5 Answer the questions:

1. What is the purpose of a transmitting antenna?

2. What is the purpose of a receiving antenna?

3. What two broad categories do antennas for communications systems fall into?

4. What is the design of a pencil beam antenna?

5. What antennas are used in communications?

6. What are the basic properties of an antenna?

7. When will the maximum power transfer occur?

8. What does a communication link consist of?

PART 2 (17.3.2 – 17.4.1)

17.3.2 Radiation from small antennas

Small antennas are needed for mobile communications operating at frequencies from HF to the low microwave region. Most of these are derivatives of the simple dipole, Figure 17.6, which is an electric current element which radiates from the currents flowing along a small metal rod. The radiation pattern is always very broad with energy radiating in all directions. An important design parameter is the impedance of the dipole which can vary considerably depending on the exact size and shape of the rod. This means that the impedance matching between the antenna and the transmitting or receiv­ing circuit becomes a major design constraint. Table 17.2 Radiation characteristics of circular apertures.

The radiation fields from a dipole are obtained by integrating the radiation from an infinitesimally small current element over the length of the dipole. This depends on knowing the current distribu­tion which is a function not only of the length but also of the shape and thickness of the rod. Many studies have been addressed towards obtaining accurate results (King, 1956; King, 1968). For most cases this has to be done by numerical integration. A simple case is a short dipole with a length when the current distribution may be assumed to be triangular. This results in radiated fields of the form given in Equation 17.9.

The electric field is plotted in polar form in Figure 17.6. The radiation resistance is calculated by evaluating the radiated power and using P = I²R to give Equation 17.10.

A dipole of length has a radiation resistance of 2.0ohms.

This is low by comparison with standard transmission lines and indicates the problem of matching to the transmission line.

The half wave dipole is widely used. Assuming a sinusoidal current distribution the far fields are given by Equation 17.11.

This gives a slightly narrower pattern than that of the short dipole and has a half beamwidlh of 78 degrees. The radiation resistance must be evaluated numerically. For an infinitely thin dipole it has a value of 73 +j 42.5 ohms. For finite thickness the imaginary part can become zero in which case the dipole is easily matched to a coaxial cable of impedance 75ohms. The half wave dipole has a gain of2.15dB.

A monopole is a dipole divided in half at its centre feed point and fed against a ground plane, Figure 17.6. The ground plane acts as a mirror and consequently the image of the monopole appears below the ground. Since the fields extend over a hemisphere the power radiated and the radiation resistance is half that of the equivalent dipole with the same current. The gain of a monopole is twice that of a dipole. The radiation pattern above the ground plane is the same as that of the dipole.