## 3 RX Phased Array Design  
As shown in Figure 6, the 2048-element RX phased array consists 16 subarrays with RF ports on each subarray combined together through an external 16:1 Wilkinson combiner. The power and the control circuit of those subarrays are arranged at the upper and the lower ends of the RX array, so that antenna elements can be arranged in a regular rectangle in middle. Actually, it can be assembled into a larger array in the \(x\)-direction.  
The detailed architecture of subarrays is shown in Figure 7. Each 128-element subarray consists of 32 8-channel silicon-based beamformer chips, a linear amplifier, some functional circuits and a multilayer PCB. Each beamformer chip is connected to 4 dual-polarized antenna elements to form a basic 2 \(\times\) 2 cell. The chips are combined into a common port through a microstrip Wilkinson power combiner on the bottom layer of the PCB. The linear amplifier with 20 dB of gain is added to the common port to compensate for microstrip line losses and increase RX electronic gain of the array. The power and control circuits are located at one end of the PCB, providing power and SPI control signals through the connectors.  
The basic 2 \(\times\) 2 cell is illustrated in Figure 8. Ports on the antenna for V\(-\) and H\(-\)polarization are connected to two polarization channels of the chip respectively. Feed ports of the four antennas are placed in mirror image symmetry, which will improve the cross-polarization level of the arrays. This is because the cross-polarized magnetic currents generated by the non-radiating edges of the 2 \(\times\) 2 patch antenna are canceled out. It can be seen that each polarization of the antenna element is connected by an RF channel containing a digital phase shifter and an attenuator. When the array receives a beamforming command, the program in the array calculates the corresponding amplitude and phase according to the coordinate position of each antenna element in the array, and then sets the values of the corresponding phase shifter and attenuator in each chip through the control line, thus realizing the beamforming.  
The stackup of the 14-layer PCB is shown in Figure 9. High-frequency base materials were used for the dual-polarized antenna elements on layers M1-M8, as well as the microstrip line and Wilkinson power combiner on layers M13-M14. FR-4 substrates were used in the middle M9-M12 layers for the control and power layouts, which were connected to the chip pins using metal vias. The beamformer chips, linear amplifier, and RF connector were mounted onto layer M14 and interconnected with one another using microstrip lines. The copper paste sintering technology was used between layers M6 and M7 to interconnect the vias with the feeding stripline, in order to avoid drilling vias in the antenna element and reduce the types of vias required.  
\begin{table}
\begin{tabular}{c c c c} \hline  &
\begin{tabular}{c} **Copper Paste** \\ **Sintering** \\ \end{tabular} & \multicolumn{1}{c}{**Normal Process**} & \multicolumn{1}{c}{**HDI Process**} \\ \hline Flexibility & More & Less & Normal \\ Lamination numbers & Less & More & Normal \\ PCB thickness & Thinner & Thicker & Normal \\ Copper stub & No exist & Exist & No exist \\ Application limitation & Unlimited & Unlimited & Only for feed section \\ Cost & High & Low & Normal \\ \hline \end{tabular}
\end{table}
Table 1: Comparison of the copper paste sintering process with the normal PCB process and HDI process.  
Figure 8: \(2\times 2\) antenna cell based on 8-channel RX beamformer chip.  
Figure 6: \(2048\)-element RX phased array for \(Ku\)-band SATCOM with 16 subarrays.  
Figure 7: Detailed architecture of the subarray.