In this chapter we will discuss the most important and well-known relations and parameters of an array antenna that are relevant for the conception of a radar system. Possible implementations for different applications will be summarised. We will start our discussion with a simple illustration, the basic principle of which is sketched in Figure 4.1. A set of antennas or an array of antenna elements is distributed on a metal ground plane, preferably on a regular grid. These antenna elements may be, for example, dipoles matched with the length of their arms to the operating wavelength l of the radar system. In the case of transmission each antenna element is the source of a spherical wavefront. As a first step we assume waves of equal phase from each antenna element. Comparatively, we observe circular water waves if a group of persons standing at a linear sea wall are throwing stones at the same instant of time into calm water. These waves superpose coherently according to the famous Huygens' Principle at each point in space. If all sources radiate in phase then at the boresight direction, orthogonal to the antenna plane, we have a linear or coherent summation of the field strength of all individual waves. That is if, at a certain distance from the antenna, the electrical field strength produced from one antenna element is E, we would have with N antenna elements the field strength NE resulting at the boresight direction, or a power density proportional to (NE)²=N²E². Outside the boresight direction the condition of in-phase superposition is not fulfilled and there is approximately, as the spatial mean over all directions, only a superposition of the power; that is we have there a power density with an order of magnitude only proportional to NE². More detailed considerations for the antenna pattern will follow. This results in a factor of N for the power density between the boresight direction and all other directions. In this example our main beam is formed in the boresight direction and in the other directions only the unwanted sidelobes are produced. We recognise that only for N approaching infinity can we expect the sidelobe to mainlobe power ratio to approach zero. That would require an antenna with an infinite antenna plane. The angular width of our main beam, the region with approximately coherent superposition of the partial waves from the antenna elements, also depends on N. The angular region for an in-phase superposition of all individual waves decreases for an increasing N, resulting in a narrower beam.

Chapter Contents:

• 4.1 Array factor
• 4.2 Array parameters
• 4.2.1 Half-power beamwidth
• 4.2.2 Bandwidth limitation with phase steering
• 4.2.3 Antenna element spacing without grating lobes
• 4.2.4 Gain of regular-spaced planar arrays with d=λ/2
• 4.2.5 Reduction of sidelobes by tapering
• 4.3 Circular array
• 4.4 Phase and amplitude errors
• 4.5 Architectures of passive and active array antennas
• 4.5.1 Comparison of efficiency for active and passive array
• 4.5.2 Radar equation for active arrays
• 4.6 Concepts for an extended field of view
• 4.6.1 Volume array for complete azimuth coverage
• 4.7 Monitoring of phased array antennas
• 4.7.1 Antenna measurement
• 4.7.2 Transmit/receive module (TRM) monitoring
• 4.8 Appendix: Taylor and Bayliss weighting
• References

Inspec keywords: dipole antenna arrays; radar antennas; antenna radiation patterns

Other keywords: power density; boresight direction; infinite antenna plane; antenna element array; circular water waves; in-phase superposition condition; linear sea wall; radar system; Huygens principle; metal ground plane; electrical field strength; spherical wavefront; antenna radiation pattern; mainlobe power ratio; regular grid

Subjects: Antenna arrays; Radar equipment, systems and applications