AN4: Millimeter-Wave Radiometers

Applications:
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:

  • Remote sensing of atmosphere
  • Remote measurement of temperature in an industrial environment
  • Contraband and weapon detection
  • Material Characterization
  • Water vapor and humidity measurement from space and ground
  • Profiling of earth’s atmosphere (or planetary atmosphere) for various molecules and radicals (ozone, chlorine monoxide, sulfur dioxide, etc.)
  • Sensors for weapon guidance and missile seekers
  • Radio Astronomy
  • Severe weather and meteorological measurements
  • Plasma diagnostics and imaging

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


  • Sensitivity or thermal resolution
  • Operating bandwidth
  • Stability or gain fluctuations
  • Calibration and diagnostics
  • Accuracy of measurements
  • Data rate or integration time

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 Applications 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:
Amplifiers (Low Noise, Power)
Antennas
Balanced Low Noise Broadband Mixers
Cryogenically-Cooled Amplifiers
Detectors
Feed horns
Frequency Multipliers
Microwave (IF) Amplifiers
Noise Sources
Oscillators (Gunn Diode Oscillator, series QTM from 18-150 GHz)
Power Dividers/Hybrids (Short Slot Coupler, Matched Hybrid Tee, Directional Couplers)
Switches (PIN, electromechanical)