Microwave NDE Equipment

When performing microwave experiments, there are a few pieces of equipment that are commonly used. We can categorize this equipment into instrumentation and probes.

On the instrumentation side, network analyzers are common pieces of equipment used to make microwave measurements. There are two primary types of network analyzers that may be used, namely: scalar network analyzers (SNAs) or vector network analyzers (VNAs) depending on whether one is interested in only measuring signal magnitude (SNA) or magnitude and phase (VNA). The latter is referred to as vector measurement. Network analyzers, particularly vector types, facilitate characterization of active and passive devices through measuring their reflection and transmission properties (i.e., S-parameters, which will be discussed more in the next section). Network analyzers are wide band, extremely accurate, and allow for calibrated measurements at specified reference planes throughout a microwave network. Network analyzers are best-suited for measurements conducted in a laboratory and for validation of models since VNAs are usually bulky and relatively expensive. However, in the past couple of decades and due to the increase in wireless communications needs, portable and even handheld VNAs have entered the market. Although these may not have the same noise floor and dynamic range characteristics as laboratory benchtop VNAs, their utility for NDE applications is ever increasing.

Example of a benchtop 4-port VNA and a handheld 1-port VNA

When making a measurement with a network analyzer, the user begins by connecting a probe, antenna, or device-to-be-tested to the network analyzer through high-quality coaxial cables. A coaxial cable is a type of transmission line that consists of an inner conductor and an outer conductor separated by some type of dielectric material. Coaxial cables can support a very wide range of frequencies and are terminated in different types of connectors and adapters. Some common types of connectors and adapters include N-Type, SMA, 2.4 mm, and APC-7. Each of these has a different frequency range of operation. 

Beyond being used to connect probes and devices to a network analyzer, coaxial cables can also be used as probes themselves. Open-ended coaxial probes have been used in applications such as surface crack detection and materials characterization, which will be discussed more in-depth in subsequent sections. 

Another common type of probe used in microwave NDE is an open-ended waveguide. Standard waveguides, with rectangular and circular cross-sections, operate in a certain range of frequencies as a function of their broad or radius dimensions, respectively. Rectangular waveguides more commonly used in practice, particular as an interrogating probe. Examples of rectangular waveguide to coax adapters and waveguide sections that are labeled with their waveguide band designation are shown in the picture below. The Table below lists the dimensions, operating frequencies, and designations of all rectangular waveguides in the microwave and millimeter wave frequency ranges. The picture and table show that as the operating frequency decreases, the waveguide size increases and vice versa.

For additional information about waveguides the reader may consult one of many available reference textbooks or web sites.

Waveguide to coax adapters and waveguide sections covering 2.6 to 110 GHz. 

Rectangular Waveguides for Microwave and Millimeter Waves

Rectangular Waveguide Band Letter Designation	Operating Frequency Range	Dimensions
	(GHz)	(cm x cm)
L	1.12-1.70	16.51 x 8.25
R	1.70-2.60	10.92 x 5.46
S	2.60-3.95	7.21 x 3.40
H or G	3.95-5.85	4.75 x 2.21
C or J	5.85-8.20	3.48 x 1.58
X	8.20-12.40	2.28 x 1.01
Ku	12.4-18.0	1.58 x 0.79
K	18.0-26.5	1.07 x 0.43
Ka	26.5-40.0	0.71 x 0.35
Q	33.0-50.5	0.57 x 0.28
U	40.0-60.0	0.48 x 0.24
V	50.0-75.0	0.38 x 0.19
W	75.0-110.0	0.25 x 0.12
D	110.0-170.0	0.17 x 0.08
G	140.0-220.0	0.13 x 0.06

While coaxial probes and waveguides are commonly used in microwave NDE due to their versatility and simplicity of use, other types of probes, antennas, and devices can also be used depending on the application. Examples of these include horn antennas, microwave cavities, and microstrip resonators. When performing microwave imaging, the measurement systems can also be quite varied as a function of the type of imaging performed (e.g., near-field, lens-focused or synthetic aperture radar (SAR)). More about equipment in the context of specific applications will be discussed in the upcoming sections.

   

Horn antenna (left) and microwave cavity (right).