The Antenna

Antenna structures can take many physical forms, from the ''whip'' antenna used mostly for in-car radios, through ''rabbit ears'' for television to the microwave parabolic ''dish'' used in satellite communication. These are only the most common examples of antennas which are used for reception of broadcast signals. The antenna is the last processor of the signal at the transmitting end and the first at the receiving end. Antennae vary widely in shape, size and complexity, depending on the frequency of operation and the desired field pattern.

Radio communication in free space is possible because, when an alternating current flows in a conductor, part of the energy is lost in the form of electromagnetic radiation into free space. When the frequency of the current is low, the radiation ''loss'' is very small, but as the frequency increases, substantial losses can occur. In designing an antenna the object is to construct a structure which will maximize the radiated energy in a given direction or over a geographic area.

A useful analog of how an antenna radiates energy is given by a body which is floating in the middle of a pond on a windless day. For the purposes of this description we may assume that the body is a beach ball. If we can get a man suspended from a crane directly above the beach ball to push the ball very slowly into the water and release it equally slowly then the energy expended in pushing the ball into the water is recovered when it is released and very little will be lost. If the man increases the frequency at which he pushes and releases the ball, an increasing amount of energy will be lost in creating waves which will radiate from the ball outwards. The amount of radiated energy at any given point on the pond will bear an inverse relationship to the distance from the ball. It is necessary to bear in mind that the waves on the pond are surface waves (two dimensional) whereas those produced by an antenna are three dimensional.

The speed of propagation of the radio wave is the same as that of light (c = 3 x 108 m/s). The wavelength and the frequency are related by f = c

where 1 is the wavelength in meters and f is the frequency in hertz. Electromagnetic radiation has two vectors; the E and H which are orthogonal to each other and to the direction of propagation.

A detailed study of antenna design is beyond the scope of this text and the interested reader may consult a suitable textbook on the subject. The discussion will therefore be qualitative.

2.9.1 Radiation Pattern of an Isolated Dipole

A simple way to gain insight into the operation of a dipole is to start from an open-circuit balanced transmission line. Such a transmission line will have a standing-wave distribution of currents in opposite directions so that the net radiation from the structure will be quite small. Figure 2.51(a) shows the transmission line and the standing-wave pattern. It is assumed that the diameter of the conductors is infinitesimally small compared to the wavelength of the signal to be transmitted.

If the transmission line is bent at right angles as shown in Figure 2.51(b), the structure is called a dipole and it has the current distribution shown. Since the oppositely directed currents are now far apart, they do not counteract each other and hence the dipole is a good radiator of electromagnetic waves.

If it is assumed that 2l = 1/2, the dipole is then called a haf-wavelength dipole. The E field of the half-wavelength dipole when plotted gives a circle tangential to the axis of the dipole touching the axis at the mid-point. In three dimensions, the pattern is a doughnut, as shown in Figure 2.52.

Now assume that l ^ 1/2: the dipole is said to be a short dipole and the field pattern is also a doughnut but the cross section is slightly distorted as if compressed vertically.

2.9.2 Monopole or Half-Dipole

The equations which describe the behavior of the isolated dipole can be used, with slight modifications, for the monopole or half-dipole by assuming that the Earth's

Figure 2.51. (a) The current standing wave pattern on an open-circuit balanced transmission line. (b) The current standing wave pattern when the line is bent at a point where l = 1/2 to form a dipole.

Figure 2.51. (a) The current standing wave pattern on an open-circuit balanced transmission line. (b) The current standing wave pattern when the line is bent at a point where l = 1/2 to form a dipole.

Figure 2.52. The radiation pattern of an isolated half-wavelength dipole.

surface, on which the antenna is to be placed, is a perfectly flat conductor. This permits the treatment of the monopole and its image on the ground plane as a dipole. Figure 2.53 shows the monopole and its image. This representation of the antenna is closer to reality than the isolated dipole.

2.9.3 Field Patterns for a Vertical Grounded Antenna

The E field pattern generated by a vertical grounded antenna resembles that of the monopole closely even though the Earth's surface is not exactly a perfectly flat

Monopole

Monopole

I Image

Figure 2.53. This diagram shows how the half-wavelength dipole can be replaced with a monopole and a flat conducting surface (ground plane).

I Image

Figure 2.53. This diagram shows how the half-wavelength dipole can be replaced with a monopole and a flat conducting surface (ground plane).

conducting surface. Figures 2.54(a)-(e) are a series of diagrams which present the expected approximate field pattern as the height of the vertical grounded antenna changes. It is important to note that the height is measured in terms of the wavelength of the signal to be radiated.

Figure 2.54. (a) Radiation pattern when h is approximately 1/10. (b) Radiation pattern when h is approximately 1/4. Note the slight elongation of the pattern along the ground plane. (c) The radiation pattern has undergone further distortion as h increases to approximately 1/2. (d) As h approaches 51/8, the ground wave gets more elongated and the sky wave appears. (e) With h approximately 31/4, the sky wave has grown considerably at the expense of the ground wave.

Figure 2.54. (a) Radiation pattern when h is approximately 1/10. (b) Radiation pattern when h is approximately 1/4. Note the slight elongation of the pattern along the ground plane. (c) The radiation pattern has undergone further distortion as h increases to approximately 1/2. (d) As h approaches 51/8, the ground wave gets more elongated and the sky wave appears. (e) With h approximately 31/4, the sky wave has grown considerably at the expense of the ground wave.

Figure 2.54. (continued)
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