## 3 Results and Discussion  
The prototype of the antenna with specification--in Figure 2, in Table 1, and with the description in second section of this article--has been manufactured using standard low-cost single-layer PCB process. The SIW-to-GCPW transition is used here, along with SEL, to facilitate the measurement of the proposed antenna. In the practical use case scenario, the SIW-to-GCPW is not compulsory and it has been used here, merely to facilitated the electrical connection between testing-equipment and the antenna under test (AUT). The SIW-to-GCPW transition is designed through simulation, inspired from the work reported in [11], and the detailed dimensions of the optimized structure are shown in Figure 4. Figure 5 present the photograph of fabricated antenna showing its front- and the back-side view--along with supporting components including SEL. Figure 4: Substrate Integrated Waveguide to the Grounded Co-Planar Waveguide (SIW-to-GCPW) transition. Figure 3: Surface current-density on the top metal surface (including circular-patch) of the proposed antenna at 34 GHz. With excitation phase of (**Left**) 0\({}^{\circ}\) and (**Right**) 90\({}^{\circ}\). The simulated and the measured return-loss performance of the design antenna, presented in Figure 5, is shown in Figure 6. The test results match well with the simulation and the measured \(S_{11}\leq-10\) dB impedance bandwidth is around 9.5%. There is slight frequency shift of about 300-400 MHz, towards lower frequency, that should be due to the changes in substrate material properties, as the parameters given in the datasheet of the Rogers RT/Duroid 5880 are extracted over 10 GHz, only [21]. To measure the far-field radiation characteristics of the proposed CP antenna, the rotating linearly-polarized horn antenna has been used as the source. The method from [22] has been adopted to extract the antenna radiation parameters from test data including the AR, and the peak-gain. The absolute gain of the CP antenna can be extracted from radiation pattern, using [23]  
\[G(dBic)=G_{max}+3+20\text{log}\left(\frac{1+10\frac{\omega}{2}}{2}\right) \tag{3}\]  
Figure 5: Photograph of the fabricated antenna with Southwest(r) end launcher (SEL). Figure 6: The simulated and the measured return-loss performance graph of the designed antenna. Figure 7 illustrate the combined measured and simulated AR as well as combined measured and the simulated gain over frequency graph. The measured AR-bandwidth is about 2.3% and wider than in simulated results. In simulation the boresight-gain of the proposed antenna is quite stable and in a range of 8 to 8.5 _dBic_. Nevertheless, the measured gain over frequency graph is not stable; at such higher frequency when the radiator is relatively smaller, any foreign object in vicinity could have unwanted influence. It is highly likely that the SEL could have caused the gain fluctuation over frequency. The normalized, measured (at 33.35 GHz) and the simulated (34 GHz), radiation pattern of CP antenna is shown in Figure 8. The reason to choose the different frequency points in measured and the in simulated radiation pattern graph, in Figure 8, is the frequency shift of around 500 MHz, towards lower frequency, observed from AR vs. frequency graph. The results shows the good agreement between simulated and the measured radiation performance, especially in the E-plane. Also, in the E-plane the 3 dB AR beamwidth is wider than 120\({}^{\circ}\). In H-plane, especially over the theta above 30\({}^{\circ}\) there is deterioration in the AR, that is also due to the foreign object (SEL) in AUT close vicinity. In simulation the FBR of the designed radiator is under \(-\)20 dB. A comparative study has been conducted between the work reported in some references and in this article, in terms of the frequency, bandwidth, gain, and the AR-bandwidth.