# A Ka-Band Circular Polarized Waveguide Slot Antenna with a Cross Iris
## 4 Experimental Results  
Photographs of the fabricated antenna are shown in Figure 12. The manufactured antenna is assembled in two parts: an upper cavity and a rectangular waveguide with a cross iris. To minimize manufacturing errors, the electroforming process was used. For measuring the reflection coefficient and the radiation pattern with the axial ratio, an Agilent N5247A vector network analyzer with Agilent R11644A calibration kit with a WR-28 coaxial to waveguide adapter were used. They were measured inside the millimeter-wave anechoic chamber equipped with MTG operating software. Figure 13 shows the simulated and measured reflection coefficients and axial ratio of the proposed antenna. The measured -10 dB reflection coefficient bandwidth is 7700 MHz (31.51 GHz-39.21 GHz) and the fractional bandwidth is about 21.7%. The measured 3 dB axial ratio bandwidth is 6810 MHz (31.72 GHz-38.53 GHz) and the fractional bandwidth is about 19.38%. There are slight discrepancies between the simulation and measurement results. The discrepancies are due to the differences in the cross iris dimensions of simulation and those of the manufactured antenna.  
Figure 11: Simulated reflection coefficients for different height (\(H_{4}\)) values of the cross iris.  
Figure 12: Photographs of the fabricated antenna: (**a**) top view; (**b**) side view; (**c**) top view of the bottom waveguide; (**d**) bottom view of the upper cavity.  
Figure 13: Simulated and measured (**a**) reflection coefficients and (**b**) axial ratio of the proposed antenna.  
Figure 14 shows the simulated and measured radiation patterns of the proposed antenna in the xz-plane (\(\varnothing\ =\ 0^{{}^{\circ}}\)) and yz-plane (\(\varnothing\ =\ 90^{{}^{\circ}}\)) at 33.5 GHz and 37.6 GHz. The simulated and measured radiation patterns agree well in both the xz and yz planes. It can be observed that the antenna has directional patterns with the inclined maximum gains due to the effect of a hexagonal shaped cavity. The simulated bore-sight gains are 8.27 dBi and 6.13 dBi, and those measured are 7.87 dBi and 5.76 dBi at 33.5 GHz and 37.6 GHz, respectively. At 33.5 GHz, the maximum gain of 9.08 dBi is achieved at 18\({}^{\circ}\) in the xz-plane. At 37.6 GHz, the peak gain of 7.33 dBi is obtained at 24\({}^{\circ}\) in the xz-plane.  
Table 2 compares the dimensions, 3-dB axial ratio bandwidth, and \(-\)10 dB reflection coefficient bandwidth values from the literature with those of the proposed antenna. The fractional 3-dB axial ratio bandwidth of the proposed antenna is much wider than the others with a broad overlapping bandwidth (88%) over which both the 3 dB axial ratio and \(-\)10 dB reflection coefficient requirements are satisfied. Even though some antennas [11; 12] have a shorter length, their overlapping bandwidths (40.5% and 50%) are much smaller than that of the proposed antenna. The antenna in [17] has a very narrow 3 dB axial ratio and \(-\)10 dB reflection coefficient bandwidth even with the size comparable to the proposed antenna. Therefore, the proposed antenna has several advantages over other waveguide CP antennas such as a good CP characteristic with a wide operating frequency and a low profile.  
Figure 14: Simulated and measured radiation pattern of the proposed antenna at (**a**) 33.5 GHz, and (**b**) 37.6 GHz.