectric resonator antenna as illustrated in Figure 13 for wideband circular polarization applications. This C-shaped dielectric resonator was excited by using coplanar waveguide with a simple straight line microstrip line. The structure itself was responsible for radiating circularly polarized field by generating orthogonal modes using single strip. The axial ratio bandwidth achieved with this setup was approximately 19%. This axial ratio bandwidth of the antenna was further enhanced by introducing an additional current path in the form of a narrow strip connected to the ground plane. Circular polarization was mainly controlled by the length ‘h’ of the additional strip as shown in Figure 13. Strength of the orthogonal mode generated in the DR is partially dependent on length ‘h’ of the strip. The additional current path introduce in the circuit extended the axial ratio bandwidth up to 51%.

Further in this article, some more circularly polarized designs are summarized in Table 2.All the CP DRA’s designs highlighted in table 2, the orthogonal modes were excited either by modification of the shape of dielectric resonator or the feeding network. Even though, the axial ratio bandwidth accumulated with the reported designs are relatively narrow, but these techniques could serve as a starting point for the antenna engineers. Some more CP DRA reported in the literature can be found in [79-85].

Recently, a hybrid rectangular dielectric resonator antenna was reported in [86], for wideband applications. The geometry and configuration of the antenna is shown in Figure 14. In this paper, a modified cross-slot structure was shown to excite the rectangular shaped DR.

Fig. 12 wideband CP rectangular ring-shaped DRA with parasitic printed loops[65]

Fig. 13 C-shaped wideband CP DRA [66]

The proposed cross-slot design also acts as a dielectric-loaded antenna. It was reported that, by optimizing all the parameters (Length of different arms, radius of the inner ring,width of the slot, dimensions of the DR, and position of the DR) of the cross-slot and rectangular DR, the resonances from the slot and the DR can be merged together for widebandCP antenna. Further, it was stated that, by combining two TE mode resonances generated in the slot and the DR at slightly different frequencies enhanced CP bandwidth of the antenna. This hybrid arrangement of a relatively simple profile antenna can accumulate axial ratio bandwidth as wide as 24.6%.

Table II Summary of selected circularly polarized DRA

Fig. 14 Cross-slot fed rectangular DRA for wideband CP [86]

Another important circularly polarized designed was reported in [87] in which the orthogonal modes in the dielectric resonator were excited by modifying the geometry of the resonator. The configuration of this design with two unequal inclined slits in the DR and tapered strips is illustrated in Figure 15. By introducing unequal slits in the diagonal of the square DR and connecting tapered strips to one edge of the DR and an external chip resistor to other edge of the DR, CP modes were excited. An axial ratio bandwidth as wide as 43-50% was achieved but the shortcoming of this design is the external impedance matching chip resistor attached to lateral side of the resonator which complicates the antenna geometry.

VI. CONCLUSION

The purpose of this article has been to provide technical review of the circularly polarized dielectric resonator antenna. A general overview of the circular polarization, its advantages over linear polarization and its importance especially in satellite communication are highlighted.Different techniques used for exciting circularly polarized field in the dielectric resonator,such as Single-point-feeding, multi-pointfeeding and sequential rotation techniques are addressed. In the last section of the paper,several CP DRAs are addressed which were designed using some special modification of the DR shape or the feeding network. Some more special designs for circularly polarized DRA are summarized in Table 2. It is concluded that the DRA has the flexibility to radiate circularly polarized field with wider axial ratio bandwidth compared to its metallic counterpart. It is believed that, further improvement in the CP DRA can be achieved by using inho-mogeneous dielectric resonators with special geometries or by modifying dielectric characteristics of the well known dielectric resonator shapes in the azimuth direction.

ACKNOWLEDGMENT

Authors would like to acknowledge Global Fellowship Scheme of Universiti Sains Malaysia and research grant number USM RUT 1001/PELECT/854004.

Fig. 15 Configuration of square dielectric resonator antenna with inclined slits

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