# SIW Cavity-Backed Antenna Array Based on Double Slots for mmWave Communications
## 3 Experimental Validation
The simulated and measured gain level is around 17 dBi and shows quite stable behavior in the desired working band (35.4 to 37 GHz). Notably, due to the transitions used for the receiving antenna and the AUT, some possible misalignments between the antennas, and slight modification of the phase center of the receiving horn with frequency, we estimates  
Figure 8: Simulated and measured H-plane radiation patterns of the proposed SIW antenna at: (**a**) 35.4, (**b**) 36.2, and (**c**) 37 GHz. Simulated and measured E-plane radiation patterns of the proposed SIW antenna at: (**d**) 35.4, (**e**) 36.2, and (**f**) 37 GHz. Figure 9: Three-dimensional (spherical) far-field radiation pattern of the proposed SIW antenna at: (**a**) 35.4, (**b**) 36.2, and (**c**) 37 GHz. In the plot, the angular turn is related to roll and the radial variation refers to the azimuth (steps of 10\({}^{\circ}\) between dashed rings). the error margin to be within 4.1%. The simulated directivity values are around 18 dBi, which means around 1.1 dB reduction due to the total efficiency of the antenna, which is directly linked to the dielectric losses. As cited in the Introduction, this is one of the main drawbacks of SIW antennas. Since the gain depends directly on the directivity and inversely on the losses, an increase in the losses leads to a reduction in the gain. However, this unfavorable effect may be compensated by obtaining a high-directivity antenna. In that case, even if the total power radiated is lower due to losses, this power would be concentrated in the desired direction. The directive radiation pattern and the gain value achieved make this antenna suitable for directive communication systems in millimeter-wave bands. Additionally, due to the use of a corporate feeding network and cavity-backed antennas, the design of the antenna array allows the possibility of increasing the directivity and reducing the beamwidth in the steering direction by simply increasing the number of elements in the array. This array enlargement only requires stacking additional SIW corporate feeding network layers. For instance, an additional one-to-four corporate feeding network, which feeds four antenna arrays identical to the one presented in this paper, would permit the easy implementation of an array of \(8\times 8\) antenna elements. In addition, some challenges for the proposed design can be conceived, such as a wider operating band extension or modification using radiated polarization (the proposed design provides linear polarization). Finally, in Table 1, the proposed SIW antenna is compared with other slotted SIW antennas reported in the literature. The proposed antenna array design provides one of the highest working frequency bands compared with the reported antennas. Only the antenna proposed in [14] presents an operating frequency band in the upper part of the Ka band; however, its relative bandwidth is smaller than that of the proposed design. Another remarkable feature of our antenna design is the maximum value of the gain achieved. This illustrates the efficiency provided by our antenna design even in frequency bands higher than those of some of the designs presented in other works with the same number of radiating elements [11; 13]. Figure 10: Simulated directivity, simulated gain, and measured gain values.