Antennas for low power applications

AN4190

14/29

Doc ID 023812 Rev 1

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Antennas for low power applications

For wireless communication systems, the antenna is one of the most critical components. A

good design of the antenna can relax system requirements and improve overall system

performance. A typical example is a TV, for which the overall broadcast reception can be

improved by utilizing a high performance antenna.

A good antenna requires it to be the right type for the application. It must also be matched

and tuned to the transmitter and receiver.

An introduction and brief discussion of some forms of the various antenna types that can be

used for low power applications is given here.

2.1

Linear antenna

Wire antennas, linear or curved, are some of the oldest, simplest, cheapest, and in many

cases most versatile, antennas for many applications. They are familiar to the layman

because they are seen virtually everywhere - on automobiles, buildings, ships, aircraft,

spacecraft and so on. There are various shapes of linear antenna such a straight wire

(dipoles, monopoles), loop, helix and so on.

2.1.1

Dipole antenna

A dipole antenna is a radio antenna that can be made of a simple wire, with a center fed

driven element. It consists of two metal wire-rod conductors, in line with each other, with a

small space between them. The radio frequency voltage is applied to the antenna at the

center, between the two conductors. These antennas are the simplest practical antennas

from a theoretical point of view.

The half-wave dipole antenna is the basis of many other antennas and is also used as a

reference antenna for the measurement of antenna gain and radiated antenna density. At

the frequency of resonance, i.e. at the frequency at which the length of the dipole equals a

half-wavelength, we have a minimum voltage and a maximum current at the termination in

the center of the antenna, as shown in Figure 8; the impedance is minimal. This is a simple

antenna that radiates its energy out toward the horizon (perpendicular to the antenna). The

resulting 3D pattern looks kind of like a donut or a bagel with the antenna sitting in the hole

and radiating energy outward (see Figure 9). The strongest energy is radiated in the plane

perpendicular to the antenna. The gain of the half-dipole is approximately 2.2 dBi.

Figure 8.

Half-wave dipole antenna voltage and current distribution

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