**1. Introduction**

Electromagnetic (EM) waves have been known to human society since the sixteenth century when mariners observed that lightning strikes could deflect a compass needle. EM waves transport both energy and momentum, where the momentum is categorized into two types: linear momentum and angular momentum. Specifically, the angular momentum has one added component in connection with field polarization, namely spin angular momentum (SAM), and the other component associated with spatial field distribution, termed as orbital angular momentum (OAM). Allen et al. [1] first introduced the concept of OAM in the optical domain. The twisted radio waves are characterized by amplitude singularity along its beam axis, and helical phase front of exp ð Þ �*jlϕ* , where *ϕ* denotes azimuthal angle around the beam axis, � indicates respectively clockwise and anticlockwise helical-phase front, and *l* corresponds to OAM index number, also called as topological charge, which suggests the number of twists in one wavelength [1]. OAM wave is widely investigated, and physical systems based on OAM wave have been implemented in the optical domain [2, 3]. Perhaps, the OAM research in microwave areas is focused on increasing the channel capacity of wireless communications by multiplexing different modes of OAM at a single frequency [4–6].

Current communications and satellite systems across the globe are based on linear and spin angular momentum (plane wave propagation). People have used all the dimensions of space, time, frequency, and polarization for multiplexing signals in the channel to achieve a maximum data rate. Foreseeing the nature of the emerging and connected world, scientists anticipate that we will need a data rate of terabit per second (Tbps) for transmission in a decade. Presently used means of wireless communication can support data rates only up to Gbps as we have utilized the maximum extent of resources to achieve this. So, there is the need for another degree of freedom to achieve the data rate of Tbps, which can be realized by multiplexing different OAM modes at a single frequency [7]. However, the practical adoption of OAM waves for long-distance wireless communication is unsuitable because of singularity (null) in the boresight direction of radiating fields [8, 9]. It has been shown through the aperture antenna theory that OAM capacity can be significantly increased for nearfield applications by using an extended OAM receiving antenna [9]. In 2012, Tamburini et al. experimented through the transmission of a signal for 442 m using a helical reflector antenna to increase the channel capacity of a wireless communication link [10]. However, many controversies arose from whether OAM multiplexing is a subset of MIMO and OAM multiplexing cannot work in the far-field [11]. In 2018, NTT Japan demonstrated a 10 m OAM multiplexing experiment with uniform concentric circular array (UCCA) and OAM–MIMO multiplexing, achieving a milestone data rate of 100 Gbps [12]. Despite some controversies, the scientific community is searching for more mature research and development in OAM, OAM antenna, and its system-level implementation has gained momentum in the past decade.

Obviously, OAM has potential applications in optical fiber communication, freespace optical communication, radio communication, and acoustic communication [13]. Its applications mainly lie in high-speed and high-capacity communication,

#### *Orbital Angular Momentum Wave and Propagation DOI: http://dx.doi.org/10.5772/intechopen.104477*

radar, imaging, and particle manipulation in the optical domain. Initially, cylindrical lenses were used to generate Hermite–Gaussian (HG) beam for the OAM wave by combining different states of the HG beam [1], then spiral phase plates were deployed to generate OAM waves in the optical domain [14]. In the radio domain, a similar trend was followed, and most research activities initially were focused on the generation of OAM waves using spiral phase plates [14–17]. Then, spiral reflectors were used to generate vortex beams [10, 18]. Also, planar antenna topology was deployed to generate OAM waves using uniform circular antenna arrays (UCCA) [19–25]. In [26], a circular time-switched antenna array was studied to generate an OAM wave. The time switched method uses high-speed RF switches to sequentially activate each array element, thus allowing the concurrent generation of OAM modes at harmonic frequencies of the switching period. Furthermore, a traveling wave circular slot was implemented to generate OAM waves [27, 28]. In addition, slot- and SIW-based OAM antennas were also considered [29, 30]. As in the optical domain, metamaterials have been used to generate OAM waves, similar to that has been implemented in the radio domain categorized as reflective metamaterials [31–40] and transmissive metamaterials [41]. OAM has also found its place in the guided-wave structure [42–47]. OAM has found applications in the acoustic domain, and analogous to antenna array in radio, uniform circular transducer antenna arrays are immensely used to generate acoustic vortex beam [48–50]. In nature, the vortex wave can well be observed as a twisting wave around a black hole [51], the most recent study of twisting wave around M87 black hole in radio astronomy [52], and the helical wave around the hurricane having a center point with null energy similar to the case of OAM [53]. **Figure 1** summarizes the state-of-the-art antenna types studied and researched for generating and detecting OAM waves from GHz to THz.

However, these previous works are dedicated to developing OAM waves using uniform circular antenna array (UCAA), spiral phase plates, and OAM multiplexing using UCCA. Minimal efforts have been invested in research to generate an OAM beam using a single patch antenna [60–62], which is set to relieve from complex feed network and space utilization. In this work, single patch-based plane wave patch antenna (PWPA) and OAM wave patch antenna (OWPA) are deployed as a demonstrator model to compare key performance parameters of PWPA and OWPA. Moreover, the main objective of this work is to show a comprehensive comparison of plane wave and OAM wave and appreciate the fundamental and distinguished properties of the OAM wave against those of the plane counterpart. Analytical, simulation and measurement comparisons between them are carried out thoroughly. In addition, it shows how the OAM wave can be useful for certain wireless applications even though point-to-point wireless connectivity with OAM is still a challenge because of its divergence pattern. Nevertheless, it is promising for short-range radar sensing applications. In this chapter, a detailed theoretical formulation of vector potentials, electric and magnetic fields for TE and TM modes, intrinsic impedances, wave vectors, propagation constants, wavelength, and pitch is devised and studied as a generic case for OAM waves. It shows clearly that the key performance parameters of plane waves are a special case of OAM waves. Furthermore, a narrative discussion of PWPA, OAM wave generation with single element patch using CMA, propagation, and characteristics comparison of PWPA and OWPA is carried out through simulations and validated through measurements.

Section 2 discusses the theoretical investigation of OAM wave and plane wave and their comparison in the rest of the chapter. Then, the OAM wave generation technique using UCCA is discussed in Section 3. This is followed by discussing simulated and measured results for PWPA and OWPA in Section 4. Subsequently, PWPA and

**Figure 1.** *State-of-the-art generation techniques of OAM wave using different antenna types from GHz to THz [54–59].*

OWPA performances are compared critically in Section 5. Finally, plane wave propagation and OAM wave propagation are investigated and compared for different scenarios in Section 6. Then, concluding remarks are observed in Section 7.
