Tham khảo tài liệu 'advanced trends in wireless communications part 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Adaptive Antenna Arrays for Ad-Hoc Millimetre-Wave Wireless Communications 95 ad-hoc or mobile network that relies on high gain antennas also requires beam scanning. The antenna beam can be steered to a desired direction with appropriate beam forming. Passive phased arrays generally suffer from losses in combining networks that are very high at the mm-wave frequencies. In a spatial power-combining phased array transmitter each individual element has a power amplifier PA . To generate a pencil beam in a particular direction the signal radiated from each element is delayed electronically in order to compensate for differences in the free-space propagation time from the different elements. In a spatial powercombining transmitter with multiple radiating elements this coherent addition increases the Effective Isotropic Radiated Power EIRP in two ways firstly via the increase in directivity due to the increased electrical aperture and secondly via the increase in total radiated power through the increased number of power amplifiers. So if we take the efficiency of the spatial power combining transmitter to be n for an array of N elements each generating an EIRP of P watts the EIRP of the transmitter is n N2 P watts. Assuming an efficiency of 100 the increase in EIRP in going from 1 to N elements is 20 log N dB. These results are plotted in Figure 1 where the equivalent EIRP of passive and active arrays is plotted versus number of array elements. Fig. 1. Active versus passive phased array transmitters It should be noted that the data for a lossless corporate feed plotted in Fig. 1 is a theoretical assumption only. It does not take into account the power combining loss for the passive array with a single PA. The combining loss is hard to predict as it largely depends on number of elements operating frequency and other parameters of a specific design and could be in the order of several dB. An example shown in Fig. 1 that uses an optimistic assumption of only loss per
