# Uniform Linear Antenna Array Beamsteering Based on Phase-Locked Loops
## 4 Hardware Required  
The radiating elements are 55 mm long with a balun of 33 mm connected to a rigid coax. The elements are mounted on a wooden platform which was covered in metal foil to create a ground plane (Figure 5a).  
Each element is fed from a dedicated source provided by separate PLLs with a common 10 MHz reference connected to all PLLs. Phase-shifting is accomplished by shifting the 10 MHz reference signal fed to each PLL. Individual PLLs were chosen over a single common source with phase shifters at RF because phase shifters will have varying losses depending on the required shift; the power to each element decreases with the number of elements; turning on off output amplifiers could affect loading of the phase shifter leading to difficulty in calibration. A narrow-band phase shifter at the reference frequency allows for phase-shifting to be accomplished across the full frequency band of the PLL, to achieve this at RF, a wideband phase shifter would be needed. Multiple PLLs also remove the need for a corporate feed network [9].  
The topology for each element is shown in Figure 6. A 10 MHz reference is applied to the PLL board and individually buffered to each PLL [10] by a CMOS hex inverter [11]. The phase of the 10 MHz reference to each PLL passes through a phase shifter based on a varactor diode [12]. The phase of the output, the RF signal from the PLL, is adjusted by varying the DC voltage on the varactor. Applying the phase shift before the PLL allows for a constant output power irrespective of phase shift applied, also the system can be expanded by adding more PLL boards. The output of the PLL is then connected to a  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Scan Angle (\(\theta\))** & **Progressive Phase (\(\beta\))** \\ \hline
0 & \(-180\) \\ \hline
30 & \(-155.9\) \\ \hline
45 & \(-127.3\) \\ \hline
60 & \(-90\) \\ \hline
90 & 0 \\ \hline
120 & 90 \\ \hline
135 & 127.3 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Setting parameters of ULA at 2.4 GHz.  
digitally controlled attenuator [13]. This allows for the power level of each PLL to be adjusted to compensate for variance in PLL output power between circuits as well as allowing for future work, where the excitation level can be adjusted as well as the phase. The output of the attenuator is then connected to an amplifier; in this case, an LNA [14], as this demonstration does not require high power transmission. The LNA increases the power output of each circuit as well as providing 30 dB of isolation between PLL and element; this isolation reduces the effect of a neighbouring radiating element with different phase, changing the phase of a PLL oscillator. In addition, the PLLs selected have frequency dividers available between the internal oscillator and RF output, this allows the PLL to be configured with the internal oscillator running at twice the RF frequency; again, this aids in isolating one PLL from any signal coupled to it from the antenna or PCB affecting its phase. Each LNA can be switched off to allow the output of each PLL to be measured independently and adjusted while still being connected to the antenna array.  
To control the array, each PLL, attenuator, LNA, and phase shifter are connected to a dedicated microcontroller [15]. All the microcontrollers are connected to a single RS485 network with a CAT5e cable used to connect this to control the computer via an RS485 to a serial adapter. A combination of custom firmware for the microcontrollers and a Windows desktop application allow each PLL to be controlled individually from a remote computer. In this experiment, a 20 m cable was used to allow the control computer to be outside of the anechoic chamber. Schematics and production files for the PLL array along with source code and MATLAB [8] scripts can be found in the supporting material [6]. The PLLs are individually bench-tested to verify their power output and phase-shifting performance before integration with the ULA (Figure 7).