**5. Comparison of PWPA and OWPA**

Until this point, the behavior of PWPA and OWPA is well understood, and it will be worth comparing the performance of these antennas. Let us begin with comparing the near-field electric field as depicted in **Figure 17(a)**, which shows a spiral motion of the electric field with null energy at the center for OWPA. At the same time, PWPA has energy at the central axis through an isoline pattern. **Figure 17(b)** also predicts similar behavior for PWPA and OWPA while showing their near-field magnetic field. It is worthwhile to visualize the E-near-field scan in **Figure 17(c)** on a cylindrical surface, which clearly indicates a helical motion of the electric field along the Z-axis

**Figure 15.** *Simulated and measured reflection coefficients of OWPA.*

**Figure 16.** *1D polar plot of simulated and measured realized gain for E-plane and H-plane: (a) realized gain in dBi and (b) realized gain in linear.*

(propagation axis) while the wave is advancing forward from OWPA. In contrast, the cylindrical scan for PWPA shows a typical plane wave pattern progressing normally in the direction of propagation. If we visualize the scenario in the cylindrical coordinate

system ð Þ *ρ*, *ϕ*, *z* , E-field is swirling along the *ρ* ! and *ϕ* ! orientation while advancing along the z-direction, precisely verified from Eq. (9) in theory.

This phase profile of the OAM beam is orthogonal to the propagation axis. The cylindrical scan is created for *<sup>h</sup>* <sup>¼</sup> 1 m, *<sup>r</sup>* <sup>¼</sup> 5 cm, and step size 1° in CST Studio Suite®. This shows us that PWPA has a uniform phase front, whereas the phase front of OWPA is changing with respect to exp ð Þ �*jlϕ* . This phase change pattern is also noted in **Figure 17(e)**, which shows twice the change 2*π* over a wavelength for OWPA, suggesting that OAM winding number or mode ð Þ*l* 2 be produced. Comparing PWPA and OWPA through **Figure 17(d), (f)** and **(g)**, E-pattern in far-field (head-on), power flow (head-on), and 3D radiation pattern, respectively, simulated in CST-MWS, all
