AN4: Millimeter-Wave Radiometers

Millimeter wave radiometers are a special class of instruments or equipment based on broadband passive receivers for measurement of physical temperature, molecular absorption or emissivity of an object, environment or a scene under study. The primary function of the millimeter wave radiometer subsystem is to measure either temperature or spectral characteristics of an object or the medium under observation. Some of the most prominent applications of millimeter wave radiometers are:

Since radiometers are passive receivers (do not transmit any electromagnetic signal), they are particularly attractive for many military, security and industrial applications.

Depending upon the application and operational requirements, the complexity of a radiometer varies greatly ranging from a basic single-channel fixed radiometer instrument to highly complex, multi-channel radiometric imagers (cameras) with elaborate scanning mechanism.

Description:

Radiometers can be configured in a wide variety of ways depending on the performance required, type of measurements or observation, speed and complexity of operation and budget. Figure 1 shows several types of radiometer arrangements with increasing complexity or rigor of measurement.

(A) Total Power Heterodyne Receiver

(B) Direct "Tuned RF" Detector

Most important parameters for a millimeter wave radiometer are:

In the majority of applications, the calibration of the radiometer is a very significant step in the operation of the instrument. The majority of subsystems incorporate highly temperature stabilized and/or controlled thermal loads or noise sources as an integral part of the instrument. A Dicke switch at the input of the radiometer is used to reduce or eliminate potential errors introduced by the normal fluctuations or time-dependent variations in the gain of the receiver. The receiver is switched between the antenna (the scene) and a thermal load at a known, stable temperature at a high rate.

Some of the most common variations or enhancements to the basic radiometer configuration are: multi-frequency operation, multi-element array, wide area/angle scanning, polarimetric measurements,

Operation and Typical Performance Characteristics:

Millimeter wave radiometers almost invariably involve the use of a suitable antenna at the input port to allow measurements or characterization of a specific area or object covered by the antenna beam (spot).

Typical Examples and Case Histories
Table and Figure

Frequency	Description	Application and Comments
In 18-35 GHz range (three channels)	Three ultra low-noise radiometers at three different frequencies in this range. Provision for noise injection and calibration load switching	Space-borne Radiometer for meteorological studies. Designed to be extremely stable and repeatable/reliable.
91-95 GHz	Broadband radiometer covering 4 GHz (IF from 0.5 to 4.5 GHz) with noise figure of 5 dB	Millimeter wave imaging and remote sensing
22-30 GHz and 51-59 GHz	Dual-Band Radiometer had two independent channels with noise figure of 3.5 dB for the 22-30 GHz channel, and 5 db for the 51-59 GHz channel. Built-in noise injection using a broadband noise source. Ferrite device- based Dicke switch incorporated in the front end together with low-noise IF amplifiers and video detector/amplifiers.	Water vapor measurement using 23.8 and 31.4 GHz, and temperature measurement using oxygen lines in the 51-59 GHz band
35, 60, 94 GHz: Triple channel radiometer	Three independent radiometer subsystems in a single package can be operated simultaneously or individually. Designed to incorporate noise sources and optional reference loads.	Instrumentation subsystem for research and development. Material and environmental measurement instrument.

QuinStar Components and Products Used