# Triple-Band Uniform Circular Array Antenna for a Multi-Functional Radar System
## 3 Results  
For the purpose of the evaluation, the proposed antenna design was evaluated using a full-wave simulation tool, HFSS. The simulated result was compared with the measured results. A photograph of the proposed array antenna is shown in Figure 2. Two substrates of Taconic TLY-5 were used for the fabricated antenna design. Two substrates were firmly attached using plastic screws. A comparison between simulated and measured reflection coefficients is shown in Figure 3. The three types of the radial array antennas provided impedance matching of less than \(-20\) dB at the different frequency bands, which were reasonably good matching characteristics. It was observed that the resonant frequency of the measurement deviated from that of the simulation by \(2.2\%\) (C-band) and \(1.4\%\) (X-band), whereas an excellent agreement between the simulation and the measurement was observed for the S-band array antenna. The shift of the resonant frequencies (C-band) might be attributed to the connector effect, which is not included in the simulated results, although it affects the measured results. It is worth of noting that the reflection coefficient of one port was measured while the other two ports were terminated. Simulated and measured mutual couplings between two ports are presented in Figure 4. Excellent isolation below \(-40\) dB was obtained throughout the frequency bands of interest. This might be attributed to the orthogonal antenna polarizations of two neighboring array antennas, as previously reported in [17]. The radiation patterns at the frequencies of \(2\) GHz, \(5.8\) GHz, and \(10.525\) GHz were measured inside a far-field anechoic chamber. The simulated and measured radiation patterns (co-pol) are shown in Figure 5. The far-field range of the anechoic chamber was enough to measure the proposed antenna at the far-field distance of the array antenna. The proposed antenna provides a simulated maximum gain of \(14.7\) dB, \(15.7\) dB, and \(12.9\) dB at each operating frequency band. The reasonably stable maximum gain was obtained owing to the identical number of element antennas that each array antenna possessed. The measured maximum gains were \(14.3\) dB, \(15.3\) dB, and \(12.5\) dB at each operating frequency band, which deviated from the simulation by less than \(0.4\) dB. It was observed that the cross-polarization level obtained from the full-wave simulation was below \(-41\) dB in the S-band, below \(-23\) dB in the C-band, and below \(-16\) dB in the X-band in terms of each co-polarization level. The deviation between simulated and measured results was mainly caused by manufacturing tolerances, RF connectors, and manufacturing error of the \(16\) vias.  
Figure 1: Configuration of the three-layered proposed antenna: (**a**) three array antennas on the top layer; (**b**) ground plane between the top and bottom layers; (**c**) feed lines on the bottom layer.  
Figure 4: Comparison of the mutual couplings obtained from the HFSS simulations and measurements among port 1, port 2 and port 3.  
Figure 3: Comparison of the reflection coefficients obtained from the HFSS simulations and measurements for port 1, port 2 and port 3.  
Figure 2: Photographs of the fabricated antenna design: (a) top layer of the prototype antenna; (b) bottom layer of the prototype antenna.