Introduction to Antennas

Introduction to Antennas

Antennas act as converters between conducted waves and electromagnetic waves propagating freely in space (see Figure 1). Their name is borrowed from zoology, in which the Latin word antennae is used to describe the long, thin feelers possessed by many insects.

The oldest existing antennas, such as those used by Heinrich Hertz in 1888 during his first experiments to prove the existence of electromagnetic waves, were in theory and in practice not so very different from an RF generator.

An antenna can be derived from a parallel circuit which consists of an inductor and a capacitor. If the plates of the capacitor are bent open, and the inductor is reduced to the inductance of the wire itself, one ends up with a dipole antenna as shown at the very right position of Figure 2. In fact, resonant circuits are still frequently used even today as a means of explaining the individual properties of antennas. It was not until around 1900 or even later, when transmitting and receiving stations were being built, that a clear distinction was made and antennas were classified as separate components of radio systems. 

In Figure 3 it can be seen that the antenna is an important element in any radio system because it acts like a link of a chain. So the overall performance is significantly influenced by the performance of transmit and receive antennas. 

At first glance, modern antennas may still look very similar to the ancient model. However they are nowadays optimised at great expense for their intended application. Communications antenna technology primarily strives to transform one wave type into another with as little loss as possible. This requirement is less important in the case of test antennas, which are intended to provide a precise measurement of the field strength at the installation site to a connected test receiver; instead, their physical properties need to be known with high

accuracy. The explanation of the physical parameters by which the behaviour of each antenna can

be both described and evaluated is probably of wider general use; however the following chapters can describe only a few of the many forms of antenna that are in use today. 

reference :

[1] Antenna Basics, White Paper, Rohde&Schwarz