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Mobile communication devices have many different requirements; namely they often have stringent size constraints, and must efficiently radiate over several frequencies in a myriad of different environments. Furthermore, the antenna is often electrically small or unintentionally loaded by environmental effects which cause unpredictable changes in antenna impedance. Therefore an agile matching network that is self-tuned to increase matching efficiency is desired. Most existing tuning approaches minimize reflections looking into the matching network. It will be demonstrated that this approach does not guarantee optimal performance due to circuit losses. A better approach is to also maximize power transmitted through the matching network. This paper presents a real-time frequency and impedance-agile tuning design that automatically matches a very wide load impedance range (0.5. < Re{ZL} < 1K. and -1K. < Im{ZL} < 1K.) from 200 to 400 MHz by the use of varactor diodes with impedance tuning stubs.
Planar low-profile antennas over high-impedance surfaces show improved performance compared to that over metal ground planes. Unfortunately, these high-impedance surfaces often operate over narrow bandwidths. This paper describes an approach to high-impedance surfaces which permits improved performance over a broader bandwidth. Current approaches to the design of high-impedance substrates typically employ identical unit cells with the same resonant frequency to produce high-impedance behavior over a relatively narrow frequency range. The wide bandwidth performance described in this paper derives from cells having a size and subsequent resonant frequencies that vary with position on the PMC substrate. This approach is explored through simulations using CST Microwave Studio which show the improved performance of these wideband structures.
Fig. 1: Cylindrically conformal antenna array system This paper discusses an example of a unique cylindrical conformal array design approach. This exemplary design is composed of six subarrays of series-fed microstrip patch antenna operating in X-band. The diameter of the conducting cylinder is 7.5 inches in diameter. Each subarray contains seven patch antennas connected in series configuration and is connected to a single coaxial probe located at the center element. Such feed arrangement greatly reduces the number of feeding cables, increases the feed-line isolations, and minimizes the feed-line radiation. The elevation pattern of each subarray is controlled by the number of patches and impedance matching tapering via varying the width of connecting microstrip lines. The azimuth pattern is controlled by the subarray height above cylinder surface, substrate width, cylinder radius, and surface treatments between subarrays. Measurement results exhibited good impedance matching and broad antenna coverage in X-band.
In this paper, a Neural Network based methodology is presented to measure the complex permittivity of materials using monopole probes. A multilayered Arti.cial Neural Network, using the Levenberg Marquardt back propagation algorithm is used to back solve the complex permittivity of the medium. The proposed network can be trained using an analytical model, numerical model, or measurement data spread over the complete range of parameters of interest. The input training data for the non linear inverse problem of reconstructing the complex permittivity comprises the complex re.ection coef.cient of the monopole probe. For the results presented in this paper, the network is trained using the analytical model for impedances of monopole antennas in a half space by Gooch et al. [1]. In addition to computational ef.ciency, the proposed approach gives 99% accurate results in the frequency range of 2.55 GHz, with the values of permittivity varying across a range of 3-10 for the real part, and 0 -0.5 for the imaginary part. The accuracy and the effective range of real and imaginary components of the complex permittivity that can be reconstructed using this approach, depends upon the accuracy and robustness of the model / system used to generate the training data. The analytical model used in this paper has a limited range for the values of loss tangent that it can model accurately. However, the performance of the back solving algorithm remains independent from any speci.c model, and the scheme can be successfully applied using any reliable analytical or numerical model, or re.ection coef.cient training data generated through a series of measurements. The methodology is likely to be employed for experimental measurements of complex permittivity of dissipative media.
Impedance and gain measurements for electrically small antennas represent a great challenge due to influences of the feeding cable. The leaking current along the cable and scattering effects are two main issues caused by the feed line. In this paper, a novel cable-free antenna impedance and gain measurement technique for electrically small antennas is proposed. The antenna properties are extracted by measuring the signal scattered by the antenna under test (AUT), when it is loaded with three known loads. The tech-nique is based on a rigorous electromagnetic model where the probe and AUT are represented in terms of spherical wave expansions (SWEs), and the propaga-tion is accounted for by a transmission formula. In this paper the measurement results by the proposed technique will be presented for several AUTs, includ-ing a standard gain horn antenna, a monopole an-tenna, and an electrically small loop antenna. A com-parison of measurement results by using the proposed method and by using other methods will be presented.
Antenna miniaturization will continue to be a key issue in wireless communications, navigation, sensors, and RFIDs. For instance, each cellular tower is often populated with many antennas to cover different angular sectors and different frequency bands. Each modern notebook computer is likely embedded with multiple antennas to provide service in WWAN (824 MHz to 2170 MHz) and WLAN (2.4 GHz and 5.5 GHz), Bluetooth, etc. Also automobiles, vessels, and aircrafts will require more antennas to compete for very limited real estate. This dire situation is changing antenna designer worldwide with a goal to develop a new generation of physically small antennas that multi-bands or wideband. This paper presents several generic miniaturize antenna design examples that applies the concept of artificial transmission line concept for artificially control phase velocity and impedance. This miniaturization approach can be applied to reduce the size of both narrowband and wideband antennas using minimal amount of materials. Thus improves antenna’s efficiency, and reduces its cost and weight.
Accurate knowledge of an RFID IC’s input impedance enables the design of performance-optimized RFID tags with a given IC. For this purpose, the most valuable information is the IC’s input impedance at its wake-up power, but as the impedance itself is power-dependent, few simple methods exist to extract this information. This paper presents a method, based on the joint use of computational electromagnetics, wireless RFID tag measurements and Monte Carlo simulations, to determine the input impedance of an UHF RFID tag chip at the wake-up power of the IC and the measurement uncertainty related to the result.
Small size ultra-wideband (UWB) antennas are attractive for aircraft and ground vehicle communication systems. In this paper, we presented a novel compact low-profile Inverted-Hat Antenna (IHA) to work from low VHF frequencies up to 2GHz. Such a large bandwidth is achieved via excitation of traveling waves between the ground plane and the top portion of the antenna. As one UWB radiator, the IHA is designed to have frequency-independent behavior by introducing a moderate number of elliptical segments. In particular, an 11-ellipse IHA is fabricated and tested to validate the concept. Fairly good impedance matching and radiation properties are achieved. In addition, quad-blade IHA is investigated to show the flexibility of both impedance and pattern control. The proposed antenna is simple and rugged for various UWB applications.
The slot line, a transmission line suitable for application to microwave integrated circuits, may be used in place of or in association with microstrip. This paper presents an alternative method based on the adaptive neuro-fuzzy inference system (ANFIS) for computing the characteristic impedances of slot lines. The ANFIS is a class of adaptive networks which are functionally equivalent to fuzzy inference systems. The ANFIS has the advantages of the expert knowledge of the fuzzy inference system and the learning capability of neural networks. Different optimization algorithms, hybrid learning, genetic, simulated annealing, and least-squares, are used to determine optimally the design parameters of the ANFIS. The algorithm performances for the optimization of the ANFIS model parameters are compared with each other. The results of ANFIS are compared with the results of a commercial electromagnetic simulator IE3D and closed form expressions (CFE) obtained by curve fitting technique to the numerical results.
Paper discusses the design, optimization and implementation of a Circularly Polarized (CP) microstrip 2 x 2 sequentially rotated phased antenna array for an X-band onboard satellite transceiver. In the final design, CP radiation is constructed by using CP elements, having unique sequential rotation along with sequential phase shift feeding–giving wider 3dB Axial Ratio (AR) Bandwidth. CP in each patch element is achieved by a perturbation segment, in this case a pair of truncated corners and with a single point feed–reducing complexity, weight and RF loss of the array feed. First analysis based on cavity model approach for the single CP patch is carried out, which is used to determine the normalized perturbation parameter. The initial dimensions are calculated using perturbation analysis. Optimization initially for individual patch and then for the array is performed using full wave analysis tools based on Method of Moments (MoM), and verified using Finite Difference Time Domain (FDTD). Finally, the measured input impedance and radiation patterns are correlated with the calculated results. It is observed that the measured Gain and 3db Beamwidth agrees well with the simulated results of the array optimized using MoM, while the measured results of Axial Ratio, VSWR and reflection coefficients Sxx follows closely the results from the simulations based on FDTD.
A device and algorithm of measuring of microwave airborne radar antenna impedance and input power level are presented. A compact five-port microwave reflectometer, p-i-n diodes switch, single microwave detector are used. The output detector signal is processed. All of that results in decreasing of the cost of equipment, elimination of instrument components non-ideality and reaching of high equipment accuracy.
This paper presents the results of an intensive investigation for trading off size vs. frequency for a large bandwidth antenna. Theoretical limits were established to determine minimum size as a function of gain and frequency. Bandwidth for the antenna developed was 50 MHz to 2000 MHz; stimulation was done with 50 ohm input impedance. The antenna broadband elements were located in a unique cavity, 6 inches in depth and 15 inches in diameter. The unique ground plane was composed of a combination of a Ferrite Region and Perfect Electric Conductor region which was implemented using Silver particles embedded in MEK. The cavity was fabricated using Carbon Composites to reduce weight. FireFly Opus antenna is in the testing phases.
Solid state diode based multipliers and sub-harmonic mixers are enjoying increasing popularity as frequency extenders for the mm-and sub-mm wave frequency bands. Nowadays, models are available with 40% bandwidth, thus covering full frequency bands, with a reasonable amount of output power. When driven by a frequency synthesizer, they exhibit excellent frequency and phase stability, in contrast to tube devices like a Backward Wave Oscillator (BWO). Drawbacks of the multipliers and sub-harmonic mixers (SHM’s) include their low efficiency, requiring high power amplifiers (HPA’s) to drive them, and the difficulty of achieving broadband impedance matching, which makes it hard to get a constant performance level over the band. For the transmit module, a single HPA driving the multiplier directly turned out to be a satisfactory solution. On the receive side, a feedback circuit regulating the LO power amplifier was introduced. This circuit is based on pilot tone injection in the IF channel of the SHM. The modules have been breadboarded and tested.
Antenna miniaturization has already been demonstrated using equal inductive and capacitive loading to improve antenna impedance at high frequencies, before and after loading. Inductive loading was introduced by coiling the antenna arms to form an inductor like coil, whereas the capacitive loading was achieved using dielectric material. However, this approach can only be applied to miniaturize wire antennas. Here, an alternative miniaturization technique is introduced, using low-loss ferrite composites. The inductive and capacitive loading is now provided by the permeability and permittivity of the ferrite composite, respectively. Of course, the ferrite should possess equal permeability and permittivity (i.e. e’r = µ’r) and must be of low loss for large bandwidths. The basic concept of this approach is to match the impedance of the material to that of free-space, and thus minimize reflections caused by impedance mismatches. In this paper a miniaturized spiral antenna is presented, using the above technique. The challenges of fabricating such a unique ferrite material will also be discussed. .
This paper describes a new approach to improving the low frequency reflectivity performance of geometric transition radar absorbent materials through the use of impedance loading in the form of one or more included FSS layers. The discussion includes theoretical predictions and measured data on modified commercially available RAM which confirm the validity of the concept.
Microwave microscopes that measure surface impedance or roughness have been demonstrated with fine spatial resolutions of less than a micron. These microwave probes are practical only for samples less than a few inches in size. However, composite materials in applications such as multi-layer radomes, embedded frequency selective surfaces, or integrated EMI shielding, have larger length-scale features embedded within a multilayer laminate. Diagnosing larger-scale, subsurface features such as joints/seams, periodic elements, imperfections, or damage is driving a need for methods to characterize embedded electromagnetic properties at mm or cm length-scales. In this research, finite difference time domain (FDTD) simulations and experimental measurements were used to investigate a probe technique for measuring sub-wavelength sized features embedded within a dielectric composite. For these applications, the probe interacted with the sample material via both evanescent and radiating fields. A dielectrically loaded, reduced size, X-band waveguide probe was designed in a resonant configuration for improved sensitivity. Experimental measurements demonstrated that the probe could characterize small gaps in ground planes embedded within a dielectric laminate. Simulations also demonstrated the possibility of detecting more subtle imperfections such as air voids.
Planar power dividers with a good match over a wideband of frequencies are designed using Klopfenstein impedance taper. To validate the proposed design procedure a 2-way stripline and a 2-way microstrip power divider are designed based on simulation, fabrication, and measurement. The measured return loss reveals better than –24 dB (from 4.3 GHz to 19.5 GHz) for a stripline configuration and –27 dB (from 2.2 GHz to 12 GHz) for a microstrip line configuration. Guidelines for accurate simulation and experimental verification are also presented.
A probe station based antenna measurement setup is presented. The setup allows for measurement of complex impedance and radiation patterns of an on-wafer planar antenna, henceforth referred to as the device under test (DUT), radiating at broadside and fed by a coplanar waveguide (CPW). The setup eliminates the need for wafer dicing and custom-built test fixtures with coaxial connectors or waveguide flanges by contacting the DUT with a coplanar RF probe. In addition, the DUT is probed exactly where it will be connected to a transceiver IC later on, such that no de-embedding of the measured data is required. The primary sources of measurement errors are related to calibration, insufficient dynamic range (DR), misalignment, scattering from nearby objects and vibrations. The performance of the setup will be demonstrated through measurement of an on-wafer electrically short slot antenna (.0/35 × .0/35, 5 mm2) radiating at 2.45 GHz.
Double-ridged waveguide horns can provide better than 10:1 relative frequency bandwidth over which they exhibit excellent impedance match and power transfer characteristics. However, the radiation pattern of such an antenna generally becomes more complex at the high end of its operating frequency range. That is, the pattern degenerates from being predominantly single-lobed at lower frequencies to a more complicated pattern exhibiting four gain maxima around the principal axis, all of which are greater than the gain on the principal axis. Here, we present some numerical simulations that appear to indicate that this behavior might not be directly related to higher order modes in the feed region and is not due to manufacturing imperfections, but rather is simply due to the overall taper of the horn itself.
The groundwave correction method of measuring the gain of a vertical antenna over a lossy ground plane is an accepted means of performing a gain measurement without the need for a standard reference antenna. However, on antenna ranges where the ground plane is not uniform, this approach may not yield accurate results over certain portions of the test band due to discontinuities in the ground. This paper shall present a method for using surface impedance methods to predict the performance of an outdoor antenna test range that has a non-uniform ground. Comparison with measured data shall also be presented over the commercial HF and VHF bands.