The treatment of single antennas in Chapter 2 introduced key concepts for receivers, including gain, directivity, effective area, impedance matching, noise figure, equivalent noise temperature, signal to noise ratio, and sensitivity. The treatment was then extended in Chapter 4 to arrays of transmitting elements. We now turn our attention to the main focus of the book, modeling high-sensitivity receiving antenna arrays.
For transmitters, the distribution of the signal power radiated by the antenna system is the primary consideration. Noise radiated by a transmitter is usually of secondary importance. For receivers, both the signal and noise response of the system are important, and the ultimate figure of merit for the performance of the system is the ratio of received signal of interest to the system noise (SNR). For this reason, the treatment of receiving arrays is more complicated than transmitters, as system noise must be brought into the analysis.We will begin by extending the network theory treatment developed in Chapter 4 to receiving arrays, and then bring in the noise modeling concepts that were introduced in Chapter 2.
Receiving Array Network Model
As a model for a receiving array, we will consider a basic narrowband active receive array architecture consisting of antenna elements terminated by low noise amplifiers, followed by receiver chains and a beamforming network which applies a complex gain constant (magnitude scaling and phase shift) to the signal from each element and sums the weighted signals to form a single scalar output, as in Fig. 5.1.
The canonical block diagram shown in Fig. 5.1 can represent many different types of antenna array receiver systems used in a variety of applications, and there are many variations on this basic architecture. Signals may be combined with an analog network, or the beamforming can be done using sampling followed by digital signal processing. For broadband applications, beamforming can be done with a time delay network, but more commonly the signal is processed digitally in frequency subbands using a narrowband beamformer architecture. Digital signal processing systems can form many simultaneous beams, so that the beamformer block is repeated in parallel for each formed beam.