Latest Trends in the Field of Printed Antennas
Dr. Anand Sharma
Motilal Nehru National Institute of Technology, Allahabad
Printed antennas are a very important part of today’s wireless communication. It is because of their inherent qualities such as compactness, conformability to planar and non-planar surfaces and easy mountability to Monolithic Microwave Integrated Circuits (MMICs) . Two or three decades before, researchers mainly focused on the bandwidth enhancement, generation of multiple frequency bands in a single radiator, and size reduction of printed radiators [2-4]. Now, the wireless communication technology is exponentially growing. Different wireless devices such as mobile phones, hand-held computers and multimedia devices are being used by millions of users at a time. In this era of wireless communication, we are talking about 5G or 6G technology, which requires not only a high data rate but also an efficient spectrum utilization. Now, wireless technology is moving towards high frequencies for getting more and more bandwidth as well as a high data rate. Due to the frequency increment, metallic losses are so large, which in turn reduces the antenna gain. That is why today’s research is also being focused on getting a higher gain. Another important research area in the field of printed antennas is the designing of super-wideband antennas, which are also helpful to get a higher data rate in both indoor and outdoor communication systems. Designing of printed antennas with circular polarization features is a hot research topic in the field of microwave antennas. It is because of their wide requirement in the field of satellite communication for maintaining the transmitter and receiver orientation independent. In the current scenario, antenna engineers focus on the designing of an efficient MIMO antenna for getting a higher data rate and maintaining the link reliability. For that purpose, envelope correlation coefficient (ECC) parameter is targeted by researchers. Frequency-selective surface- based Fabry-Perot cavity method is highly effective in the case of ECC (using far-field parameters) reduction.
In this chapter, the latest research trends in the case of printed antennas are discussed. It is divided into six subsections: (i) discussion on the latest techniques for enhancing the gain of printed antennas; (ii) MIMO-based printed antennas with RCS reduction features; (iii) design requirements of printed antennas for 5G applications; (iv) different techniques of circular polarization development; (v) design procedure of super-wideband antennas; and (vi) ECC reduction techniques in the case of printed MIMO antennas.
Latest Research Areas in the Field of Printed Antennas
High-Gain Printed Antennas
Gain is the very important parameter of any antenna. One can say that it is a figure of merit for any antenna. It is a well-known fact that an antenna is a passive element. Then, the question is what the significance of the term gain of an antenna is. Actually, the antenna gain basically indicates how much input power is converted into radiated power in some specific direction. We measure the enhancement (gain) in radiated power in a particular direction with respect to hypothetical antennas such as isotropic antennas. Antenna gain is directly related to the efficiency as well as the directivity of the radiator (G = rD) .
This has been a highly attractive topic of research in the field of antennas for several decades. The reason for attraction can be easily understood by the Friis equation :
From eq. (2.1), it can be said that the received power (Pr) can be improved by enhancing the gain of transmitting (G,) and receiving antennas (G,) without enhancing the transmitted power (within the given range). Nowadays, due to strict government regulations, one cannot enhance the transmitting power for getting a better range or signal power. So, gain enhancement has become a hot topic of research in the past few decades. A low value of gain (2.0-3.0 dBi) is a big problem in the case of conventional printed antennas because of high metallic losses and lesser directivity . Today’s communication world is moving towards higher frequencies, which makes the problem of low gain more and more dominant. Therefore, in order to get the full advantages of printed antennas (i.e. along with compactness), it is very important to develop some techniques for getting a higher gain value.
The research on gain enhancement, in the case of printed antennas, has started three to four decades ago. There are large numbers of techniques available in the literature for the enhancement of gain of printed antennas. As the relationship between directivity and gain suggests, there are two ways to get a better gain value: either by increasing the radiating power density in some specific direction or by reducing the losses associated with the antenna. In 1985, D.R. Jcakson and N.G. Alexopoulos, two US-based researchers, invented a new technique for the gain enhancement with the help of a superstrate . Figure 2.1 shows the arrangement proposed by the aforementioned researchers.
In this technique, a very high permittivity substrate is used in the antenna structure, which reduces the half-power beam width of the antenna | в,, <*—!—I and
makes the pattern more directive. It is helpful to get a better gain value. This phenomenon can be understood mathematically as follows :
In eq. (2.1), le2’ and ‘e,’ are the permittivity of the superstrate and the antenna substrate, respectively. 'ц2' and H are the permeability of the superstrate and the thickness of the antenna substrate, respectively. In this technique, a very high permittivity superstrate (£,. > 100) with low loss has been used. Such types of materials are impractical. Therefore, H.Y. Yang and N.G. Alexopoulos revised the technique and used a multi-superstrate arrangement in place of a single superstrate. They arranged the multiple superstates in two ways, i.e. electric-magnetic-electric (type-1 resonance) and magnetic-electric-magnetic (type-2 resonance) . Figure 2.2 shows the E-plane pattern for type-1 and type-2 resonances with two, four and six layers of dielectric. In both the cases, the gain value increases largely. Type-2 resonance provides a better gain value as compared to type-1 resonance. But, the superstrate method suffers from two drawbacks, i.e. reduction in bandwidth and increased antenna thickness. In 1988, R.Q. Lee and K.F. Lee proposed the concept of an electromagnetically coupled microstrip radiator antenna for gain improvement . Figure 2.3 displays the proposed concept graphically. They took the idea from the Yagi-Uda antenna,
FIGURE 2.2 E-plane pattern for type-1 and type-2 resonances with two, four and six layers of dielectric .
FIGURE 2.3 Multilayer electromagnetically coupled microstrip antenna .
where parasitic directors are used to improve the directivity and gain. An electromagnetically coupled microstrip antenna is generally divided into three regions: The first region is used to enhance the bandwidth, the second region is used for radiation purposes, and the third region is used for gain enhancement. All these classifications are based on the separation between the fed patch and the parasitic directors. The authors fed the patch with the fundamental TM01 mode. Two parasitic patches are placed above the driven element at a distance of 0.352 and 0.822, respectively, for the enhancement of directivity. Figure 2.4 displays the 2D far-field variation in both principle planes with single-patch, two- and three-layer electromagnetically coupled antennas. Figure 2.4 clarifies that the maximum directivity is obtained with three layers in broadside direction. This technique enhances the gain value from 4.7 to
10.6 dBi with less than 1.0% reduction in bandwidth. In E-plane, the beam width is reduced from 103° to 30°, and it is reduced from 70° to 35° in H-plane.
FIGURE 2.4 Far-held pattern in E- and H-planes with single-patch, two- and three-layer EMCP .
FIGURE 2.5 (a) 3x3 U-slot array antenna layout; (b) gain and efficiency variation .
Printed array antennas are also an important technique for gain enhancement, in which a 3-dB power divider is used to feed the different radiating elements. This feed arrangement provides the current distribution in the same phase in each radiating element. The coupling between radiating elements is also less in the array antenna design. This method is able to give a large gain value without (or sometimes minimally) affecting the impedance bandwidth.
Chen et al. proposed two different U-shaped slot-loaded patch array antenna arrangements, i.e. 3x2 array and 3x3 array. Figure 2.5 shows the geometrical layout of the 3x2 U-shaped slot array and the gain variation of the given array. As the authors move from the 3x2 array to the 3x3 array arrangement, the value of gain is increased by 1.8 dBi. Similarly, the bandwidth also increases by 14%-20%. This antenna arrangement is able to get a maximum gain value of 19.4 dBi . The losses associated with the feed line are more in the array case, which in turn reduces the efficiency of the antenna at higher frequencies.
Another important way of gain enhancement is the loading of meta-material-based resonating structure with the printed radiating structure. The loading of the meta-material structure enhances the refractive index of the substrate electrically, which will bend the radiation beam in certain direction (as per Snell’s law). It will enhance the directivity of the antenna, which in turn enhances the gain value. This concept was well used by Wang et al. in 2014. They used H-shaped resonating structures periodically at the top of the antenna substrate. The value of refractive index, after applying resonating structures, is calculated as follows :
The antenna design and its gain variation are shown in Figures 2.6 and 2.7, respectively. The antenna is designed vertically above the ground plane. An H-shaped
FIGURE 2.6 Vertical printed antenna with H-shaped resonating structure .
resonating structure is used to enhance the gain value. This phenomenon improves the gain value by more than 5.0 dBi. The loading of frequency-selective surfaces (FSS) over the printed antenna is also an important and latest technique in the field of gain enhancement. Frequency-selective surfaces are periodic arrangements of unit cells. This type of structure is placed over the printed antenna, which makes a cavity-like structure. EM waves strike the FSS and are reflected back to the antenna. Finally, a radiation beam leaks through the FSS. This beam has a high directivity, which in turn improves the gain value .
Shalini et al. proposed a FSS-loaded dual-band slot antenna with an improved gain, which is displayed in Figure 2.8. In the aforementioned antenna design, the FSS is designed in such a way that it can be used for both the operating frequency ranges. After applying the FSS structure, the value of gain is improved by 8.0 dBi in the working frequency range . Figure 2.9 presents the gain variation with and without the use of frequency-selective surfaces. From Figure 2.9, it can be observed that the gain value is enhanced by 6-7 dBi in both the frequency ranges.
However, in FSS technique, the gain improvement is large as compared to other discussed methods, but it affects the main advantage of the printed radiators, i.e. compactness. Therefore, Prateek Juyal and L. Shafai proposed a printed radiator with an improved gain value with no (or very little) effect on the antenna size. The authors proposed the superposition of two modes in order to enhance the directivity and gain w'ith less effect on physical size. Initially, they added the radiation in TMn and TMB modes. These modes are created by two circular discs placed in a staked configuration. The circular discs are arranged by size in such a way that both modes have the same resonant frequency. In this antenna design, the value of gain is about
13.06 dBi . But, the staked configuration still creates the problem of size. In order to overcome it, the same authors proposed a new printed antenna based on the same concept, i.e. superposition of two modes (TMI2 and TMI4 modes), but the radiators are placed in the same plane. In this case, the value of gain is larger, i.e. 15 dBi . Antenna geometry of the staked configuration and the single-layer configuration is shown in Figure 2.10.
FIGURE 2.9 Gain variation with and without using frequency-selective surface over the printed radiator .
FIGURE 2.10 Dual-mode printed antenna configuration: (a) staked geometry ; (b) single-layer geometry .
The current wireless communication world has a wide requirement of a large data rate for rapidly increasing data traffic. To fulfil such type of requirement, mm-wave spectrum has been utilized by the wireless communication engineers. At mm-wave spectrum, a wide bandwidth is available as compared to lower frequencies, which is helpful to get a higher data rate. But, at mm-wave frequencies, metallic losses are very high, which in turn reduces the antenna gain effectively. As discussed above, the gain of an antenna can be enhanced by several ways, such as the use of reflectors and antenna arrays. But, at mm-wave frequencies, these techniques are not used because of certain reasons. For example, the use of reflectors along with radiators limits the impedance bandwidth and the frequency of operation is sensitive to the gap between the reflector and the antenna. Antenna arrays are not very effective at mm-wave frequency because they suffer from a large feed loss. Recently, a new technique has been developed, i.e. substrate integrated waveguide (SIW) antenna, for gain enhancement at mm-wave frequencies. SIW structure consists of two metallic plates separated by a dielectric material. The two metallic plates are connected through conducting vias. The conducting vias connect the surface current of upper and lower metallic plates in order to maintain guiding structures. SIW antennas work based on the principle of leaky wave antennas such as waveguide antenna. Leaky wave antennas generally have a high directivity. The feed network loss is minimum in the case of SIW antennas. This is the main reason for getting a high gain value. Wahab et al. proposed an aperture-coupled microstrip antenna design at 60.0 GHz. The aperture is fed by substrate integrated waveguide. The authors discussed two different antenna designs based on aperture orientation: (i) transverse slot and (ii) longitudinal slot. Figure 2.11 displays the antenna design, its reflection coefficient, and gain variation. In both the configurations, the antenna gain is about 6.0 dBi at
FIGURE 2.11 Millimetre-wave microstrip antenna: (a) radiator geometry; (b) reflection coefficient; (c) gain and efficiency variation .
FIGURE 2.12 SIW antenna at mm waves: (a) antenna geometry; (b) reflection coefficient variation; (c) gain variation .
60.0GHz . Similarly, M. Asaadi and A. Sebak proposed a high-gain low-profile circularly polarized SIW antenna. In this antenna design, periodically arranged slots are excited by SIW. Figure 2.12 shows the SIW-based antenna design, its reflection coefficient features and antenna gain variation. This radiator gives approximately 16.0 dBi gain at 28.0GHz frequency .