Welcome to the AMTA paper archive. Select a category, publication date or search by author.
(Note: Papers will always be listed by categories. To see ALL of the papers meeting your search criteria select the "AMTA Paper Archive" category after performing your search.)
The Air Force Research Laboratory (AFRL), Information Directorate, has served as an Air Force center for antenna measurements for over thirty years. AFRL's Newport Research Facility consists of multiple far field outdoor test ranges with 3-axis positioner towers. The range supports a wide variety of test activities including measurements on simple antennas, complex active phased arrays, avionics, communications and electronic countermeasure systems. The trend towards increasingly complex antenna systems led to a requirement for a faster, more adaptable data acquisition system. To support the changing test requirements, AFRL developed a multi-port 16-RF-switch controller as part of our data acquisition system. In a typical antenna test at Newport, multiple aircraft antennas are multiplexed into a single RF output through a programmable matrix of solid state RF switches. The switch controller is installed inside the aircraft under test which is mounted on a 3-axis positioner. We can then configure and reconfigure each aircraft for a variety of antenna tests at Newport.
Measurement of E and H plane far field patterns for an open-ended rectangular waveguide in the free air operating between the frequencies of 16 and 19 GHz are shown and compared with the simulated patterns derived by several authors. Although the theoretical expressions give a broader pattern for the E-plane than for the H-plane, which is observed, measurements exhibit a sharper decay in the E-plane than the one obtained by simulation. In this work, we calculate the errors associated with the use of the different models that fail to correctly approximate the E-plane. Finally, we introduce a parameter in the best model to adjust the effective aperture dimensions in order to obtain a more realistic representation of the measured far field.
A technique is proposed to measure the permittivity and permeability parameters of a sample of biaxial material placed into a rectangular waveguide. By constructing the material as a cube, only a single sample is required to find all six material parameters. The sample is inserted into the waveguide in three different orientations, and the transmission and reflection coefficients of the sample region are measured using a vector network analyzer. The material parameters are then found by equating the measured S-parameters to those determined theoretically using a mode-matching technique. The theoretical details are outlined and the extraction process is described. Comparison of the mode-matching S-parameters with those obtained using the commercial finite element solver HFSS validates the theory.
Ground to air imaging poses numerous technical challenges, a number of which relate to target motion throughout its inverse synthetic aperture. A well-tracked target benefits not only its illumination, but provides an accurate description of the target’s position as a function of time. Tracking may be accomplished using a monopulse tracking radar or precision GPS/telemetry techniques; either of which are sufficiently accurate for coarse translational motion compensation. However, without target attitude telemetry, the inverse synthetic aperture may still be inferred from the target’s spherical coordinates and their first derivatives, and coarse rotational motion compensation may be performed. Further refinement is available from the in-phase/quadrature data. Residual translational motion may be characterized and corrected by considering both intra-chirp (i.e., within stepped chirps) and inter-chirp phase migration, accommodating fine translational motion compensation. The data become reasonably bandlimited, allowing rotational motion compensation to be performed via bandlimited resampling, yielding a focused ISAR image.
All of us involved with antenna measurements or radar cross section measurements are familiar with the absorber seen on the walls, ceiling, and floor of anechoic chambers. It helps simulate free-space conditions. It comes in various shapes and lengths, and it reduces the reflections, or unwanted energy, from encroaching on the quiet zone. But what makes one absorber better than another? Further, what advances in composition have been made over the last 50 years to improve the simulation of free space? This paper will address differences in geometry and differences in materials and “ingredients” for optimizing performance. Also, it will discuss the advantages in using different materials to create stronger absorber to help maintain performance and for creating clean and safe environments, for such endeavors as measurements involving flight hardware.
The use of adaptive acquisition techniques to reduce the overall test time in planar near-field antenna measurements was presented in [1] & [2]. In those publications the concept of a decision function to track the uncertainty of a measurement as the data acquisition proceeds and also to adapt the acquisition region dynamically, was introduced. In this publication we build upon that work and present the concept of near-field array initialization. This is tested on different antennas and simulation results are presented. We also present actual measurement results to validate simulations that have to date been used to demonstrate advantages of the adaptive techniques.
A Plane Wave Synthesis Approach for mitigating errors in antenna measurements caused by stray signals and imperfections in the measurement range illuminating fields has been demonstrated previously for a 2D scalar source [1]. This paper presents algorithms developed for the Range Transfer Function (RTF) method for a 2D vector source. Vector basis functions for both the field representation and the AUT representation are implemented to provide a robust numerical solution. The new algorithms are more stable because the plane wave angles and the antenna measurement angles may be completely general, provided that Nyquist rules of sampling are observed during both the field probing (to obtain the plane wave coefficients) and the antenna measurement (to obtain the raw pattern data).
This paper addresses the difficulties in measuring the broadband complex permeability of thin films using conventional stripline or microstrip permeameters and outlines a novel methodology to solve them. It is shown using full-wave simulations that several of the conventional assumptions made for extracting permeability from a permeameter are not justified. In particular, the proportionality factor used to relate the measured effective permeability to the actual film permeability is shown not to be a constant. Another typical drawback is the need for a known reference sample for calibration. By exploiting the analyticity of the function relating effective to true permeability we have come up with a general methodology to derive this proportionality function for permeameters free of the problems mentioned before. The validity of the method is confirmed with fullwave simulations. Moreover, this general approach can be applied to other similar test devices. A key issue in measuring a thin film’s permeability over a broadband frequency range is assuming that its permittivity is known. More often than not, this data is not available. We show a method to extract a film’s permeability without the need to assume or know its permittivity value. This is done measuring two identical films of equal widths.
As a novel concept for field probes, RCS measurements on long rigid objects rotated within a small angular range about the broadside condition are reported. The rotation was maintained either in a horizontal (H) plane or in a vertical (V) plane containing the center of the quiet-zone (QZ). Processing the RCS data by DFT yields a spectrum which is recognized as the field distribution along that object. This spectrum compares extremely well to traditional field-probes taken earlier by translating a sphere across the QZ in H- or V-direction. Preliminary results at several S-band frequencies are presented and discussed.
Traditionally Taper chambers are constructed using a square based pyramidal shaped taper. The taper is then shaped into an octagon and finally transformed into a cylindrical launch section. This approach is related to the manufacturability of different absorber cuts. This presentation introduces a chamber where the conical shape of the launch in continued through the entire length of the taper chamber. The results are of the free space VSWR test over a 1.5m diameter quiet zone are presented at different frequencies. The conical taper appears to have a better illumination wave front and better QZ levels even at frequencies above 2GHz than the standard traditional approach. The implementation of this design was done in Singapore and the actual chamber was designed to have a secondary Near Field range for Planar and spherical scans.
An adaptive approach for optimized sampling in cylindrical and spherical near-field antenna measurements is described. The presented technique applies higher sampling density in rapidly varying near-field regions and skips data points in the smoother regions. Abrupt changes in the near field are detected by comparing the extrapolated and the measured near-field values at coarser sampling points during the measurements. A decision function based on signal-to-noise ratio of the measured value is used to determine the threshold difference between the extrapolated and the measured near-field value for skipping the sampling point. The reduced near-field data collected is processed using the fast irregular antenna field transformation algorithm (FIAFTA). FIAFTA is computationally efficient and capable of handling data on irregular grids with full probe correction. Several test cases are then presented on the applicability of the given approach and significant reduction in the number of measurement points is observed, thereby reducing measurement time and the computational effort.
Far-field uncertainty due to probe errors in planar near-field measurements is analyzed for the fast irregular antenna field transformation algorithm. Results are compared with the classical technique employing two dimensional Fast Fourier Transform (2D FFT). Errors involving probe's relative pattern, alignment, transverse and longitudinal position, interaction with AUT etc. have been considered for planar measurements. The multiple reflections error originating from the interaction of the probe and the AUT tends to deteriorate the radiation pattern to a greater extent. Therefore, a novel technique which utilizes near-field measurements on two partial planes is presented to reduce the multiple reflections between the probe and the AUT.
In this paper, we present an overview of UHF RFID tag performance characterization. We review the link budget of RFID system, explain different tag performance characteristics, and describe various testing methods. We also review state-of-the art test systems present on the market today.
This paper describes the development of a method based on measurements of the radar cross section (RCS) in the forward direction to determine the extinction cross section for the 2.5-38GHz frequency range using the optical theorem. Forward RCS measurements are technically complicated due to that the direct signal has to be subtracted from the total signal at the receiving antenna in order to extract the forward RCS. The efficiency of this subtraction as a function of time is evaluated. A traditional calibration method using a calibration target and a second method that does not require a calibration target are investigated and compared. The accuracy of the forward RCS measurements is determined using small spheres of different sizes. The spheres have a forward RCS that is straightforward to calculate with good accuracy. The method is also extended to polarimetric measurements on a small helix that are compared to theoretical calculations.
This paper details a recent advance that, for the first time, enables the Mathematical Absorber Reflection Suppression (MARS) technique to be successfully deployed to correct measurements taken using plane-polar near-field antenna test systems with reduced AUT-to-probe separation. This paper provides an overview of the measurement, transformation, and post-processing. Preliminary results of range measurements are presented and discussed that illustrate the success of the new planepolar MARS technique by utilising redundancy within the near-field measured data that enables comparisons to be obtained and verified by using two existing, alternative, scattering suppression methodologies.
This paper presents a comparative investigation of two versatile error mitigation techniques applicable to general antenna near field measurement scenarios with echo signals of unknown origin. Both techniques are based on spatial filtering of the measured field taking advantage of the apriori knowledge of the antenna size. The first approach takes advantages of the spatial filtering properties of the spherical waves expansion of the measured field. The second approach is based on the reconstruction of equivalent currents and implements the spatial filtering as a direct consequence of the selected size and shape of the reconstruction surface. The investigation is performed using measured data on two different horns in both planar and spherical near field scanning geometries. The presence and levels of echo pollution in the measurements are controlled by introducing known scattering objects in the anechoic chambers and comparing to reference situations without disturbance.
Spiral antennas have been well established as good radiators of circularly polarized radiation that are capable of achieving very large bandwidths. Though traditionally used for receiving applications, this paper will show that printed spiral antennas are also capable of performing as high-power radiators. Several printed 4-armed spiral antennas are presented, along with measured data that attest to their ability to handle hundreds of watts of continuous wave (CW) power at microwave frequencies. This high-power data includes temperature, electric field, and return loss readings recorded during the tests. Such high power performance is achieved through the use of a high-power capable substrate, lossless cavity, multi-arming, and applying a dual-layering technique which serves to reduce the current density and improve the spiral antenna’s match to 50O. Radiation and impedance measurements are taken to fully verify wideband performance. Analysis of the current densities from simulations is also presented. Data from the high-power tests indicate that the chief factors limiting the spirals’ power handling are their beamformers and resistive terminations.
The goal of this project was to design and test a UWB S21 measurement system for less than $2,000. We use a synthesized source and a coherent demodulator. The bandwidth extends from 0.5 to 4.5 GHz with frequency steps = 100 MHz. The selected synthesized source is a Windfreak SynthNV module based on the Analog Devices wideband fractional-N synthesizer chip. This chip can output signals in the 137-4,400 MHz range. It has enabled a number of very low cost modules to be developed, and the selected module is a USB controlled and powered synthesizer. The I/Q mixer is a Polyphase Microwave quadrature demodulator with a bandwidth from 0.5 to 4.0 GHz with built in LO amplifier and I/Q low pass filters. We will show the design, performance parameters and cost of this radar and show results of the use of this radar to detect and characterize the human heartbeat.
This work concerns the experimental validation of a probe compensated near-field – far-field transformation technique using a spherical spiral scanning, which allows one to significantly reduce the measurement time by means of continuous and synchronized movements of the positioning systems of the probe and antenna under test. Such a technique relies on the nonredundant sampling representations of the electromagnetic fields and makes use of a two-dimensional optimal sampling interpolation formula to recover the near-field data needed to perform the classical spherical near-field – far-field transformation. The good agreement between the so reconstructed far-field patterns and those obtained via the classical spherical near-field – far-field transformation assesses the effectiveness of the approach.
In addition to steady state performance, antennas also have transient responses that need to be characterized. As antennas become more complex, such as active phased arrays, the transient responses of the antennas also become more complex. Transient responses are a function of internal antenna interactions such as coupling and VSWR, active circuitry, and components such as phase shifters and attenuators. This paper will show techniques for measuring antenna transient responses. The first measurements utilize standard instrumentation capable of sampling at up to 4 MHz, giving 250 nS time resolution of the transient effect. Recognizing that some transient measurements require finer time resolution, a higher sampling rate prototype receiver was developed with 1 nS time resolution. After verification of its performance, the prototype receiver was used to measure the transient effects of a 50 nS pulse through a broadband antenna. The spectrum of the pulse yields information on the time and frequency domain responses of the antenna. Phased arrays may exhibit transient signals when switching between beam directions as well as switching between frequencies. The methods presented in this paper are applicable to both.