Chapter 4
Antennas
An antenna is a device that is used in both wireless transmitters and receivers. In a wireless transmitter, an antenna converts guided electromagnetic signals (usually, from a transmission line) into propagating electromagnetic wave signals. In a wireless receiver, an antenna converts propagating wireless electromagnetic wave signals (arriving at the receiver) into guided electromagnetic signals.
When used for transmitting wireless signals, instead of letting the same amount of power be radiated in every direction, antennas often direct the signals, for more efficient communications. The directivity, antenna gain, and so on, are ways of quantifying this phenomenon, which is also known as the antenna pattern. In Section 4.1.8 we take a further and more quantitative look at these directional characteristics. When used for receiving wireless signals, the directional characteristics are the same as for transmitting. This reciprocity between the patterns for transmitting and receiving is one of the reciprocity characteristics of antennas.
By reciprocity principles, the following are the same:
1. The impedance for transmitting and receiving
2. The directional characteristics/patterns when used for transmitting and for receiving
To improve our understanding of antennas and their use, we need to examine different ways to characterize them, only part of which are their directional characteristics. Thus, we examine various characterizations of antennas in Section 4.1. ...
