**4. Antenna structures**

The use of antenna arrays in communication handsets is feasible in mm-Wave communications due to the inherently small size of the antennas. However, applying diversity techniques in these systems require specific properties of the antennas, like stable and controllable radiation patterns, and the ability to produce a desired polarization with low cross polarization level. In this section, we present different antenna designs covering the

*Ridge Gap Waveguide Beamforming Components and Antennas for Millimeter-Wave Applications DOI: http://dx.doi.org/10.5772/intechopen.105653*

possible polarizations, namely linear, circular, and dual polarizations as candidates for mm-Wave applications. The presented designs show promising performance in terms of beam stability, wide bandwidth, and low cross polarization level.

## **4.1 Linearly polarized antenna**

As shown in **Figure 12**, a linearly polarized planar aperture antenna is presented, based on a similar design [69]. The antenna aperture has a cross-shaped patch in the middle, all in one top layer, fed by capacitive coupling from a differential feeding line in a bottom layer [58, 69]. The benefit of using planar radiating aperture is the ability to produce highly directive beam without need for increased dimensions. The feeding line is tapered and loaded with stubs to achieve acceptable matching level. The dimensions of the radiating element and the feeding structure are tuned to achieve optimum bandwidth, in terms of return loss and beam stability over the band [58]. The final dimensions are given in **Table 14**, while simulated and measured reflection coefficient and realized gain are plotted in **Figure 12c**. To obtain the measured results, the fabricated antenna was fed by the rat-race directional coupler, described in the previous section, to provide the differential feeding signal [64]. The results, as shown by **Figure 12c**, reveal a wide bandwidth


**Table 14.**

*Dimensions of the planar aperture antenna and its differential feeding layer shown in Figure 12a and b.*

#### **Figure 13.**

*Radiation properties of the linearly polarized planar aperture antenna. (a) simulated and measured co- and x- polarization patterns in E-plane at 30 GHz. (b) simulated and measured co- and x- polarization patterns in H-plane at 30 GHz.*

over 25.6–34.3 GHz band with more than 10 dB return loss and with 12.28 dBi maximum gain. Moreover, the 3 dB gain bandwidth covers the range from 25.6 GHz up to 33.7 GHz, indicating a stable beam of the antenna. Beam stability is further revealed by measuring the radiation pattern at multiple frequencies over the band [58]. The simulated and measured patterns at the center frequency of 30 GHz are shown in **Figure 13a** and **b**. These patterns illustrate a very low cross polarization level in both E- and H- planes.

#### **4.2 Circularly polarized antenna**

Alternatively, the antenna design illustrated in **Figure 14** provides circular polarization (CP) at the same frequency band around 30 GHz. Unlike typical CP antenna designs, which depend on feeding the radiating element by two equal amplitude and quadrature phase signals, this design uses differential feeding to a planar aperture loaded with a polarizer [63]. The polarizer consists of an annular ring with two opposite cuts adjacent to the feeding position. These cuts perturb the excitation and cause the formulation of two orthogonal modes. The circular polarization is then adjusted through the circular patch added at the center to tune the amplitude and the phase relation of these orthogonal modes [63]. This tunning is performed by adjusting the patch size, and the orientation of the two introduced non-radiating edge slots. The antenna is designed on

*Ridge Gap Waveguide Beamforming Components and Antennas for Millimeter-Wave Applications DOI: http://dx.doi.org/10.5772/intechopen.105653*

#### **Figure 14.**

*Design and performance of the CP antenna: (a) design details. (b) simulated and measured* ∣*S*11*dB: (c) simulated and measured gain and axial ratio over the band. (d) simulated co- and x- polarization patterns (RH and LH CP's) in E-plane. (e) simulated co- and x- polarization patterns (RH and LH CP's) in H-plane.*

the Rogers RT5880 substrate with 0.127 mm thickness and 2.2 relative permittivity [63]. All the design dimensions are given in **Table 15**, for the planar aperture, the polarizer, and the patch in the center. Since this antenna is mainly aimed for use in communication arrays, the performance is investigated for an array of 4 elements shown by the 3D view of the design layers in **Figure 14a**. The simulated and measured reflection coefficients of that array are given in **Figure 14b** and indicate below 10 dB reflection over the 28–32.5 GHz frequency range, which is equivalent to 15% relative bandwidth at 30 GHz. Simulated and measured axial ratio illustrated in **Figure 14c** is showing 3 dB axial ratio


**Table 15.**

*Dimensions of the circularly polarized antenna illustrated by Figure 14a.*

over 28.5–31.5 GHz range, a slightly smaller relative bandwidth of 10%. However, these achievements are greater than the usually narrow bandwidths for CP antennas reported at 30 GHz. Regarding the cross polarization level and the beam angle, simulated and measured radiation patterns at 30 GHz are plotted in **Figure 14d** and **e**, showing less than 20 dB cross polarization levels at the direction of maximum radiation [63].
