ll three planes and this is often termed the radiation pattern. To fully describe the performance of an antenna a full three-dimensional representation would be required, but normally this is not needed. Instead, a diagram known as a polar diagram is used to plot the performance of the antenna in a particular plane. Essentially this plots a curve around the antenna, showing the intensity of the radiation at each point. Normally a logarithmic scale is used SO that the differences can be accommodated on the plot. A simple dipole Directions of maximum radiation Axis of antenna Figure 4.2 Polar diagram of a half wave dipole 80 Newnes Guide to Radio and Communications Technology antenna may have a pattern like that shown in Figure 4.2. From this it can be seen that the maximum radiation for transmission, and hence the maximum sensitivity to received signals, occurs when the signal is at right angles to the antenna. When an antenna is designed to be directive, its polar diagram may look more like that shown in Figure 4.3. In this diagram, it can be seen that the antenna radiates far more signal in one direction than another. The fact that it 'beams' the power in a particular direction means that antennas of this nature are often called beams. Direction of the main 'beam' Major lobe Minor lobes Figure 4.3 Polar diagram of a directive antenna or beam The region of maximum radiation is called the major lobe. However, there are other areas around the antenna where there are significant levels of radiation. These are called minor lobes. They are always present to some extent, and generally the largest is in the opposite direction to the main lobe. One of the major design parameters of any beam antenna is its gain. This has to be compared to another antenna. The most common antenna used for comparisons is called a dipole. The gain is simply the ratio of the signal from the beam antenna compared to the dipole expressed in decibels. Sometimes another type of antenna may be used. Called an isotropic source it is an imaginary an