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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.
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.
As part of a target simulator [1], a linearly polarized signal was required with a variable tilt angle that could be controlled electronically and changed at a 1 kHz rate. However, microwave components available in the 33.4 – 36 GHz operating range were inadequate to achieve the desired performance. A novel approach was developed to downconvert the input signal to a lower frequency range and use vector modulators available in this band to produce the appropriate phase and amplitude changes in each path, then upconvert back to the desired operating frequency to drive an orthomode transducer. A calibration and measurement procedure was developed to determine the vector modulator input settings that produced the most accurate tilt angles and best cross-polarization performance. By iteratively measuring cross-polarization and tilt angle, then adjusting the vector modulator controls, a tilt angle accuracy of +/-1 degree was achieved with a crosspolarization of -25 dB, exceeding the required performance. This paper provides an overview of the concept, a block diagram of the design, discussion of the calibration and measurement procedure, and a summary of the results achieved.
This paper addresses the difficulties of achieving lower cross-polarization EM field transmission (or reception) levels by employing wideband electromagnetic polarization filters (EMPF). These EMPFs are applied as add-on screens to reduce the cross-polarization levels of standard gain horns (SGH) and diagonal horns (DH). Cross-polarization level reduction as much as 19 dB is presented for diagonal horns with add-on screens. Similarly, more than 9 dB cross-polarization reduction is shown for standard gain horns across the operational bandwidth. Later, an alteration is done on the add-on screens by making use of Ludwig's 3rd coordinate definition. This modification results in further cross-polarization suppression in the vicinity of boresight direction. For instance, on X-Band SGH, near boresight angular region where cross-polarization level keeps below -60 dB increases by 26% with use of this modified add-on screen.
Method of moments (MoM) codes have become increasingly capable and accurate for predicting the radiation and scattering from structures with dimensions up to several tens of wavelengths. In an earlier AMTA paper [1], we presented a network model (NM) algorithm that uses a Gauss-Newton iterative nonlinear estimation method in conjunction with a MoM model to estimate the “as-built” materials parameters of a target from a set of backscatter measurements. In this paper, we demonstrate how the NM algorithm, combined with the basis pursuits (BP) l1 minimization technique, can be used to locate unknown defects (dents, cracks, etc.) on a target from a limited set of RCS pattern measurements. The advantage of l1 minimization techniques such as BP is that they are capable of finding sparse solutions to underdetermined problems. As such, they reduce the requirement for a priori information regarding the location of the defects and do not require Nyquist sampling of the input pattern measurements. We will also show how the BP solutions can be used to interpolate RCS pattern data that is undersampled or has gaps.
We present exact solutions to antenna holography problems based on planar, spherical, or cylindrical nearfield data. Full field distribution information in the source region is determined exactly, from two tangential field components over a planar, spherical, or cylindrical surface. Stated in so many words, all three components of both electric and magnetic fields in the antenna aperture are obtained exactly from two-component near-field data. Conventional antenna holography relies upon back transformation for planar near-field data, and upon optimization schemes for both spherical and cylindrical near-field data. It is both acknowledged and accepted that the back transform is only an approximate solution due to its far-field nature, whereas optimization algorithms are vulnerable to convergence instability and, moreover, are computationally intensive. Our approach tackles holography by solving an inverse scattering problem, with exact solutions derived on the basis of three common types of near-field data. A mapping algorithm is proposed herein which determines the field everywhere, in both interior and exterior regions, based on a single-slice nearfield data capture. It provides exact antenna holography solutions analytically, with the full electric and magnetic fields disclosed throughout the source region. The field mapping algorithm is a direct, closed-form solution which is numerically straightforward and efficient. Verification is carried out and demonstrated by analytic examples and numerical simulations, as well as by hardware measurements. Nine test examples are given. Analytic examples include dipole arrays deployed across planar, spherical, and cylindrical regions, and a narrow azimuthal slot on a conducting sphere. The simulation example exposes the structure of a slotted array antenna based upon its near-field data as generated by a commercial software package. The hardware measurements address themselves to a concrete embodiment of that same slotted array antenna, an elongated sector antenna, and to a patch antenna. Excellent agreement is found in all test cases.
A balanced PIFA-inspired antenna design is presented for use with the 2:45 GHz ear-to-ear radio channel. The antenna is designed such that the radiated electric fields are primarily polarized normal to the surface of the head, in order to obtain a high on-body path gain (jS21 j). The antenna structure can be made conformal to the outer surface of a hearing instrument, such that the bandwidth of the antenna is optimized given the available volume. The radiation patterns, ear-to-ear path gain and available bandwidth is measured and compared to the simulated results. It is found that the antenna obtains a relatively high ear-to-ear on-body path gain, as well as a bandwidth that is large enough to cover the entire 2:45 GHz ISMband.
A fast algorithm is presented for the generation of 3D current density images of antennas utilizing arbitrary antenna measurement data. The images represent a broadband equivalent current distribution which radiates the same fields as the true current distribution on the antenna structure in a broad frequency band. These equivalent currents convey important information about the antenna. The imaging algorithm can efficiently handle arbitrary measurement geometries and probe characteristics. It is inspired by the Multi-Level Fast Multipole Method (MLFMM). The near-field compensation and probe correction is realized using a modified adjoint operator based on adaptive fast multipole translations. Due to the modifications, a priori knowledge about the field observation density can be exploited and the generated image becomes more accurate. The complexity of the proposed approach is identical to a fast Fourier transform (FFT) based imaging algorithm. Numerical examples are given to prove
Over-the-air (OTA) testing is an established technique used to measure the wireless system performance of mobile devices in addition to characterizing the antenna subsystem. 3D radiation patterns of transmit power and receive sensitivity are used to determine a figure of merit (FOM) for the transmitter and the receiver performance, i.e., Total Radiated Power (TRP) and Total Isotropic Sensitivity (TIS), respectively. For LTE, most attention is focused on the MIMO receiver chain evaluation. Discussions have been ongoing for quite some time on how OTA testing can be updated to support LTE MIMO. The MIMO OTA decomposition approach [1], [2] determines separate FOMs for the key MIMO receiver chain subsystems. The conducted MIMO test is utilizing a fading simulator to introduce dynamic fading and is used to determine a FOM for the MIMO receiver performance. The radiated test is performed without the introduction of fading profiles inside the anechoic chamber and is used to determine a FOM for the MIMO receive antenna pair. The FOM for the overall MIMO performance of the DUT is a combination of the FOMs from each test (conducted and OTA). Splitting up the MIMO wireless system performance testing into two straightforward and cost effective tests provides more information about the DUT performance than performing a complex single test. The presented approach differentiates itself from competitive MIMO OTA approaches due to its simplicity, reduced complexity, and low cost.
The Electronic Proving Ground's Antenna Test Facility at Fort Huachuca, Arizona has some of the most interesting testing structures in the world. These structures include a wooden Arc measurements system with a 23 m radius, a 30 m tower, and a compact range with an 18 m quiet zone. All of these structures are outdoors and support testing from UHF to mm frequencies on antenna systems mounted on large land and air vehicles. This paper describes the ranges supported by these structures (some of which were built in the late 1960’s) and the efforts made to keep these ranges current. This paper also describes an economical approach to arc range design which moves the arc instead of the vehicles. This paper discusses plans to build one of these systems outdoors at EPG within a limited budget.
This paper details a systematic procedure of the evolution of an electromagnetic band-gap (EBG) meta-surface from a simple ground plane. The main aspect of the paper is to understand the behavior of these EBG surfaces for low profile antenna design solutions. Reflection phase diagrams are used as a criterion to understand the flat reflection phase response of these surfaces for all angles of incidences and all polarizations. The evolution of EBG from a PEC ground plane to a via-loaded PEC ground plane to a planar patch-type surface and finally to a Mushroom-EBG surface is presented in a novel way. In addition, uniplanar compact-EBG (UC-EBG) properties are also investigated. It is observed that the EBG structures are most robust to different incident angles and polarizations making it a powerful candidate for low profile antenna design solutions. Additionally, the band-gap properties of planar patchtype, Mushroom-EBG and UC-EBG are studied and representative designs are provided.
A dual-band GPS L1/L2 (1.575/1.227 GHz) RHCP antenna is presented for small cylindrical platforms where the antenna must have small size and height. The antenna is a single-layer, dual probe-fed patch with meandered slots and is 4cm 4cm 5.08mm (?/6 ?/6 ?/50 at 1.227 GHz). The antenna was recess mounted on the circumference of various diameter metal cylinders (60-160 mm) and its performance was simulated. For all cylinders, it was found that the antenna has good gain bandwidth performance (1.2-2.7 dBi peak RHCP gain and 25 MHz 3 dB bandwidth) to receive C/A-, P(Y)-, and M-coded GPS satellite signals. Importantly, the antenna does not need to be retuned when placed on different diameter cylinders. Finally, the performance of the antenna is measured when the antenna is mounted on a 117 mm diameter cylinder. The measured performance shows very good agreement with the simulated results.
The increasing complexity and stringent performances in RF instruments and payloads often demands that the final RF functional verification is performed on the integrated satellite. In order to minimize the overall time and cost of future Antenna Integration Verification and Test campaigns (AIV/AIT) it is necessary to investigate and develop advanced test methodologies to minimize the test duration. This paper reports the preliminary results of a functional testing solutions for RF end-to-end antenna testing. The proposed approach is based on the intelligent and innovative use of existing measurement capabilities and advanced numerical modeling tools. The scope of the activity is to demonstrate through the implementation of a demonstrator and measurement on suitable hardware the possibility to achieve accurate and fast measurement results using a radical measurement under-sampling with respect to the conventional Nyquist criteria.
In this paper, a broadband EM wave absorptive structure using a periodic structure is proposed and its reflectivity characteristic examined. Based on the proposed structure, the design technique for improving absorption bandwidth of original structure is additionally proposed. Finally, the method for measuring the reflectivity of the absorber is presented, and the measured results of the absorbers, which show a broadband reflectivity response with a fractional bandwidth of approximately 76% below -10 dB and an increase of 17% of the bandwidth because of the proposed design technique, are presented.