Radiometer antennas

Abstract

Microwave radiometry is concerned with purely passive sensing of naturally generated microwave radiation of thermal origin. Microwave radiometers are corresponding measuring devices typically designed and built as a very low-noise receiver followed by a signal recording unit. Usually, radiometers contain an antenna as the first reception component collecting the incoming radiation, and they measure radiation power expressed in an apparent temperature called brightness temperature. The observable brightness temperature of any object or surface depends on various chemical and physical quantities, whose concurrence is expressed by the objects' emission (absorption), reflection, and transmission properties and its true temperature. Since the Earth has a temperature typically close to 300 K and the universe close to 3 K, a nearly arbitrary mixture of these two extreme temperatures can be expected. Consequently, our environment can show quite different brightness temperature values depending on the direction of actual observation. On the one hand, radiometer measurements are carried out stationary with respect to the antenna pointing direction in order to observe time-dependent variations of the brightness temperature. On the other hand, the brightness temperature of a whole scene is scanned in order to acquire locally changing one- or two-dimensional profiles, while the latter ones are assembled as a two-dimensional image comparable to a conventional photograph. Depending on the specific application, various antenna types are considered, where usually hard requirements with respect to beam width, side-lobe level, scan capability, and losses have to be addressed (chapter "Transmission Lines"). Radiometric measurements are performed for Earth or planetary observation in space (chapter "Space Antennas Including Terahertz Antennas"), from aircraft platforms on the Earth's surface and the atmosphere, or on the ground, either sensing the environment or sensing the universe, the latter being performed in radio astronomy (chapter "Antennas in Radio Telescope Systems"). Usually, the brightness temperature is rarely used as the physical quantity of interest. More often, it is transferred via adequate physical models to other secondary or third quantities for more direct use in the case of Earth observation (e.g., soil moisture, ocean salinity, rain rate, snow cover, etc.), being performed already since the 1950s of the last century. However, in the last decades, microwave radiometry is as well used in many safety- and security-related applications, for which often only sufficient temperature contrast between an object and its surrounding is required besides spatial resolution for detection and recognition purposes. In this chapter relevant fundamentals of microwave radiometry are outlined for better understanding of antenna requirements, followed by an overview of typical types of radiometer antenna systems. Some existing antenna systems are discussed in order to illustrate the variability with respect to applications. A section on basic antenna quantities addresses key figures for practical design and verification and illustrates the results exemplarily for selected cases. Finally, a brief summary and an outlook on possible future implementations and other frequency ranges are given.