AN2: Millimeter-Wave Reflectometers

Millimeter wave reflectometers are used to measure reflection characteristics of various objects, plasma, materials and scenes to obtain some of their specific features or properties at millimeter wavelengths. Reflectometers are somewhat similar to radars and interferometers in their architecture and function, but are often treated as an independent class of instruments or equipment. Millimeter wave reflectometers find applications in many diverse fields, such as:

  • Plasma diagnostics
  • Dielectric and other material properties measurement
  • Imaging
  • Process Monitoring/Control
  • Radar Signature Characterization and Data collection

Reflectometers are used in specific applications as diverse as plasma density measurement, determination of thickness of ash and carbon build-up and modeling of aircraft and military vehicles.

Many possible configurations and architectures are used to realize millimeter wave reflectometers for specific applications. A generic reflectometer arrangement is shown in Figure 1. The equipment or instrument consists of a millimeter wave transmitter as the illumination source for the object or scene under study. Suitable antenna(s) and optical arrangements are employed to optimally illuminate the scene or item being characterized. A quasi-optical realization of millimeter wave and submillimeter wave reflectometers is also practical for characterization of specific materials or objects. One or more receivers may be used to collect the reflected signal from the object/scene using appropriate optics or antenna configuration. The angular location and/or physical distance to the scene/object can be varied, if desired. Depending on the type of reflectometer and the purpose of the measurement, the local oscillator for the receiver is either derived from the same master oscillator that produces the transmitter/illumination signal or created independently. The received signal may be obtained either as a scalar (amplitude of the return signal) or vector (amplitude and phase using I-Q detector/mixers).

Figure 1

Most typical applications use a fixed frequency CW signal for illumination. However, a multi-frequency or swept signal is also employed in some cases. Dual frequency reflectometry is useful in obtaining material properties and related information. Generally, for most applications and measurement scenarios, the transmitter is a stable (but not phase-locked) source.

Operation and Typical Performance Characteristics:

Typical Examples and Case Histories
Table and Figure

Frequency, GHz Application Subsystem description
35, 60 GHz, 140 GHz Plasma Density Measurement System Real-time measurement of reflection properties of plasma using a stable phase-amplitude measurement subsystem.
70 GHz (any suitable millimeter wave frequency) Liquid characterization by measuring its loss and dielectric properties Characterization of loss in liquids at millimeter wavelengths.
47 and 94 GHz Moisture content measurement in paper and other materials Measurement of transmission and reflection coefficient at two frequencies to determine moisture content and other properties
94, 140 GHz Measurement of deposits of carbon and ashes in chimneys Measures reflection from deposits.


QuinStar Components and Products Used: Amplifiers (Low Noise, Power) Detectors Frequency Multipliers High Power IMPATT oscillators at 34-36, 43-47, 58-62, 92-97 GHz)) I-Q Mixer Oscillators (Gunn Diode Oscillator, series QTM from 18-150 GHz Power Dividers/Hybrids (Short Slot Coupler, Matched Hybrid Tee, Directional Couplers)