This paper directly compares a laboratory focused beam system to an alternative measurement system based on recently developed RF spot probes.

Characterizing Materials Using Dielectrically Loaded Antennas

In a laboratory, microwave characterization of materials is often accomplished with a free-space focused beam, which uses either lenses or shaped reflectors to focus RF energy onto a specimen. For the 2-20 GHz band, 60 cm diameter lenses can be spaced 77 cm from the specimen to form a Gaussian beam, simulating a plane-wave at the specimen location. An alternative method uses dielectrically loaded antennas near a specimen, which is a more compact and lower cost fixture. That said, the probe method has the disadvantage of slightly reduced accuracy. These are specially designed antennas encapsulated in a dielectric and optimized to provide a small illumination spot 7 to 8 cm in front of the probe. The resulting comparisons show that the spot probe method can be ‘almost’ as good as the higher fidelity, laboratory focused beam method.

The post SP324 RF Spot Probes – Lab Quality Materials Measurements in a Small Package appeared first on Compass Technology Group.

This paper introduces a new, non-traditional, method for measuring intrinsic dielectric properties at UHF and VHF frequencies.

Traditional fixtures for UHF and VHF measurements of dielectric properties require specimens cut or machined with precision tolerances to avoid air gap errors. They also require significant handling and multi-step calibration procedures, complicating their use in non-laboratory applications. This paper introduces a new, non-traditional, measurement method for obtaining complex dielectric properties at UHF and VHF frequencies. It applies an open-ended stripline sensor in a handheld device that non-destructively measures reflection from a material surface. 

One-Step Calibration Process

A novel computational electromagnetic (CEM) inversion methodology is applied to translate from reflection amplitude and phase into complex permittivity or sheet impedance. Because of this CEM inversion, the calibration is a simple one-step process. The CEM inversion method also enables the characterization of multilayer structures and anisotropic materials. In addition to general principles for the device, example measurements and measurement comparisons to conventional dielectric measurement systems are presented. 

The post A New Handheld Sensor for Measuring Intrinsic Dielectric Properties at 100 to 1000 MHz appeared first on Compass Technology Group.

We present a new method for measuring thin, polymer sheet permittivity using a slotted rectangular coaxial transmission line (R- Coax). This method allows a sheet of material to be inserted into the R-Coax slot, greatly simplifying the measurement procedure over traditional waveguide methods. The permittivity inversion is performed with the aid of computational simulations of the R- Coax conducted across a range of expected dielectric properties. 

How Does the Slotted R-Coax Device Work?

In particular, the slotted R-Coax device was optimized to enhance signal strength but has no simple analytical solutions for inversion. This new measurement technique is demonstrated on several thicknesses of commercial polyethylene terephthalate (PET) films, with a maximum thickness of 10 mils (0.254 mm). Due to the coaxial geometry, this technique does not have an intrinsic lower frequency cutoff and has an upper-frequency cutoff near 3 GHz from over-modeling within the transmission line, though this frequency range could be extended by shrinking the fixture. However, the signal strength and calibration stability limit the useful range of permittivity measurement to 0.5-3 GHz for 10 mil thick specimens (and a range of ~1 GHz-3 GHz for 0.5 mil thick specimens). Repeatability for the real part of the permittivity ranged between 2-5% and loss tangents of ~0.006 were measured. Thus, this paper demonstrates the R-Coax measurement technique as a potential QA tool for microwave frequency electrical properties of thin polymer films.

The post New Method for Determining Permittivity of Thin Polymer Sheets appeared first on Compass Technology Group.

This paper presents a new flat lens antenna technology, which enables significant reductions in size and weight compared to conventional wide bandwidth horn antennas. 

Free space material measurements at VHF and UHF bands require antennas that are necessarily large and heavy to accommodate the long wavelengths in these bands. Large antennas make measurements less practical and more expensive.

How this New Technology Works

These new antennas utilize artificial dielectric loading combined with lossy materials to give directivities similar to much larger and heavier horns. This paper also presents the direct application of these antennas for free space dielectric material characterization. Example measurements of dielectric specimens are shown with a pair of 200 MHz to 4 GHz antennas.

The post Flat Lens Antenna Technology for Free Space Material Measurements appeared first on Compass Technology Group.

This paper on new methods for improving the accuracy of broad band free space dielectric measurements provides several new corrections that improve the loss tangent accuracy of these methods over an order of magnitude.

Recent interest in communication and sensing at millimeter wave frequencies has led to a need for accurate characterization of dielectric materials for antennas and components. Historically, broad-band free space methods have suffered from poor accuracy when determining the loss tangent of very low-loss materials. 

Corrections for Free-Space Probe Measurements

Within this paper corrections that account for beam shift and focusing effects are described and demonstrated on several dielectric materials known for their low loss. These corrections are demonstrated on both focused beam and non-focused, free-space probe measurements.  

The post New Methods for Improved Accuracy of Broad Band Free Space Dielectric Measurements appeared first on Compass Technology Group.

We present a non-destructive testing (NDT) method for detecting aircraft radome defects. The aircraft technique is based on the analysis of microwave spectra measurements captured from handheld spot probes to detect mechanical issues in the radome. The measurements are processed using microwave transmission line theory, and empirical lineshape fitting, with the results of the processing providing the likelihood of the presence of defects. 

Examples of Defect Detections Using Spot Probes

Here, we show measurements and modeling to detect delaminations between the core and shell of an A-sandwich radome, as well as water trapped in the radome core. We also show proof of principle results indicating that a similar NDT defect detection method can be developed for use from 20 to 40 GHz, for both X-band dielectric radomes, as well as higher frequency millimeter wave radomes.

The post A Microwave Spot Probe Method for Scanning Aircraft Radomes appeared first on Compass Technology Group.

Microwave properties of dielectric and magnetic materials can be obtained non-destructively in a laboratory setting with a free-space focused beam device or an admittance tunnel lined with an absorber. However, these systems are far from portable and may be impractical for use in a manufacturing environment. Instead, a portable and rugged microwave sensor is of interest for characterization of larger structures that don’t easily fit within the geometry of a laboratory system. 

Using a Compact Device Out in the Field

This paper describes the use of a new microwave probe design that is optimized to interrogate a small area of a material or component and determine reflection or transmission properties in the 2 to 20+ GHz range. This probe is ruggedized for use in harsh environments and is optimized to have a standoff of 25 to 100 mm from the material under test. Additionally, this paper describes the integration of this probe with industrial robots to spatially map the dielectric properties of flat or curved structures. Example measurements are shown of a multilayer structure of dielectric constituents, where one or two-dimensional maps show spatially dependent properties.

The post Ruggedized Compact Microwave Probes for Mapping Material Properties of Structures appeared first on Compass Technology Group.

Intrinsic magnetic and dielectric properties of magneto-dielectric composites are typically determined at microwave frequencies with both transmission and reflection data. An iterative method, such as root-finding, is often used to extract the properties on a frequency-by-frequency basis. In some situations, materials may be manufactured on a metal substrate that prevents transmission data from being obtained. This happens when the materials are too fragile or too strongly bonded to the substrate for removal and must be characterized with the metal substrate in place. 

Comparing Two Different Free-Space Extraction Algorithms

This paper compares two different free-space extraction algorithms, developed for the simultaneous extraction of complex permittivity and permeability from metal-backed reflection. One of the algorithms relies on reflection measurements of the same material with two known thicknesses. The second method takes advantage of wide bandwidth measurements to fit the reflection to analytical models (e.g. Debye). The accuracy of these methods is evaluated and the stability criteria for the techniques will be discussed, as well as the tolerance of the techniques to various measurement errors.

The post Extraction of Magneto-Dielectric Properties from Metal-Backed Free-Space Reflectivity appeared first on Compass Technology Group.

The post New Developments in Microwave Materials Measurements appeared first on Compass Technology Group.

Measuring the RF, microwave, or millimeter wave performance of materials and components often requires a substantial length of transmission lines or cables to connect the microwave source/receiver to the test apparatus. Such cables may be subject to environmental variations (e.g. temperature or pressure) that change the overall phase delay and amplitude of signals that travel through said cables. Furthermore, some testing requires the physical motion of the cable, which is another source of phase and amplitude error. 

New Phase and Amplitude Correction Method

This paper describes a new correction method, which determines and corrects for phase and amplitude errors in transmission line cables (patent pending). Unlike previously published methods, the present technique does not require any specialized circuitry at the device under test (DUT). Instead, it utilizes in-situ reflections that already exist in the measurement apparatus to obtain a reference phase and amplitude signal. The described algorithm combines these reflections with frequency and time-domain signal processing to compensate for erroneous phase and amplitude shifts that occur during a measurement. This paper demonstrates the correction methodology with materials measurement examples. Additionally, this phase and amplitude correction may be applicable for scatter and antenna measurements. It can be applied to either reflection or transmission measurement data. 

The post Correction of Transmission Line Induced Phase and Amplitude Errors in Reflection and Transmission Measurements  appeared first on Compass Technology Group.