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Microwave Hyperthermia has been used successfully in non-invasive treatment of tumors on the surface or just below the surface of the human body. The precise focusing of beams in the near-field zone of the applicator is quite complex, and conformal antennas, owing to their flexible contour, can blend with the local body surface area. This paper details experimental studies on a K band waveguide array (18-26 Ghz), operating in different geometries: 1-dimensional linear arc array, 2- dimensional planar array or 3-dimensional spherical volumetric array. The array has N directing waveguides (2 = N = 8) and one focusing waveguide, and conformal shape can be achieved by movement of primarily the focusing element. This paper presents both linear z-axis near-field measurement, and planar x-y axis measurement. In this particular application, the primary motivation is to get a focused beam in the near-field, and to control the axial movement of the beam by the position of the focusing element of the array.
New taper chamber feed section was created for numerical analysis. To launch the undisturbed electromagnetic wave into the test zone, newly designed dual polarized aperture-matched blade mode bowtie (ABB) antenna was designed and implemented at the vertex of the feed section of the tapered chamber. For the accurate calculation, wall type absorber samples are obtained and measured. These values are included for realistic configurations. From the simulated time domain result, field distributions at the aperture of the feed sections are investigated. Determination of the usable spaces for different frequencies is discussed. Also, cross-talk levels are presented since the feed antenna designed for dual polarization.
In this paper, the electromagnetic (EM) characteristics of various antennas implanted in both the human head and the human body are analyzed for biomedical applications such as hyperthermia and biotelemetry. The implanted antennas are studied in two ways: the near- and far-field patterns of the antenna are calculated and the potential effects on the human body are observed. To ensure the correctness of the results, we apply two simulation methodologies: dyadic Green’s function (DGF) expansions and finite difference time domain (FDTD). We characterize the performances of the low profile antennas designed for biomedical applications in terms of specific absorption rate (SAR), radiation patterns, maximum available power and safety issues. These results should also provide a good basis for validating the results of experimental data.
The Air Force Research Laboratory (AFRL) Materials Measurement Laboratory (MML) is a state of the art facility for the characterization of the electromagnetic properties of materials at radio frequencies. The two-fold mission of the MML is to provide material characterization services to AFRL and to conduct R&D to develop or improve RF material characterization technology. The goal of the MML is to perform—or develop the ability to perform—material property measurements to the highest degree of accuracy possible with state of the art test equipment. Characterization measurements range from determination of RF reflection or transmission loss to the extraction of the dielectric permittivity and magnetic permeability of material samples. The MML has the ability to characterize material samples from below 100 MHz to above 18 GHz over material test sample temperatures ranging from – 150oC to greater than 1000oC. While maintaining capabilities using ‘standard’ material measurement techniques (circular coax and rectangular waveguide), the MML’s most highly utilized system is based on the GTRI focused arch apparatus. The MML also employs resonant cavity fixtures, open-ended coax probes and impedance meters to provide a capability to evaluate material samples of a wide variety of shapes and sizes.
The microwave slotted line is an accurate single frequency instrument for determining the dielectric properties of materials. The broadband application of materials requires accurate dielectric characterization across wide bands, which is a labor intensive procedure with a manually operated slotted line. The subject effort improves upon the manual slotted line dielectric measurement procedure by incorporating a steppermotor drive mechanism and automated measurement software with the standard slotted line carriage. A comparison of results obtained with the automated slotted line is made with those derived using a network analyzer system.
Measurement of material properties in the radio frequency (RF) regime, including microwave and millimeter-wave frequencies, is a critical issue for RF polymer research and development. RF polymers are polymer composites specifically designed for specified electrical and magnetic properties within a required frequency band of operation. In this paper, we investigate the methods required to determine the permittivity and permeability in the RF band for RF polymers.
The microwave characterisation of the electromagnetic parameters of lossy materials is an essential part of the work of the BAE SYSTEMS Advanced Technology Centre, Stealth Materials Group at Towcester (UK). The electromagnetic parameters of lossy materials change rapidly with frequency below 5GHz, therefore for stealth applications it is vitally important to be able to characterise materials at these frequencies. This paper describes a unique quasi-optical free space focused beam system for the measurement of microwave electromagnetic material parameters. The system employs two spherical reflectors which are illuminated from the side by gaussian beam forming antennas. The frequency range of 1.7GHz to 5.8GHz is covered in three bands with three pairs of corrugated feed antennas. An advantage of this system is that a parallel beam is formed between the reflectors whose beam waist diameter (or illumination area) is essentially the same across each frequency band. The measurements from the system are taken using a vector network analyser under computer control. The parallel beam enables a “Through, Reflect, Line” calibration technique to be used. After calibration the sample under test is placed in the beam (mid way between the reflectors) and the four microwave ‘S’ parameters are recorded automatically in complex form. The permittivity, permeability or lumped admittance ( if the sample is very thin
A Technique is presented that allows for broadband nondestructive material electrical parameter measurements. Electrical parameters of a large number of materials are not readily available over extremely broad bandwidths (multiple octaves as an example). This information is required for accurate modeling of microwave circuits and antenna(s). These parameters consist of complex permittivity and complex permeability that result in loss due to the types and thickness of materials to be used. A Method is required that allows for fast, accurate and low cost measurements of the materials under test. The technique of using dual coaxial probes provides a solution that can be applied to numerous materials including thin films. It takes advantage of the full frequency extent of the network analyzer. This measurement uses dual coaxial probes, as compared to the implementation of cavity resonators, coaxial lines, waveguides and free space measurements, and performs the measurement in a 2-port calibration procedure. The resultant analytical solution is a transcendental equation with complex arguments. The Coaxial probes are described and can be easily made with available components where the only limitation is the valid component frequency bandwidth. Several material examples show the expected accuracy versus frequency range of this measurement technique.
Microwave measurement of intrinsic material properties can be performed with transmission-line fixtures such as waveguides or free-space focused beams. However, analyses of measured data usually assume idealized sample geometries. In this paper, Finite Difference Time Domain (FDTD) calculations are used to study the systematic error from non-ideal geometries, in free-space and waveguide measurements of impedance sheets. Analytical models of these errors are developed. FDTD analysis can be used to numerically invert intrinsic material properties from measured freespace transmission coefficients. The focused beam is simulated in FDTD with a sum of weighted plane waves with a Gaussian spectral distribution. The transmission coefficient is predicted by propagating the focused beam through a material slab or sheet; and the dielectric or impedance properties are derived from the transmission coefficient. The focused beam diameter is preferably several wavelengths, which requires large sample size (>1 square meter) at low frequencies. A modified focused beam technique is described that incorporates a finite aperture in a metal groundplane to measure samples with reduced dimensions, even at low frequencies. Calculations are compared to laboratory measurements. FDTD calculations are also applied to study the effect of gaps in waveguide fixtures, since gap and edge effects in both waveguide or free-space aperture fixtures contribute to measurement error.
Electromagnetic material characterization is the process of determining the complex permittivity and permeability of a material. Rectangular waveguide measurements involving frequencies greater than several gigahertz require only a relatively small test sample. In an X-Band (8-12 GHz) waveguide, for example, sample dimensions in the crosssectional plane are only 0.9 by 0.4 inches. However, for lower-frequency applications waveguide dimensions become progressively larger. Consequently, larger quantities of materials are required leading to possible sample fabrication difficulties. Under these circumstances, a waveguide sample holder having a reduced aperture may be utilized to reduce the time and cost spent producing large precision samples. This type of holder, however, will cause a disruption in the waveguide-wall surface currents, resulting in the excitation of higher-order modes. This paper will demonstrate how these higher-order modes can be accommodated using a modal analysis technique, thus resulting in the ability to measure smaller samples mounted in large waveguides and still determine the constitutive parameters of the materials at the desired frequencies.
In a previous AMTA paper [1], we presented a firstprinciples algorithm called wavenumber migration (WM) for estimating a target’s far-field RCS and/or far-field images from extreme near-field linear (1-D) or planar (2-D) SAR measurements, such as those collected for flight-line diagnostics of aircraft signatures. However, the algorithm assumes the radar antenna has a uniform, isotropic pattern on both transmit and receive. In this paper, we describe a modification to the (1-D) linear SAR WM algorithm that compensates for nonuniform antenna pattern effects. We also introduce two variants to the algorithm that eliminate certain computational steps and lead to more efficient implementations. The effectiveness of the pattern compensation is demonstrated for all three versions of the algorithm in both the RCS and the image domains using simulated data from arrays of simple point scatterers.
The task of performing reliable RCS measurements in complex environments under near-field conditions is gaining more and more interest, mainly for a rapid assessment of RADAR performance of constructive details. This paper describes a low-cost compact measurement system fully developed by IDS, that allows fast and effective acquisition of diagnostic images under nearfield conditions and far-field RCS estimation in a nonanechoic environment. The hardware of the system is composed of a planar scanner, two horn antennas, a Vector Network Analyzer and a computer. The two axes scanner allows 2D scanning of antennas in a vertical plane. For each point of a predefined grid along the scanned area, the Analyzer performs a frequency scan. The acquisition software synchronizes scanner movements with data acquisition, transfer and storage on the computer’s HDD. The software has post-processing capabilities as well. A number of focusing algorithms permit to produce 2D and 3D diagnostic images of the target as well as 2D backprojection. It is moreover possible to reconstruct the RCS starting from near-field images. Along with system features, a summary of performances and some simple targets images are presented.
Monostatic backscatter measurements made in the near-field have been used to generate high resolution images of complex targets; however, the appropriate use of this data for obtaining far-field RCS values needed further examination. In this paper we comment on some of the available methods, and discuss in some more detail the concept that Fourier Transform of monostatic backscatter data collected over a planar array indeed provides samples in Fourier Space directly.
A fully-functional Fresnel Zone (FZ) antenna was designed and measured using PO simulation programs and the bi-polar near-field facility. The results from these measurements and simulation are presented in this paper. First, a detailed description of an FZ antenna and its operation is given. Then, a discussion of the design and construction procedure for both the FZ antenna and supporting structure is included. The resulting far-field pattern, near-field plots, and holographic images are shown in this paper. The antenna was measured with different feed positions to observe how it affects the overall antenna performance.
Sub-millimeter wave holograms can be used as collimating elements in compact antenna test ranges. The fabrication of very large holograms can be facilitated using a modified hologram illumination with amplitude taper. The modified illumination also removes the current polarization limitation to a vertical polarization in the hologram operation. Shaped beam illuminating the hologram is achieved with a dual reflector feed system with two shaped hyperbolic reflectors. In this paper, the design of the quasi-optical reflector feed system with developed ray-tracing based reflector synthesis procedure is described. Simulation and measurement results of the dual reflector feed beam at 310 GHz are presented. The measured quietzone of a demonstration hologram fed with the dual reflector feed system is also shown.
This paper will discuss the capability of spherical near-field test ranges using probe arrays to measure electrically large directive antennas. More particularly, the operating range of the existing Stargate equipment in terms of antenna dimensions has been identified. The advantages of using such equipment to measure quasi isotropic antennas will be first reminded. Then a study that aims to give a limit of the dimensions of the antenna under test will be presented. The sources of error that contribute to the limits will be described. Finally it will shown how an extension of Stargate equipment can be implemented in order to increase its the capability to the measurement of directive antennas, i.e. largest in dimension. This paper will be illustrated with real measurements of directive antennas. A comparison with both probe array near-field scanner and a far-field test range will be commented.
The National Institute of Standards and Technology (NIST) characterized three phase retardation plates for the National Oceanic & Atmospheric Administration (NOAA). These plates are used to produce known polarized signals needed to calibrate polarimetric radiometers. The plates were tested at 10.7 and/or 18.7 GHz. The plates produce a phase shift between perpendicular field components of transmitted waves. The planar nearfield measurement technique was used to determine the phase shift. This paper will discuss the measurement procedure and results.
A new spherical near-field probe positioning device has been designed and constructed consisting of a large 5.0 meter fixed arc. This arc has been installed in a near-field test facility located at Alenia Marconi Systems on the Isle of Wight, UK. As part of the nearfield qualification, testing was performed on a ground based radar antenna. The resultant patterns were compared against measurements collected on the same antenna on a large outdoor cylindrical near-field test facility also located on the Isle of Wight [1]. These measurements included multiple frequency measurements and multiple pattern comparisons. This paper summarizes the results obtained as part of the measurement program and includes discussions on the error budgets for the two ranges along with a discussion on the mutual error budget between the two ranges.
Modern radome measurements often involve scanning the radome in front of its antenna while the antenna is actively tracking an RF signal. Beam deflections caused by the radome are automatically tracked by the antenna and its associated positioning system, which is typically a two-axis (pitch & yaw) gimbal. The motion required to accurately track the beam can be very demanding of the gimbal. High structural stiffness, zero drivetrain backlash, and extremely accurate angle measurement are all necessary qualities for radome beam deflection measurement. This paper describes a new, advanced, two-axis gimbal that embodies those qualities. The new gimbal incorporates direct-drive motors to achieve zero backlash. The motors are mounted directly to the rotating gimbal elements, thereby eliminating the usual causes of drivetrain compliance. Rated torque of the motors is not high, and the antenna is therefore fully counterweighted. Each of two optical encoders is mounted on the same rotating gimbal element as its associated motor. The encoders are directly mounted; no flexible coupling is used. The antenna is mounted to those same rotating elements. Antenna positioning error due to windup of the structure and drivetrain is virtually eliminated. Eccentricity of the encoder disk, which is the primary source of direct-drive encoder errors, is adjusted by virtue of a remarkable in situ process.
A technique to identify failures in an antenna array using corporate feeding is presented. This technique combines near-field imaging close to the radiating elements and the printed transmission lines forming the feeding network. Planar near-field probing is done with a loop and a miniature dipole probe less than 0.1 free-space wavelength above the antenna under test. Two cases are considered, one where the feed lines would be separated from the radiating elements by a metal ground plane and another where the lines and the elements are on the same metal layer. In the first case, precise diagnostic based on extraction of vectorial forward and reverse wave complex coefficients on each line segment is possible. In the second case, it is not possible to extract these coefficients. However, defect localization is still possible by relying on symmetries present in the near-field maps of selected line segments.