**3. Generation of OAM vortex beam using uniform circular antenna array**

First, we have analyzed a circular antenna array with eight elements for generating the OAM wave propagation at a frequency (f = 2.45 GHz), which has been widely used in literature, as shown in **Figure 3**. The number of elements in UCCA determines the largest mode ð Þ*l* generated by the array. The mode number is governed as per theory �*N=*2 <*l* < *N=*2, where N is the number of elements in the array. Therefore, for the eight elements array, the mode number that can be produced is �4<*l* <4. For an N-element UCCA, each patch element is fed by the same amplitude but with an incremental phase shift, as shown in **Figure 4**. The generation of a particular mode in UCAA is achieved by an interelement phase shift as 2*πl=N*, where *l* is the OAM winding number.

The simulated reflection coefficient is well below �15 dB at *f* ¼ 2*:*45 GHz, as depicted in **Figure 5**. A simultaneous excitation of each element with constant amplitude and differential phase shift, as shown in **Figure 4**, is incorporated in simulation to obtain the reflection coefficient. The simulated 3D radiation pattern in both dBi and linear scale is shown in **Figure 6**. It can clearly be visualized the amplitude null in the boresight direction along the z-axis.

**Figure 7(a)** depicts the orthographic view of the power flow of an antenna, which shows annular intensity cross sections with null energy at the center, and maximum

**Figure 4.**

*Amplitude and phase excitation pattern of an eight-element array.*

**Figure 5.** *Reflection coefficient of eight-element UCAA.*

**Figure 6.** *3D radiation pattern. (a) Realized gain (dBi) and (b) realized gain (linear).*

energy is exhibited on the ring, satisfying the key characteristics of an OAM wave. This is shown for OAM mode *l* ¼ 1, whereas for mode *l* ¼ 0, beam power is at the center, forming the plane wave case. The twisted radio beams can be observed from the phase fronts of an OAM, as shown in **Figure 7(b)**.

**Figure 7.**

*(a) 2D plot of power flow of an antenna and (b) 2D plot of phase fronts of OAM mode 1 at cutplane z* ¼ 32 mm*.*

**Figure 8.** *1D polar plot of gain of a UCCA antenna. (a) Realized gain (dBi) and (b) realized gain (linear).*

It is clearly observed in **Figure 8** that half-power null size from �33<sup>∘</sup> to <sup>þ</sup>33<sup>∘</sup> , and maximum gain of 9.15 dBi or 8 (linear scale) at 23<sup>∘</sup> . The design of a uniform circular antenna array (UCCA) and uniform concentric circular antenna array (UCCAA) for generating particular OAM modes require a complex feed network. Its design becomes further complicated if multimodes OAM is generated simultaneously. Henceforth, in another section of this chapter, our discussion is on generating OAM waves using a patch antenna, which reduces the complexity of feed design.
