Microwave Material Measurements without Cables

by John W. Schultz, PhD – Compass Technology Group

Measuring the performance of microwave materials requires a test apparatus that includes a measurement fixture and a microwave analyzer, which are usually connected with microwave or radio frequency (RF) cables. These RF cables are the bane of many measurement scenarios and can be a significant source of measurement error. A lesson that is learned early in material measurement laboratories is to avoid bumping the RF cable because of the phase and amplitude errors that result. In fact, some RF cable manufacturers specifically include polymer outerjackets with increased surface stickiness to minimize movement when the cable is inadvertently bumped.

Even if cable flex errors are carefully controlled, there can still be substantial errors from environmental temperature drift. RF cables are generally a coaxial design with solid or stranded inner and outer conductors made from metal and separated by a dielectric spacer. Teflon™ is commonly used for this purpose and is subject to thermal expansion or contraction as the temperature changes. In the case of Teflon, there is also a material phase transition that occurs near room temperature which significantly increases its thermal expansion coefficient. In the RF measurements industry, this phenomenon is known as the “Teflon knee” [1]. Because of this room temperature Teflon knee, RF cables can experience sufficient thermal expansion to cause undesired phase drift even with just a couple of degrees of ambient temperature change. In other words, temperature variation due to normal air conditioning cycles can cause significant measurement error.

When material measurements are required in a factory or production environment, these problems are exacerbated. The usual paradigm in a materials measurements laboratory is to insert a material coupon into a measurement apparatus. Conversely, in a factory environment, the need is for measuring materials in-situ. This means that the measurement apparatus is brought to the production line and the part being manufactured, rather than viceversa. For example, measurement of materials that are incorporated into large components, such as a microwave radome, may require robotic actuation of a sensor over the surface of that structure. Traditionally this requires an RF cable that connects the sensor to an analyzer to be routed along the robot arm. As the sensor is moved over the part under test, the cable flexes and creates phase and amplitude errors that are not necessarily repeatable. The RF cable also becomes a significant wear item and must be replaced as continuous motion eventually results in cable failure. Furthermore, depending on the production environment, ambient temperatures may vary significantly more than in an air-conditioned laboratory, further increasing measurement errors.

Software Solution

One potential solution to cable induced errors is through software analysis of the measured data. This method requires quantifying the phase and amplitude errors induced by the cable, and then applies a correction to the measured signal that cancels the cable error. In a recent provisional patent, Compass Technology Group developed just such a method [2]. In particular, the method leverages extra reflections that exist within a measurement apparatus and that are usually ignored or subtracted. To illustrate this idea, Figure 1 shows a microwave spot probe attached to the end of a robotic arm. A microwave network analyzer (not shown) excites the apparatus at a series of frequencies stepped from 2 to 20 GHz. Also included is a 25 foot (7.5 meter) RF cable that connects the network analyzer to the microwave probe. To protect the cable from excessive wear, it is contained within a flexible cable management tube.

During operation, the robot positions the spot sensor just above the surface to be measured. The signal of interest is from the energy that is emitted from the probe and then reflected by the material beneath it. However, the probe itself also reflects some of the microwave energy. It is this otherwise unwanted reflection from the probe that can serve as a phase and amplitude reference signal. In particular, any change in the phase and amplitude of the probe antenna provides a measurement of the phase and amplitude offset due to cable flex [2]. These offsets are applied before doing the calibration calculation, thereby minimizing them as an error source.

Measured data from an electromagnetic interference (EMI) absorber is shown in Figure 2 to illustrate the effects from cable flex. This particular material is designed to maximize absorption near 14 GHz. All the data shown in this figure were calibrated using a “response and isolation” methodology [1]. The response measurement was of an ideal microwave reflector—in this case a flat metal plate. The isolation measurement was of no specimen or free space. The calibration procedure includes a vector-subtraction of the isolation measurement from both the response data and from the specimen measurement. When the additional cable correction method is used, the phase and amplitude correction is applied before each vector subtraction step. In other words, the isolation measurement is subtracted from the corrected specimen-under-test data or from the corrected response data, as appropriate. The final calibrated reflectivity of the specimen is then the ratio of the subtracted specimen data to the subtracted response data.

The thin solid line of Figure 2 shows the result when care was taken not to move the RF cable between calibration and specimen measurement. The thick solid line shows the same specimen measured after the RF cable was moved, but without any phase or amplitude correction applied. The dashed line shows the specimen measurement after the cable was moved but with an additional phase and amplitude correction by the described method. The corrected data after cable movement overlays the calibrated data for when the cable was not disturbed. As these results show, cable movement significantly degrades the accuracy of an RF measurement, but the described correction method can account for these errors.

Cable-Less Hardware

While the use of a software algorithm to reduce cable errors is very helpful under most situations, it does not eliminate the practical limitations of RF cables. Cables still remain a significant wear item in measurement systems where motion is necessary, and cables can easily degrade to the point that no amount of software correction will fix the phase and amplitude error. Thus, the ultimate solution to dealing with RF cables is to eliminate them completely. A couple decades ago, microwave analyzers were very large and heavy, occupying full sized racks. Over the last decade or so there has been a steady progression of size reduction and currently you can find network analyzers from a number of companies that are in the form of a single, rack-mounted component. However, even these reduced-size analyzers are still relatively big and heavy so they require the sensor to be connected with an RF cable.

Most recently, there have been significant advances in compact microwave circuitry and components.  This is thanks in part to the constant drive for miniaturization of RF transceivers in consumer electronics. Following this trend, a new line of ultra-compact lab-grade vector reflectometers have been developed and made commercially available by Copper Mountain Technologies. With form factors that easily fit in your hand, these analyzers enable the idea of RF cable-less measurements, and they were recently awarded a U.S. patent related to this idea [3].

The idea of cable-less RF measurements is especially powerful because it eliminates the various errors discussed above. More importantly, it allows for a shift in the usual paradigm of RF material measurements. With conventional size microwave equipment, “witness” coupons must be made and brought to the measurement apparatus for testing. On the other hand, introduction of RF analyzers that are not much bigger than a cellular phone provides the new possibility of measuring materials and components in-situ and eliminates the need for witness coupons. This concept brings the materials measurement capability out of the laboratory and into the much more challenging environment of the factory floor. An added benefit is realized from separating the microwave measurement module from data processing which is done on a separate PC, minimizing the amount of work performed on the factory floor and improving productivity.

Another key ingredient for handheld measurement is a compact RF sensor or spot probe. The use of spot probes for microwave material measurements goes back to at least the mid 1970s. Musil, Zacek, et al. used dielectric antennas to measure transmission through a material specimen and their sensors consisted of dielectric rods inserted into the ends of metal horn antennas [4]. They used their sensors to successfully determine the complex dielectric permittivity of silicon specimens at millimeterwave frequencies. More recently Diaz et al. designed “polyrod” antennas using computational simulation tools. Their sensor included multiple dielectric layers inserted into a metal horn antenna [5], and their innovation was to use computational tools to optimize the inserted polymer material and optimize impedance match of the probe antenna.

The spot probe described in this paper also includes both metallic elements and dielectric material. However, while previous spot probes designed dielectric inserts into conventional horn antennas, the present probe design was conceived by optimizing both the dielectric shape and metallic elements into an integrated unit. Figure 3 includes a photograph of these integrated spot probes showing their compact shape. The probes are fed with a single SMA port in the rear, and they transmit and receive with linear polarization from 2.5 to 20 GHz for the larger model and from 4 to 24 GHz for the smaller model. These probes are 18 cm (7 inches) and 10.2 cm (4 inches) in length for the larger and smaller variants, respectively. Also shown in Figure 3 is the measured voltage standing wave ratio (VSWR) for two different large probes, which shows that the VSWR is better than 3 for the entire frequency band of use, and is better than 2 for most of that same band. The smaller probes have similar VSWR characteristics, except that they work to higher frequencies.

These probes have been shown to have measurement accuracies similar to larger laboratory measurement systems when measuring materials at normal incidence [6]. The illumination area is approximately round and has a diameter that depends on both standoff distance and frequency. For the measurement examples discussed in this article, a standoff distance of approximately 7 cm (2.75 inches) was used and the illumination area diameter is approximately 5 cm (2 inches) at 10 GHz, with the diameter being larger at lower frequencies and smaller at higher frequencies within the band.

The sections that follow review the use of a miniaturized analyzer that has been integrated with this spot probe sensor. Two examples are presented: i) monitoring reflectivity of materials used in electromagnetic interference (EMI) mitigation with a handheld device, and ii) non-destructive detection of defects in fiberglass composites with robotic scanning.

Application Example: EMI Absorber

The proliferation of high-speed computing and wireless communication has resulted in a crowded electromagnetic environment. Signals generated within a device or between devices can inadvertently interfere with the functionality of those devices. One technique for reducing this mutual interference is the use of materials to block or absorb signals. For example, the housing for a given component may be lined with or have embedded materials that are designed to absorb RF energy. An active component may even have absorbing material directly applied to it to minimize interference issues. Screens designed to block and/or absorb energy may be placed between adjacent components.

Figure 4 shows an integrated reflectometer that includes a Copper Mountain Technologies’ RP180 combined with Compass Technology’s SP218 spot probe. This system is hand-held and requires cables only for power and communication (USB) with a data acquisition computer. Calibration of this device is straightforward, requiring only two additional measurements: a “response” measurement and an “isolation” measurement. The response measurement is a reference standard such as a flat metal plate. The isolation standard is simply measuring the probe while it is pointed at free space. This isolation measurement allows subtraction of background and foreground from the signal of interest, including the probe response. Additionally, time-domain processing is used to further isolate the signal of interest from other unwanted reflections (such as room reflections). Time domain processing involves transforming the wide-band frequency data into time domain and isolating the reflection of the sample under test from the rest of the detected reflections.

Figure 5 shows measured data from two different absorber material samples measured by the reflectometer system shown in Figure 4. These specimens were of a commercial magnetic absorber made by mixing iron particles with an elastomer. The iron loading and thickness of the 0.070 inch (1.8 mm) thick absorber enables it to optimally absorb around 9 GHz, as shown by the data. A second curve is also shown that has two 70 mil layers placed on top of each other, thereby doubling the thickness. As a result, the reflection null occurs at a frequency close to half that of the original absorber sheet. With the compact size of this measurement system, it is conducive to use in a factory environment where the materials are being manufactured. This is much more convenient than having to bring specimens back to a laboratory for testing. Additionally, this very portable device can be used when and where materials are applied to components or parts. While the measurements shown here are of a magnetic absorber, this reflectometer can also measure microwave performance of resistive materials such as EMI shielding and dielectric materials such as radomes and microwave windows.

Application Example: NDE of Composites

The modern factory is automated and industrial robots are commonly employed to improve production efficiency and quality. In this factory setting, direct measurements of manufactured components are desired for quality assurance (QA) and feedback purposes so that defects are identified early in the process and manufacturing processes are corrected. Catching problems early saves cost. Furthermore, in the manufacture of large or expensive parts, QA requirements may necessitate characterization of every part that is produced. A compact reflectometer device such as the integrated analyzer and spot probe described above enables this when integrated into factory automation systems. Figure 6 shows a picture of the compact reflectometer system mounted on an industrial robot. In this configuration, the reflectometer is scanned over the surface of a material to map the positional dependence of microwave properties. Physics-based models can then be used to determine information about the material under measurement.

For example, one of the problems that can occur either during manufacture or in use of composite parts is delamination. Composites are typically of a laminate construction where different layers of fiberglass and resin are consolidated into a single material. For various reasons, there may be air gaps or separations between layers within a part. Since fiberglass is generally opaque, there may be no visual indication of this delamination, and it only shows up when the part fails under mechanical load. Figure 7 shows a measurement of a 0.5 inch (12.7 mm) thick composite with and without a controlled delamination. In actuality, the part consists of two 0.25 inch thick composite panels sandwiched together. To simulate a delamination, a 0.018 inch (0.46 mm) spacer is placed at the top of the part. In both cases (with and without spacer), the base of the panel is mechanically clamped together to eliminate any delamination gap at the base.

The data shown in Figure 7 are calculated by comparing the measured reflectivity in the 2-18 GHz band to a singlelayer model of a dielectric slab. Plotted is the amount of deviation from the ideal slab-model as a function of measurement location while the robot scans from bottom to top. In both cases, there is a low level of residual model-fit error due to natural inhomogeneity of the fiberglass. However, when a controlled delamination is induced in the middle of the specimen, the residual fit error shows a clearly increasing trend with the increasing width of the delamination gap.

A second measurement example is provided in Figure 8, which shows an alternate analysis of reflection data measured from a composite panel. In this case, the measured data is also compared to a simple dielectric slab model. The dielectric composite permittivity is assumed to be known and constant. The model is then used to compute the thickness of the panel being measured based on this model measurement comparison. The y-axis is therefore the computed thickness in units of mils or thousandths of an inch. Consistent with physical caliper measurements, the thickness is approximately 0.5 inches. Because the composite is not perfectly flat, there is some observed variability of the thickness as the reflectometer is scanned across the panel.

A second curve is also shown in Figure 8, in which 6-inch wide painter’s tape was applied over a part of the fiberglass panel. Furthermore, the tape was applied to the rear part of the panel, opposite to the side being measured by the reflectometer. As the data show, the tape extended from about 7 to 14 inches of the measured area and is clearly evident. The painter’s tape was approximately 3 mils thick, which is consistent with the increased thickness determined by the microwave reflectometer. Detection of this 0.003 inch thick layer of tape is somewhat remarkable in comparison to the wavelength of interrogating microwave energy, which varies from 6 inches at 2 GHz to 0.66 inches at 18 GHz. Thus, this method can be used to detect small thickness changes of fiberglass composites and other dielectric materials. Furthermore, since it is easily portable, such testing can be done in any environment, such as a factory or in the field.

Conclusion

Traditionally, RF measurements of materials have been dominated by the paradigm of taking witness samples into the laboratory. This is because of the historically large size of both the microwave analyzer equipment and the fixturing. Recent technology developments in both compact spot probes and compact microwave analyzers are now enabling a dramatic shift in this paradigm. In particular, this article discussed the concept of handheld or robot mounted reflectometers for measurement of microwave relevant materials. The technology described here integrates the microwave analyzer and sensor, thereby eliminating the need for RF cables.

RF cables add errors due to thermal drift and flexing. With elimination of the RF cable, measurement reliability and accuracy is significantly improved. RF cables are also a wear item and must be replaced periodically, especially if they are regularly moved or flexed. Therefore, elimination of the RF cable can decrease measurement cost and reduce the need for maintenance. This article briefly discussed two example applications for a compact microwave reflectometer. The first example is of a handheld device that can measure the reflection coefficient of absorber materials for EMI mitigation. The second example application mounts the integrated reflectometer system onto an industrial robot and scans fiberglass composites. With appropriate data processing, a microwave reflectometer can detect defects and determine thickness of non-conductive materials. Both of the examples shown demonstrate that RF cable-less reflectometer technology has arrived. This now enables in-situ measurements in either field or factory environments, which used to be impractical.

Acknowledgements:

This work was supported in part by a Cooperative Research and Development Agreement (CRADA) between the Air Force Research Laboratory (AFRL/RX) and Compass Technology Group.

References:

1. JW Schultz, Focused Beam Methods, Measuring Microwave Materials in Free Space, ISBN 1480092851, 2012.

2. JW Schultz, R Schultz, J Maloney, K Maloney, “Correction of Transmission Line Induced Phase and Amplitude Errors in Reflectivity Measurements,” U.S. Provisional Patent 20160103197.

3. SA Zaostrovnykh, VI Ryzhov, AV Bakurov, IA Ivashchenko, AI Goloschokin, “Measurement Module of Virtual Vector Network Analyzer,” U.S. Patent 9291657B2.

4. J Musil, F Zacek, A Burger, J Karlovsky, “New Microwave System to Determine the Complex Permittivity of Small Dielectric and Semiconducting Samples,” 4th European Microwave Conference, 66-70, 1974.

5. R Diaz, J Peebles, R Lebaron, Z Zhang, L Lozano-Plata, “Compact Broad-Band Admittance Tunnel Incorporating Gaussian Beam Antennas,” U.S Patent 7889148, 2007.

6. JW Schultz, J Maloney, K Cummings-Maloney, R Schultz, J Calzada, B Foos, “A Comparison of Material Measurement Accuracy of RF Spot Probes to a Lens-Based Focused Beam System,” Proceedings of the 2014 AMTA, Tucson AZ, 2014.

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