Design, implementation, and measurement of planar microstrip antenna sub-array

In this paper a novel design of planar microstrip 4-by-4 sub-array is presented. This sub-array can be integrated into different antenna arrays to reach suitable specifications for different applications such as TV receivers, and direct broadcast by satellite DBS systems. It has been implemented on Rogers substrate using printed circuit board PCB technology. The present design should satisfy premium specifications such as working in a dual polarization mode, high isolation between the two orthogonal polarizations (S12=S21<-20 dB), impedance bandwidth of 17% centered around 11.75 GHz of the frequency band 10.75–12.75 GHz, low cross polarization level (<-10 dB) in both principal planes, high radiation efficiency, low side lobe levels (<-10 dB), and finally offering high gain of about 30 dBi. This required value of gain can be achieved by constructing a larger number of sub-arrays in the whole antenna array. Different works for dual polarized planar antennas have recently been reported. The works done in [1]–[4] have the objective of enhancing the isolation between the two ports of the dual polarized antenna. Another trend of research is directed towards enhancing the bandwidth of such antennas using the parasitic patches [5], L-probe feeding [6], and capacitive-coupled feeding [7]. A low side lobe dual-polarized planar antenna array for KU-band satellite communications is introduced in [8]. The new design has been simulated using Agilent ADS/Momentum simulator [9]. The simulation results include both the scattering parameters and the radiation patterns. Two metal layers have been used, reducing the overall cost. Fig. (1) shows the two metal layer structure used in this design with the computed patch and slot copper layers thicknesses. Fig. (2) shows the designed 4-by-4 sub-array. It is formed by integrating 4 blocks of the 2-by-2 sub-array. The integration here is done using two corporate feeding networks. The network attached to port 1 is used to excite the vertical polarization, while that attached to port 2 is used for horizontal polarization excitation. The ADS calculated S-parameters of the proposed 4-by-4 sub-array are plotted versus frequency in Fig. (3). From this figure, it is found that the return losses S11 and S22 of port 1 and port 2 are, respectively, less than −10 dB over the frequency band (10.58–12.98) GHz which corresponds to over 20% impedance bandwidths for both ports at 11.75 GHz center frequency, which is also more than the required value of 17%. The insertion losses of ports 1 and 2, S12 = S21, each ranges from −18 dB to −24 dB over the working band. It is seen that the maximum value of insertion loss is better than that (-15 dB) of the 2-by-2 sub-array [10]. For each port, the far field components are plotted versus the elevation angle θ at two values of the azimuth angle φ, 0°, and 90°, corresponding to plane 1, and plane 2, respectively, as shown in Fig.(4). The scattering parameters have been measured using the network analyzer. The simulated results together with measured ones of S22 are shown in Fig. 5. It is seen from this figure that the behavior of both simulated and measured results are predicted correctly. However, the shifting up of the resonance frequency is mainly due to the probable disabling of narrow edge mesh processing. The radiation patterns of port 1 are shown in Figs. 7(a) and 7(b) for both planes 1 and 2, respectively. It is seen that, the cross polarization levels are −14 dB and −11 dB for both planes, respectively. The side lobe levels are less than −10 dB, and −12 dB in planes 1, and 2, respectively. Similarly, from the radiation patterns at port 2 for both planes 1, and 2, the cross polarization levels are less than −15 dB and −10 dB, and side lobe levels are of −9 dB and −11 dB, respectively. The calculated directivity, gain, and radiation efficiency for port 1 are 15.861 dBi, 15.829 dBi, and 99.266%, respectively. Similarly, for port 2, the corresponding values are 16.646 dBi, 16.586 dBi, and 98.628%, respectively. The remarkably high values of radiation efficiencies are due to the use of low dielectric constant and thin thickness substrates. Photographs of the the fabricated patch layer of front-side substrate, and the fabricated slot layer of back-side substrate, are shown in Figs. (6), and (8), respectively. It is concluded that, the present sub-array offers a very good isolation between feeding ports, dual-polarization radiation, low levels of cross polarization, low side lobes, wide impedance bandwidth, and very high radiation efficiency. It is suitable as a building block for larger arrays as replaceable to parabolic dish reflector in satellite TV receivers.