## 4 Measurement Setup and Results  
The measurements scenario of the proposed system is illustrated in Figure 12. The setup was placed in an anechoic chamber, and included a power supply, providing the DC voltage to the RF up/downconverter's boards and the PLL LMX2595 (LO frequency of 12.08 GHz). In addition to the frontend boards and the PLL, the setup also included the Host-PC, the Tx and Rx antenna arrays, and the USRP N310, as can be identified in the Figure 12.  
Before starting the measurement process, and after carefully placing all the elements of the setup, the first task was to calibrate the system, in order to ensure that all signals reached the antennas in phase, and all signals received maintained their properties until they were read by the USRP. The principle of calibration is the same as beamforming, and consists of adding certain weight (phase and amplitude) to each channel with the purpose of compensating for the hardware impairments in the upconverter/downconverter modules, the phase differences in the USRP daughterboard and the temperature drifts. Using the Python/Matlab interface, a calibration routine was developed to compensate for the phase and amplitude differences of the channels.  
The four elements of the transmitter phased array antenna were placed in the rotor arm of the anechoic chamber, 65 cm away from the receiver (ensuring the far field distance). In Matlab/Python, a function that calculates the correct phase to apply to each channel based on the desired direction of beamforming \(\theta(^{\circ})\) was developed. The USRP modulated the baseband signal into two orthogonal carrier waves, thus generating a 3.84 GHz IF signal. This signal was then applied to an upconverter module that converted it to 28 GHz. The resultant signal was fed into one element of the antenna array. Since there is a channel digitally controlled (in amplitude and phase) for each element of the array, it is possible to use various beamforming algorithms. In the tests carried out, the radiation beam varied (in relation to the perpendicular plane of the array) between \(-40^{\circ}\) and \(+40^{\circ}\) with one degree increments. This procedure was repeated for nine different positions of the transmitter. For each step, the received signal in the antennas were captured, downconverted to 3.84 GHz, and then the IQ demodulation  
Figure 12: Experimental setup to measure antenna radiation pattern.  
was performed in the USRP to obtain the IQ samples. From these received samples, it was possible to measure the amplitude and phase of the signal coming from each channel.  
The first measurement using the system was related to beam steering, in which the phases estimated in Table 1 were applied to the four channels, to steer the beam to different angles. Figure 13a shows the normalized measured radiation patterns of the received signal for nine different positions of the transmitter, using uniform amplitude in the feeding of the array.  
The second measurement was devoted to the beam width of the radiation pattern. It is possible to observe good agreement between the estimated curves of the array factor (Figure 11) and the measured values of the radiation pattern of Figure 13b. It is noticeable that the radiation pattern with the widest beam is the one that uses the binomial distribution (27\({}^{\circ}\)) and the narrowest beam is related with those that use uniform distribution (17\({}^{\circ}\)).  
Table 3 compares the information of the measured beam widths of radiation patterns from Figure 13b and those taken from the theoretical results of Figure 11. It is possible to verify good correspondence between the evolution of these values.