ANTENNA AND NOISE CONCEPTS
The function of a communication link is to transmit information from one location to another. If wireless radiowave propagation is to be used, the signal must be radiated from the transmitter site by a transmitting antenna. It then propagates toward the receiver site where a receiving antenna captures the signal and delivers it to a receiver system. As a result, it is difficult to specify the propagation process without reference to the antennas that transmit and receive the signal. The ability of a signal to be properly received at the receiver will depend not only on the power carried by the incoming electromagnetic wave, but also on the sensitivity of the receiver and the presence or absence of other (interfering) signals. Consideration of noise effects is therefore very important in the design of any communication system. The aim of this chapter is to develop the basic antenna concepts necessary for a discussion of propagation effects in wireless communication systems and to introduce standard methods for describing noise in communication systems so that prediction of signal-to-noise ratios (SNR) for realistic systems becomes feasible.
4.2 ANTENNA CONCEPTS
The field radiated from an antenna is always more complicated near the antenna than at a large distance from it. If one considers free-space propagation at distances sufficiently far from the transmitting antenna, the field appears to be a spherical wave emanating from a point ...