## 3 Fabrication and Measurements  
The proposed antenna was fabricated by etching both sides of the same Rogers substrate used in the simulation and shown in Figure 7a. A U.S. quarter was placed for size comparison. The impedance resonance characteristic was measured using an Anritsu MS46522B vector network analyzer. As shown by the dashed line in Figure 4, it resonated at 2.47 GHz, showing very good agreement with the computed expectation with only negligible deviation. The gain, radiation efficiency, and patterns were measured in an anechoic chamber. The two-dimensional electric field radiation pattern measurement was carried out in the far-field measurement chamber as shown in Figure 7b. It had dimensions of \(14\times 7\times 6.7\) m and was available for the frequency range from 200 MHz to 20 GHz with an E5071C ENA series network analyzer (Agilent Technologies). The three-dimensional gain and efficiency measurements were carried out in the mobile antenna measurement chamber shown in Figure 7c. Its dimensions were \(8\times 4\times 4\) m and it covered the frequency range from 200 MHz to 6 GHz with an E8362B PNA network analyzer from Agilent Technologies. The proposed antenna was used as a receiver and the standard horn antenna was used as a source antenna. The radiation pattern of each plane was measured for both horizontal and vertical polarization by rotating the orientation of the source antenna. The simulated and measured peak gains and efficiencies are plotted in Figure 8. The figure shows that the measured maximum peak gain and efficiency are 0.8 dBi and 79.2%, respectively, which shows similar performance to the simulation results.  
The normalized simulated and measured radiation patterns along the xy, yz, and xz planes are shown in Figure 9. The figure shows that the radiation patterns in the xy- and yz-planes are nearly uniform. The pattern in the xz-plane contains two orthogonal omni-directional E\({}_{\theta}\) and E\({}_{\phi}\) patterns generated by the x-oriented electric dipole and the z-oriented magnetic dipole, respectively (Figure 2a), and the total radiated field shows a quasi-isotropic radiation pattern. The measured result is generally in good agreement with the simulation results. Finally, the measured gain deviation of the built antenna is shown in Figure 10. The maximum gain deviation is 5.2 dB with the highest and lowest gains of 0.8 and -4.4 dB, respectively. It differs by 2.2 dB from the computed expectation but is still acceptable and shows a similar distribution as compared to Figure 5. The non-perfect shielding of the measurement cable in anechoic chamber that is perpendicular to the antenna could have caused the deviation. Note that the vertical current component has a non-negligible effect on the gain deviation.  
There is considerable research on the planar quasi-isotropic antenna focusing the frequencies around 900 MHz and 2.4 GHz. In Table 3, the performance of the proposed design is compared with the previously published measured results around the 2.4 GHz frequency. Table 3 shows that the proposed design is electrically smaller and lower-profile. Even though the antenna size is small, it still shows comparable performance in comparison with electrically larger quasi-isotropic antennas. This proposed compact and planar quasi-isotropic antenna could be an effective design for miniature devices.  
Figure 8: Simulation and measurement results of the peak gain and radiation efficiency.  
Figure 7: Photos of the built antenna and the radiation performance measurement setup. (**a**) The built prototype of the proposed antenna. (**b**) The anechoic chamber for the radiation pattern measurement. (**c**) The anechoic chamber for the three-dimensional gain and efficiency measurement.  
Figure 10: Measurement result of the gain deviation over the entire space.  
Figure 9: Simulation and measurement radiation pattern of the proposed antenna: (**a**) xy, (**b**) yz, and (**c**) xz planes.