Product Application Notes – Millimeter-Wave Reflectometers
Millimeter-wave (MMW) reflectometers measure reflection characteristics of various objects, plasma, materials, and scenes in order to obtain some of their specific features or properties at millimeter wavelengths. While reflectometers are similar to radars and interferometers in their architecture and function, they are often treated as an independent class of equipment. Many diverse fields have applications for MMW reflectometers such as:
- Plasma diagnostics
- Dielectric and other material properties measurement
- Process Monitoring/Control
- Radar Signature Characterization and Data collection
Reflectometers serve specific and diverse applications. For example, reflectometers measure plasma density, determine thickness of ash and carbon build-up, and enable modeling of aircraft and military vehicles.
MMW reflectometers comprise many configurations and architectures based on the specific application. To illustrate, Figure 1 shows a generic reflectometer arrangement.
Firstly, the equipment consists of a MMW transmitter as the illumination source for the object or scene under study. Suitable antennas and optical arrangements optimally illuminate the scene or item being characterized. Additionally, quasi-optical realization of MMW and sub-MMW reflectometers is practical for characterizing specific materials or objects. Then, one or more receivers collects the reflected signal from the object or scene using appropriate optics or antenna configuration. The angular location and physical distance to the scene or object is adjustable as needed. 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. Finally, the received signal may be either a scalar (amplitude of the return signal) or vector (amplitude and phase using I-Q detector/mixers) signal.
Operation and Typical Performance Characteristics
Most typical applications use a fixed-frequency CW signal for illumination. However, a multi-frequency or swept signal is also used 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.
Typical Examples and Case Histories
|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 MMW frequency)
||Liquid characterization by measuring its loss and dielectric properties
||Characterize loss in liquids at millimeter wavelengths.
|47 and 94 GHz
||Moisture content measurement in paper and other materials
||Measure 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.
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: QTM from 18-150 GHz; Power Dividers/Hybrids (Short Slot Coupler, Matched Hybrid Tee, Directional Couplers)