# W-Band Modular Antenna/Detector Array for the Electron Cyclotron Emission Imaging System in KSTAR
## 4 Fabrication and Measurement
### Measurements  
To measure the performance of the unit antenna/detector module for the KSTAR ECE imaging system, a measurement setup is configured, as shown in Figure 8. Two W-band sources are used as RF and LO sources, each of which consists of a 12.5-18.3 GHz source, a 6x frequency multiplier, a W-band amplifier, and a W-band horn antenna. The RF source (WR10 AMC-I of Virginia Diodes) with the output power of +20 dBm radiates onto a tilted beam splitter. In the perpendicular direction, the LO source (AMC-10-RFHB0 of Millitech) impinges onto the same beam splitter. One path of the combined RF-LO signal propagates towards the modular antenna/detector array. The down-converted IF signal by the Schottky diode passes through the antenna balun, amplified, and then analyzed by the  
Figure 7: Fabricated modular antenna/detector array: (**a**) top and bottom view of the modular antenna/detector array consisting of the unit antenna/detector modules, (**b**) top and bottom view of the unit antenna/detector module. spectrum analyzer (Agilent E4407B). The radiation patterns of the unit antenna/detector module are also measured by rotating the unit/antenna detector module from \(-30^{\circ}\) to \(30^{\circ}\). The radiation patterns are measured with the test set-up, and the results are shown in Figures 9 and 10. In Figure 9, the measured radiation patterns in the E-plane are presented at RF frequencies of 75, 85, 95, 100, and 105 GHz with IF frequencies of 2 and 9 GHz. In Figure 9a, the E-plane radiation patterns at various RF frequencies with the 2 GHz IF signal are shown. The frequencies of the LO signal are selected as 2 GHz higher than the corresponding RF frequencies. As the RF frequency is increased, the 3 dB beamwidth is decreased: i.e., \(8^{\circ}\) at 75 GHz, \(8^{\circ}\) at 85 GHz, \(6^{\circ}\) at 95 GHz, and \(5^{\circ}\) at 105 GHz. The left and right side-lobe levels are \(-22.4\) dB and \(-23.3\) dB at 2 GHz, respectively. In Figure 9b, the E-plane radiation patterns are shown for the IF frequency of 9 GHz, where the LO frequencies are set to 84, 94, 104, and 96 GHz. It is observed that, as the RF frequency increased, the 3 dB beamwidth decreased as \(7^{\circ}\), \(7^{\circ}\), \(6^{\circ}\), and \(5^{\circ}\). The left and right side-lobe levels are \(-25.3\) dB and \(-21.9\) dB at 9 GHz, respectively. Figure 10 exhibits the H-plane radiation patterns with IF frequencies of 2 GHz and 9 GHz. With the 2 GHz IF frequency, the frequency of the LO signal is again selected as 2 GHz higher than the RF frequencies. As shown in Figure 10a, the radiation patterns with RF frequencies of 75, 85, 95, and 105 GHz are presented. As RF frequency increases, 3 dB beamwidth is slightly decreased: i.e., \(6^{\circ}\) at 75 GHz, \(6^{\circ}\) at 85 GHz, \(6^{\circ}\) at 95 GHz, and \(5^{\circ}\) at 105 GHz. The left and right side-lobe levels are \(-20.2\) dB and \(-26.5\) dB at 2 GHz, respectively. The left side-lobe level may have been increased due to the interference caused by the signal propagating through the side of the antenna balun. In Figure 10b, the H-plane radiation patterns are shown with the IF frequency of 9 GHz. The LO frequencies are selected as \(84\), \(94\), \(104\), and \(96\) GHz. By increasing the frequency, the 3 dB beamwidth decreased as \(7^{\circ}\), \(6^{\circ}\), \(5^{\circ}\), and \(5^{\circ}\) at 75, 85, 95, and 105 GHz, respectively. With the IF frequency  
Figure 8: Block diagram of the measurement setup. Figure 9: Measured radiation patterns of the proposed unit antenna/detector module in the E-plane: (a) IF frequency = 2 GHz, (b) IF frequency = 9 GHz. of 9 GHz, the left and right side-lobe level are \(-18.3\) dB and \(-27.4\) dB, respectively. In a similar fashion to the radiation patterns with the 2 GHz IF frequency as shown in Figure 10a, all radiation patterns with the 9 GHz IF frequency show asymmetric H-plane patterns, possibly caused by the presence of the antenna balun.