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A tutorial on radio link (Part 2)

Posted: 24 Oct 2011     Print Version  Bookmark and Share

Keywords:Antenna  radiation  power pattern 

Here's part 2 of this series, an excerpt from the book Introduction to Wireless Systems. It brings together the theoretical and practical knowledge readers need to participate effectively in the planning, design, or implementation of virtually any wireless system. This section covers antenna radiation patterns.

Antenna radiation patterns
A rock dropped into a still pond will create ripples that appear as concentric circles propagating radially outward from the point where the rock strikes the water. Well-formed ripples appear at some distance from the point of impact; however, the motion of the water is not so well defined at the point of impact or its immediately surrounding area.

A physically realisable antenna launches both electric and magnetic fields. At a distance sufficiently far from the antenna we can observe these fields propagating in a radial direction from the antenna in much the same fashion as the ripples on the surface of the pond. The region of well-defined radially propagating "ripples" is known as the far-field radiation region, or the Fraunhofer region. It is the region of interest for most (if not all) communication applications.

Much nearer to the antenna there are capacitive and inductive "near" fields that vary greatly from point to point and rapidly become negligible with distance from the antenna. A good approximation is that the far field begins at a distance from the antenna of

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where l is the antenna's largest physical dimension.1 Often in communication applications, the antenna fs physical dimensions, though greater than λ/10 , are less than a wavelength, so that the far field begins at a distance less than twice the largest dimension of the antenna. For cellular telephone applications, the far field begins a few centimeters away from the handset.

A polar plot of the far-field power density as a function of angle referenced to some axis of the antenna structure is known as a power pattern. The peak or lobe in the desired direction is called the main lobe or main beam. The remainder of the power (i.e., the power outside the main beam) is radiated in lobes called side lobes. "Directive" antennas can be designed that radiate most of the antenna input power in a given direction.

Figure 1: Typical antenna power pattern showing main loba and side lobes.

Figure 1 illustrates the beam pattern of a directive antenna with a conical beam. The antenna is located at the left of the figure from which the several lobes of the pattern emanate.

The plot represents the far-field power density measured at an arbitrary constant distance d from the antenna, where d > dfar field. The shape of the beam represents the power density as a function of angle, usually referenced to the peak of the main beam or lobe. For the antenna represented in the figure, most of the power is radiated within a small solid angle directed to the right in the plot. In general, a convenient coordinate system is chosen in which to describe the power pattern. For example, one might plot the power density versus the spherical coordinates Φ and φ for an antenna oriented along one of the coordinate axes as shown in figure 2.

Figure 2: An antenna pattern in a spherical coordinate system.

For each angular position (Φ, φ), the radius of the plot represents the value of power density measured at distance d. Since d is arbitrary, the graph is usually normalized by expressing the power density as a ratio in decibels relative to the power density value at the peak of the pattern. Alternatively, the normalisation factor can be the power density produced at distance d by an isotropic antenna, see equation (2.2). When an isotropic antenna is used to provide the reference power density, the units of power density are dBi, where the i stands for "isotropic reference." Figure 3 shows an antenna power pattern plotted versus one coordinate angle Φ with the other coordinate angle φ held constant. This kind of plot is easier to draw than the full three-dimensional power pattern and is often adequate to characterise an antenna.

Figure 3: An antenna power pattern plotted versus Φ and a fixed value of φ.

Example
A "half-wave dipole" or just "dipole" is a very simple but practical antenna. It is made from a length of wire a half-wavelength long, fed in the centre by a source. Figure 4 shows a schematic representation of a dipole antenna. A slice through the power pattern is shown in figure 5. The dipole does not radiate in the directions of the ends of the wire, so the power pattern is doughnut shaped.

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