Free-space Measurement System Research Articles

Polarization characteristics of a composite stealth radome with a frequency selective surface composed of dipole elements

In this work, a radome was fabricated using a composite sandwich constructions composed of an E-glass fabric/epoxy composite, a PVC foam, and a frequency selective surface (FSS) for stealth performance. The polarization characteristics of the stealth radome were investigated using measured electromagnetic (EM) transmission characteristics with respect to the polarization angle between the direction of the electric field of incident EM waves and the direction of the dipole length by way of a free space measurement system. The dielectric properties of the E-glass fabric/epoxy composite and the PVC foam were also measured with respect to the polarization angle.

A Low-Profile Third-Order Bandpass Frequency Selective Surface

We demonstrate a new class of low-profile frequency selective surfaces (FSS) with an overall thickness of lambda/24 and a third-order bandpass frequency response. The proposed FSS is a three layer structure composed of three metal layers, separated by two electrically thin dielectric substrates. Each layer is a two-dimensional periodic structure with sub-wavelength unit cell dimensions and periodicity. The unit cell of the proposed FSS is composed of a combination of resonant and non-resonant elements. It is shown that this arrangement acts as a spatial bandpass filter with a third-order bandpass response. However, unlike traditional third-order bandpass FSSs, which are usually obtained by cascading three identical first-order bandpass FSSs a quarter wavelength apart from one another and have thicknesses in the order of lambda/2 , the proposed structure has an extremely low profile and an overall thickness of about lambda/24 , making it an attractive choice for conformal FSS applications. As a result of the miniaturized unit cells and the extremely small overall thickness of the structure, the proposed FSS has a reduced sensitivity to the angle of incidence of the EM wave compared to traditional third-order frequency selective surfaces. The principles of operation along with guidelines for the design of the proposed FSS are presented in this paper. A prototype of the proposed third-order bandpass FSS is also fabricated and tested using a free space measurement system at C band.

Improvement of Bonding Characteristics Between the Frequency Selective Surface and E-Glass/Epoxy Composites for Stealth Radomes

Radomes of weapon systems such as aircraft, warships, and missiles have been manufactured using FSS (Frequency Selective Surface) for the stealth performance and E-glass/epoxy composite to protect the radar antenna from weather and mechanical impact and to minimize the loss of EM (electromagnetic) waves transmitted through the radome. In this work, the FSS was manufactured from an FCCL (Flexible Copper Clad Laminate) composed of a copper foil of 20 μm thickness and a polyimide film of 40 μm thickness. The bonding characteristics between the FSS and E-glass/epoxy composite were determined for different surface treatments of copper such as atmospheric pressure plasma treatment and nitric acid/ferric chloride treatment. The EM transmission characteristics of the FSS were also measured by the free space measurement system before and after surface treatments. Nitric acid/ferric chloride treatment of the FSS was found to be a promising method to enhance the bonding characteristics between the FSS and the E-glass/epoxy composite without change in the EM transmission characteristics of FSS.

Composite sandwich constructions for absorbing the electromagnetic waves

RAS (radar absorbing structures) is a key component for weapon systems such as aircrafts, warships, and missiles to achieve both the stealth performance by absorbing EM (Electromagnetic) waves incident on and load bearing capability. In this work, the RAS was fabricated as sandwich constructions composed of nanocomposite, carbon fabric/epoxy composite, and PVC foam. The nanocomposite composed of E-glass fabric, epoxy resin, and CNT (carbon nanotube) was adhesively bonded to the outside of the sandwich construction in order to absorb EM waves. The carbon fabric/epoxy composite had the dual roles as the reflection layer of incident EM waves and load bearing face material of sandwich constructions. Using the fabricated sandwich constructions, the EM absorbing characteristics were measured by the free space measurement system and the bonding characteristics between nanocomposites and carbon fabric/epoxy composites also were investigated.

Nanocomposite stealth radomes with frequency selective surfaces

The stealth function of the radome (Radar + Dome) is to transmit or reflect the EM (electromagnetic) wave selectively through the radome. In this work, the stealth radome for aircrafts and warships was developed with the FSS (frequency selective surfaces), PVC foam, and nanoclay-dispersed E-glass fabric/epoxy composite. The water diffusivity of nanocomposites, which changes the stealth characteristics, was measured with respect to the contents of nanoclay . The EM transmission characteristics were measured by the free space measurement system in the X-band frequency range (8.2–12.4 GHz) with respect to the content of nanoclay. Also, the flexural strength of the sandwich construction composed of the nanocomposite, PVC foam, and FCCL (flexible copper clad laminate) was measured by the 3-point bending test.

Low-observable radomes composed of composite sandwich constructions and frequency selective surfaces

In this work, low-observable stealth radomes composed of composite sandwich constructions and FSS (Frequency Selective Surfaces) were developed for selective wave transmission and reflection characteristics. The FSS was composed of a copper foil and a polyimide film. The sandwich constructions were composed of plain weave fabric E-glass/epoxy composites and PVC foams, which protect the radar antenna in the radome from external loads. The electromagnetic transmission characteristics of the low-observable radome wall were numerically calculated with 3-dimensional electromagnetic analysis software, and compared with the measured results by the free space measurement system. Also the mechanical properties of the sandwich constructions were investigated.

A Multilayer Negative-Refractive-Index Transmission-Line (NRI-TL) Metamaterial Free-Space Lens at X-Band

A volumetric free-space negative-refractive-index (NRI) transmission-line (TL) metamaterial lens is described that employs fully printed interdigitated capacitors and meandered inductors designed to exhibit NRI properties at X-band (8-12 GHz). The volumetric topology is realized in a layer-by-layer fashion without any vias, which facilitates easy and rapid fabrication. The fabricated lens was tested for its transmission and dispersion properties using a free-space X-band measurement system consisting of an Agilent network analyzer, standard gain horn antennas, and Rexolite dielectric lenses fabricated in-house, and showed good correspondence with simulations. The focusing ability of the multilayer NRI-TL lens was also verified using a free-space field probing system based on small shielded-loop antennas affixed to a computer-controlled xyz-translator apparatus. Arguably, these results represent the first experimental evidence of coupling between TL-based metamaterials and sources in free space.

Calibration methods for microwave free space measurements

Abstract. In this article calibration methods for the precise, contact-less measurement of the permittivity, permeability or humidity of materials are presented. The free space measurement system principally consists of a pair of focusing horn-lens antennas connected to the ports of a vector network analyzer. Based on the measured scattering parameters, the dielectric material parameters are calculable. Due to systematic errors as e.g. transmission losses of the cables or mismatches of the antennas, a calibration of the measurement setup is necessary. For this purpose calibration methods with calibration standards of equal mechanical lengths are presented. They have the advantage, that the measurement setup can be kept in a fixed position, for example no displacement of the antennas is needed. The presented self-calibration methods have in common that the calibration structures consist of a so-called obstacle network which can be partly unknown. The obstacle can either be realized as a transmissive or a reflective network depending on the chosen method. An increase of the frequency bandwidth is achievable with the reflective realization. The theory of the calibration methods and some experimental results will be presented.

内装用建材の複素比誘電率とその変動に関する検討

Complex relative permittivity, which is one of the important electrical constants of materials for electromagnetic compatibility inside and/or around building, is estimated by iteration calculation using four measurements of the magnitude of reflection coefficient from Free Space Measurement System. The estimation conducts for 1.55GHz to 6.5GHz frequency range including wireless LAN frequency, and the complex relative permittivity of the 9 kinds of interior building materials, including plaster boards and fiber reinforced cement boards are presented. The results of the work have good agreement to the previous works. The variation for frequency and samples of the complex relative permittivity also conduct for these samples. It is very important to estimate electrical constant including complex relative permittivity accurately for the efficient discussion of electromagnetic compatibility inside and/or around building. Various methods have been reported for the determination of the complex relative permittivity of non-magnetic materials, such as Resonator Method, Waveguide Method and Free Space Method. However, few works conduct for building materials, especially interior building materials. So, we estimate the complex relative permittivity for interior building materials by using four measurements of the magnitude of the reflection coefficient from the Free Space Measurement System and iteration calculation. Fig.1 shows the outline of the measurement system. Before the estimation of the complex relative permittivity, we confirm the effectiveness of the measurement system by using acrylic plate (Fig.2). Four measurements are follows, backed by a metal plate from front side (front-short), samples only from front side (front-open), samples backed by a metal plate from back side (back-short), and samples only from back side (back-open). The calculation starts to put above four measurements data and the calculated reflection coefficient by initial guess complex relative permittivity into equation (9). The calculated reflection coefficient can be conducted by using equation (1) to (8) for open and short measurement. The iteration calculation perform to minimize the value of Δ|Γ| under the certain value of the standard deviation for the complex relative permittivity, and the target complex relative permittivity should be obtained when the value of Δ|Γ| achieve to minimum. The measurement of the reflection coefficient performs for 1GHz to 13.5GHz, 501 points, 25MHz steps, and the iteration calculation for 1.55GHz to 6.5GHz, 100 points, 50MHz steps in this work. Table 1 shows samples for the estimation (9 kinds of materials and 23 samples). These samples put into standard humidifier (temperature is 20℃ and humidity is 60%) in certain days to make samples homogeneous with respect to water content. Fig.3, Fig.4 and Fig.5 show the frequency distribution of the complex relative permittivity for Fiber Reinforced Cement Boards, Plaster Boards, Sound Absorbing Boards, and Woods. The real part of the complex relative permittivity decreases for the higher frequency for all samples, and the imaginary part of the complex relative permittivity for Fiber Reinforced Cement Boards increases for the higher frequency. These results have good agreement to the work done by Rhim for the measurement of mortal and concrete from 1GHz to 20GHz and Chiba for the simulation of the concrete. And the estimated complex relative permittivity for Plaster Boards are agree to the value presented by Hashimoto by using Free Space Transmission Method. Table 2 shows the maximum frequency variation of the complex relative permittivity (the ratio to the average value) for 1.55GHz to 6.5GHz. The variation increases for the lower density. The value of the frequency variation of Fiber Reinforced Cement Board Type I and II is about 5% for the real part of the complex relative permittivity and 6-10% for the imaginary part of th

Accurate phase measurement of passive non-reciprocal quasi-optical components

A method for accurate measurement of the phase of passive non-reciprocal quasi-optical components is presented. The measurements are performed using a two-port free-space measurement system that is composed of two horn-fed axially aligned lenses calibrated using a through-reflect-match (TRM) technique. The proposed method for accurate phase measurement consists in measuring the component twice, i.e. once facing the first port and then facing the second port. A simple relationship is derived between the phase at the desired location, recorded phase values and the phase due to the thickness of the sample. The method was verified by measuring two different structures at Ka band, i.e. a polarisation-sensitive frequency-selective surface (FSS) and a multilayer isotropic dielectric sample. Another method requiring two identical components is used to measure the phase of the reflection coefficient based on a Fabry-Perot interferometer (FPI) architecture. Neither time-domain gating nor data smoothing were applied to the results. The S-parameter magnitude and phase results are in good agreement with simulations.

An estimate of the error caused by the plane-wave approximation in free-space dielectric measurement systems

In a free-space measurement system, the electrical constitutive parameters of a plane sheet of material are determined using incident, transmitted and reflected electromagnetic beams. The electrical parameters are almost always obtained from the measured transmission and reflection coefficients using a procedure that approximates each beam as a single plane wave whose wavefront is parallel to the surfaces of the sheet (the plane-wave approximation). A theoretical model is constructed to investigate, in a very general way, the limitations of the plane-wave approximation. The field of the incident beam is taken to be a Gaussian function at the center of the sheet and analytic expressions are obtained for the transmitted and reflected beams using the plane-wave spectrum technique. Power transmission and reflection coefficients are calculated and series expansions of these coefficients are used to determine under what conditions the plane-wave approximation applies. Graphical results are presented to show how the error associated with the plane-wave approximation depends on the various parameters for the problem: electrical size of the beam waist, electrical thickness of the sheet etc.

Designand development of electromagnetic absorbers with carbon fiber composites and matching dielectric layers

Radar absorbing materials are designed and developedwith carbon fibers and suitable matching layers. Complexpermittivities of carbon fiber composite are predicted on thebasis that the modulus of permittivity obeys a logarithmic lawof mixtures and the dielectric loss tangents are related through a linear law of mixtures. Linear regression analysisperformed on the data points provides the constants which areused to predict the effective permittivities of carbon fibercomposite at different frequencies. Using the free spacemeasurement system, complex permittivities of the lossy dielectric at different frequencies are obtained. These complex permittivities are used to predict the reflectivity of a thinlossy dielectric layer on carbon fiber composite substrate. Thepredicted results agree quite well with the measured data. Itis interesting to note that the thin lossy dielectric layer,about 0.03 mm thick, has helped to reduce the reflectivity ofthe 5.2 mm thick carbon fiber composite considerably.

Nondestructive Permittivity Profile Retrieval of Non-Planar Objects by Free Space Microwave Techniques

In this paper, a nondestructive method for the non-contact characterization of non-planar dielectric objects is presented. Traditionally, this kind of measurement is achieved by using spot-focused antennas associated to a vector network analyzer. In this study, we propose a free space measurement system, fitted out with a classical horn antenna, which demonstrates its usefulness in many practical cases. The permittivity has been computed from the measurement of the reflection coefficient of metal-backed samples of different shapes at 2.45 GHz. Signal processing tools, based on deconvolution techniques, are developed to enhance the accuracy of the permittivity measurement.

In situ microwave characterization of nonplanar dielectric objects

In this paper, a novel experimental solution is presented for the nondestructive, noncontact, and in situ characterization of dielectric objects of curved shape using a spot-focused freespace measurement system. Measurements were made on Plexiglas and glass samples of cylindrical shape with different radii of curvature, and the complex permittivities were computed from the measured S/sub 21/ and S/sub 12/. Comparing the results with planar samples shows that the curvature does not significantly affect the accuracy of the measured permittivity of cylindrical surfaces if the radii of curvature are large compared to the size of the focusing spot. Results for a number of curved samples agree with the published data and this demonstrates the usefulness of a spot-focused free-space system for in situ characterization and evaluation of materials and complex structures during processing and fabrication. The other benefit of this approach is the noncontact nature of the method, which permits measurement of solids and liquids in high/low-temperature environments. The spot-focused beam permits characterization of small or large samples.

A broadband free-space dielectric properties measurement system at millimeter wavelengths

A free-space system operating in the 75 GHz to 95 GHz range for measurement of the complex permittivity of matter is presented. The complex transmission coefficient of a parallel polarized quasiplane wave through a planar sample is determined for different incidence angles. The quasiplane wave is realized by a collimated Gaussian beam. The complex permittivity is subsequently calculated from the measured data by applying a nonlinear least-squares method. Errors induced by multiple reflections between the transmitting and receiving horn antennas are removed by a time-domain gating technique. The design of the quasioptical measurement setup is based on Gaussian beam optics. A dielectric lens collimates the beam of the transmitting horn antenna for minimization of diffraction effects at the edges of the sample. In order to verify the Gaussian beam and thin lens approximations at millimeter wavelengths, the far-field pattern of the horn antenna and the dielectric lens system has been measured. The performance of the measurement setup has been tested using glass samples.

High power and narrow lateral far-field divergence 1.5-?m eye-safe pulsed laser diodes with flared waveguide

We fabricated high power and narrow far-field angle, 1.5 /spl mu/m eye-safe pulsed lasers with a flared ridge waveguide. High power (9.6 W) with narrow lateral far field divergence (3-4/spl deg/), have been obtained. These devices will be applicable for free-space optical measurement systems as "eye-safe" light sources. Additionally, by decreasing perpendicular far field divergence, it is easy to collimate the output beam.

Magnetic field measurements using magneto-optic Kerr effect sensors

A novel optically based magnetic field sensing technique is developed that uses a thin magnetic film as the sensing element and the longitudinal and transverse magneto-optic Kerr effects in an optical read technique. The linear system response is tested for small magnetic fields using a simple free-space ellipsometry measurement system. Measurements taken at 8 mHz showed a magnetic field sensitivity of 4 x 10^-3 Oe, with a system dynamic range of over 60 dB. The noise spectrum was dominated by 1/<i>f</i> noise. Calculations for a shot-noise-limited measurement system indicate that a magnetic field sensitivity of 10^-6 Oe/Hz^1/2 may be possible through enhancement of the sensing element parameters and 100-mW light sources.

A broadband free-space millimeter-wave vector transmission measurement system

We report both broadband monolithic transmitter and receiver IC's for MM-wave electromagnetic measurements. The IC's use a nonlinear transmission line (NLTL) and a sampling circuit as a picosecond pulse generator and detector. The pulses are radiated and received by planar monolithic bow-tie antennas, collimated with silicon substrate lenses and off-axis parabolic reflectors. Through Fourier transformation of the received pulse, accurate 30-250 GHz free space gain-frequency and phase-frequency measurements are demonstrated. Systems design considerations are discussed, and a variety of MM-wave broadband transmission measurements are demonstrated. >

A free-space bistatic calibration technique for the measurement of parallel and perpendicular reflection coefficients of planar samples

A free-space bistatic measurement system suitable for operation in the frequency range of 5.85-40 GHz is calibrated to measure the parallel and perpendicular reflection coefficients of metal-backed planar samples for obliquely incident waves. The measurement system consists of transmit and receive antennas in the bistatic configuration, mode transitions, precision coaxial cables, and the network analyzer. Diffraction effects of the edges of the sample are minimized by using spot-focusing horn lens antennas, which focus most of the energy on a one-wavelength-diameter circular section of the sample. A new free-space bistatic calibration technique is developed to eliminate errors due to multiple reflections between transmit and receive antennas via the surface of the sample. The effect of defocusing due to the obliquely incident plane wave with focused antennas is minimized by introducing correction factors which modify measured reflection coefficients. Details of the calibration procedure and a discussion of the experimental results obtained for planar samples of Teflon and Eccogel 1365-90 in the frequency range 12.4-18 GHz are presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Free-space, broadband measurements of high-temperature, complex dielectric properties at microwave frequencies

A free-space microwave measurement system that is used for the high-temperature measurement of dielectric constants and loss tangents of homogeneous materials and that is applicable to composite materials as well is discussed. The system is capable of operating in the 5.85-40-GHz frequency range and ambient to 850 degrees C temperature range. A computer is used to control and coordinate furnace temperature, network analyzer functions, and data storage. Dielectric constants and loss tangents of the materials are calculated from the measured values of S/sub 21/. The measurement system, including the high-temperature furnace and the calibration technique, is described. Dielectric constants and loss tangents are presented for fused quartz and boron nitride grade HP in the frequency range 13.0-17.4 GHz and the temperature range from ambient to 850 degrees C. >