Antenna In Free Space Research Articles

A single‐FED slot‐aperture hybrid antenna for broadband circular polarization operations

AbstractThis article presents a microstrip‐fed slot‐aperture hybrid antenna for wideband circularly polarized (CP) operations. The antenna consists of a U‐shaped slot, a rectangular aperture, and a probing strip fed by a microstrip line. In general, the U‐shaped slot and the rectangular aperture resonate at two separate frequencies. When properly arranged, the proposed antenna may resonate over a single wideband of CP radiation. The 3‐dB axial ratio (AR) bandwidth of the presented antenna in free space is about 27% with the gain level about 34 dBic. When a planar reflector is incorporated for the desirable unidirectional pattern, the presented antenna has an enhanced gain about 7 dBic with a 3‐dB AR bandwidth of 25% and a front‐to‐back ratio of about 14 dB. Good agreement between measured and simulated results was achieved. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:412–415, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26569

A small circularly polarized tag antenna on a high-dielectric substrate

The reading range of a tag antenna usually decreases markedly as the antenna size decreases, because of the antenna's reduced gain. This letter describes the use of circular polarization to reduce the size of a tag antenna without decreasing its reading range. This approach resulted in a 33 × 33-mm antenna with a substrate thickness of 4 mm that maintained reading ranges of about 3 m for the antenna in free space and 7 m for the antenna on a metallic surface. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett 53:2423–2425, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26270

An Improved Method for Free-Space Antenna-Factor Measurement by Using the MUSIC Algorithm

This paper proposes a simple, fast, and accurate method for calibrating the free-space antenna factors of broadband electromagnetic compatibility (EMC) antennas. This method adopts a fixed-height calibration configuration and a multiple-signal-classification-based (MUSIC) data processing algorithm. Because this configuration does not require height scans, it significantly shortens measurement time and removes height-dependent calibration errors. In addition, the MUSIC-based data processing algorithm can remove unexpected reflections from the ground plane or any other reflecting objects, and transfer calibration into the free-space transmission model, from which the free-space antenna factors are obtained. The algorithm of this method can also automatically compensate for the phase center shift, which makes measurement easier and more convenient. To verify this method, numerical simulations for half-wave dipoles and experimental measurements for log-periodic dipole array (LPDA) and horn antennas are conducted. The simulated results for the half-wave dipoles indicate that the error from the theoretical AF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">FS</sub> can be within ±0.5 dB. In the case of the experimental measurements, the calibrated results are compared with other published standard methods: the mean differences can be as low as 0.25 dB for the LPDA, and as low as 0.36 dB for the horn antennas. The good agreement between results confirms that the proposed method can yield accurate estimations of free-space antenna factors for broadband EMC antennas. This method also provides a new alternative for obtaining fast and accurate antenna calibration.

UHF RFID H-Shaped Tag Antenna using Microstrip Feed Design on Metallic Objects

A novel UHF RFID antenna structure that can stick on metallic surface is presented. A printed H-shaped antenna by using microstrip feed structure on system ground which has the same size as antenna and operated at its halfwavelength (λ/2) mode as the fundamental resonant mode is designed. The printed H-shaped antenna generates about 925 MHz operations. Further, the proposed antenna has a simple planar structure and occupies only a small area of 13 × 66 mm2 on the system circuit board. By using the inductive property of the microstrip line with short end to match the capacitive reactance of tag chip, the performance in free space on metallic surface was measured. The matched impedance bandwidth (VSWR < 2) of the proposed antenna in free space was measured in the frequency range 830–951 MHz. The proposed antenna with good matched impedance, high direction radiation patterns, compact size, low-profile, low cost and measured reading range (5–6 m) in free-space can be suitable for UHF RFID applications.

Modified Dual Segment Rectangular Dielectric Resonator Antenna Terminated in a Bio-Medium

In this paper, the simulation studies of the radiation characteristics of modified dual segment rectangular dielectric resonator antenna (DSRDRA) in free space and in presence of bio-medium alongwith the absorbed power distribution in a homogenous bio-medium (muscle layer) in direct contact with the proposed antenna are reported at different WiMAX band frequencies. The specific absorption rate (SAR) distributions in muscle layer for different antenna-to-muscle layer spacings are also presented at resonant frequency of antenna in WiMAX bands. The results of return loss, input VSWR and radiation characteristics of proposed antenna in bio-medium are also compared with results in free space. The simulation study has been carried out using CST Microwave Studio software.

Reconstruction of Antenna Radiation Patterns From Phaseless Measurements in Nonanechoic Chambers

Antennas are generally measured in anechoic chambers, where walls are covered with appropriate absorbing materials so that reflections are minimized. However, different methodologies have been proposed to process the data measured in nonanechoic conditions with the aim of reconstructing the free-space antenna radiation pattern. The objective of this communication is to evaluate the conditions under which those methods can be extended to the case where only amplitude data is available and phase is estimated using the Hilbert transform.

Vertically Polarized Dipoles and Monopoles, Directivity, Effective Height and Antenna Factor

The effective length (Le) of linear antennas like dipoles and the effective height (He) of monopoles shorter than a wavelength are determined using a transmitting Hertz Dipole as a reference. Effective Receiving Area (AeR) has been calculated in the receiving mode and hence the Antenna Receiving Directivity (DR) or Gain (GR)) can be determined. From these calculations, it can be seen that the Received Power (WR) in a link between two dipole antennas in free space or between two monopole antennas over a perfect ground is of the same value in the far field region. As a result, the directivity or gain of the ideal dipole antenna in free space has the same value in transmission and reception; but, a different value for monopole and dipole antennas installed over a perfect ground plane. This result is fulfilled in the ideal as well as the real case where losses and mismatchings are involved.

Efficient loop antenna modeling for zero-offset, off-ground electromagnetic induction in multilayered media

Retrieval of the subsurface electrical properties from electromagnetic induction (EMI) data using inverse modeling relies in particular on the accuracy of the considered EMI model. We have developed a new EMI approach whereby a zero-offset, off-ground loop antenna is efficiently modeled using frequency-dependent, complex linear transfer functions and the air subsurface is described by a Green’s function for wave propagation in 3D multilayered media. To ensure proper calibration of the system, vector network analyzer (VNA) technology is used as the transmitter and receiver. An optimal integration path is proposed for fast evaluation of the spatial Green’s function from its spectral counterpart. We validated the antenna model in laboratory conditions with measurements performed with a loop antenna in free space and at different heights above a perfect electrical conductor. Provided that the loop antenna is high enough above the reflector (off-ground condition), the measured and modeled Green’s functions agreed remarkably well. In addition, inversion of the EMI data resulted in accurate estimates of the antenna heights. Yet, as expected, signal-to-noise-ratio issues occurred for the higher antenna heights and frequencies away from the loop resonant frequency. The method appears to be promising for accurate and robust soil characterization, but needs high VNA dynamic range and antenna gain.

Varactor-loaded compact folded dipole antenna for digital terrestrial radio reception

A varactor-loaded folded dipole antenna is proposed for digital terrestrial radio reception. The antenna consists of a folded dipole with two pairs of varactors and controls resonant frequency without additional DC bias circuit. It has a compact structure with its arms bent. A prototype antenna with dimensions of 332 mm by 30 mm was printed on a polyethylene terephthalate film. The antenna in free space exhibited a frequency tuning range from 187 to 230 MHz, with a voltage standing wave ratio (VSWR) of less than 1.4 and gain from 1.4 to ?3.1 dBi. The antenna put on glass maintained a VSWR of less than 1.3 from 188 to 192 MHz for digital terrestrial radio band.

Balanced and single‐ended handset antennas: Free space and human loading comparison

AbstractA study of the electromagnetic properties of balanced and single‐ended handset/wireless antennas in free space and human body scenario is presented. The article compares two particular cases, a single monopole and a balanced dipole, in terms of return loss, efficiency, and radiation pattern when changing the ground plane size. The same experiment is carried out taking into account a holding scenario, that is, when a phantom hand is holding the ground plane. © 2009 Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 2248–2254, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24525

Transmission of RF Signals Between MICS Loop Antennas in Free Space and Implanted in the Human Head

This communication characterizes the transmission of radio frequency (RF) signals in Medical Implant Communications Service (MICS) band (402-405 MHz) between the loop antennas placed in free space and implanted into a human head (beneath the skull) for medical applications. The investigation is carried out using a commercial EM simulator XFDTD based on a finite-difference time-domain (FDTD) method with a numerical human head model, and verified by the measurements with a phantom. The study shows the effects of human head on the transmission of RF signals between the antennas inside and outside a human head. The information derived from this study is useful for the link budget of medical systems with the wireless connection between implanted devices in human heads and external central controllers.

Uncertainty of Free-Space Dipole Antenna Factor From 1 GHz to 2 GHz

The free-space antenna factor (AF) of a dipole antenna above 1 GHz using the three-antenna method (TAM) and associated uncertainty is presented. Theoretical site insertion loss containing antenna alignment and terminal impedances is newly given for the TAM and the uncertainty analysis. Higher order terms of Taylor series expansion are considered to include the effect of the alignment angle on uncertainty. Finally, the AFs are obtained with an expanded uncertainty of 0.4 dB (k = 2) in the entire frequency range.

Measurement and Uncertainty Analysis of Free-Space Antenna Factors of a Log-Periodic Antenna Using Time-Domain Techniques

We have proposed a method for evaluating the free-space antenna factor of a log-periodic dipole antenna (LPDA) with a time-domain technique. The proposed method is based on a time-domain analysis and a pulse-compression technique. The proposed method enables the direct wave to be isolated from reflected waves on the ground plane of an open-area test site (OATS) and evaluation of the free-space antenna factor. In a measurement, two LPDAs are held on masts at a height of 6 m above the ground and are separated by 10 m. The direct wave is separated using a Wiener filter obtained from frequency-domain data at 3 m antenna separation. The proposed method can completely separate the direct wave from other waves from the ground plane of an OATS. The expanded uncertainty ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> = 2) of LPDAs was estimated to be from 0.38 to 0.45 dB over the frequency range from 300 to 1000 MHz.

A Novel Indirect Method to Determine the Radiation Impedance of a Handheld Antenna Structure

A novel indirect measurement technique is developed in which the input impedance of an unknown antenna structure is determined by measuring the change in the measured input impedance of a second probe antenna, while the unknown antenna is loaded at its input by a set of known impedances. The feasibility of the approach is explored using method-of-moment boundary element simulations of the interaction between two dipole antennas in free space. Examples of the measurement of the helical antenna of a terrestrial trunked radio (TETRA) handheld radio and a small portable device with an electrically small antenna are presented. The significant advantage of the technique in the accurate measurement of electrically small antennas is confirmed.

FAST HYBRID FEM/CRE-UTD METHOD TO COMPUTE THE RADIATION PATTERN OF ANTENNAS ON LARGE CARRIERS

A hybrid method is developed to compute the radiation pattern of antennas on large complex three-dimension carriers. The hybrid method involves computing the radiation fields of the antenna in free space with FEM, characterizing the reflection and diffraction of the carrier to the radiation fields with CRE (Complex Ray Expansion) and UTD (Uniform Theory of Diffraction). The ray technique of SBR using traditional hybrid method is employed by CRE. The shortcomings of the SBR, such as great number of ray trace, distortion and partly shadowing of the rays etc., are overcome by the use of CRE, and the time consuming physical-optics-type integration is replaced by the paraxial approximation of the complex rays. A dipole placed on different carriers are taken as the examples to show the validity of the hybrid method, and the radiation patterns computed by the proposed method are in good agreement with those by FEM. By using the proposed method, the computation of the three dimension radiation pattern of an antenna in a large ship is finished by a PC in 1671.20 seconds.

Propagation characteristics of a 2.45 GHz microwave radio frequency identification system

The propagation characterisation of a 2.45 GHz microwave semi-passive radio frequency identification system in real indoor environment is presented and the effects of three system parameters: the reader orientation, tag antenna designs and propagation environment are evaluated. The unmodulated forward propagation is evaluated by reader signal strength measurements at different tag positions while the modulated backscattered propagation evaluation employs tag read rate as a function of range under horizontal or vertical reader polarisation in an uncluttered or cluttered indoor environment for different tag antenna types. Measured results are used to quantify the variations introduced by the reader polarisation, indoor propagation environment relative to free-space and tag antenna design on the tag response.

Stub- and Capacitor-Loaded Folded Dipole Antenna for Digital Terrestrial TV Reception

A bandwidth enhancement method for a folded dipole antenna is proposed for digital terrestrial television (TV) reception. The antenna has a pair of stubs inside a folded dipole and a capacitor loaded on the folded dipole. A cylindrical wire model with a lumped element capacitor is analyzed by the commercial simulator ldquoFEKO.rdquo The analysis shows the effect of the stubs and capacitor on bandwidth enhancement, using current distributions and a comparison of dipoles without stubs and/or capacitor. A prototype antenna printed on polyethylene terephthalate (PET) film has dimensions of 15 mm by 280 mm with a line width of 1 mm. The capacitor is implemented with a slit on the line for ease of fabrication. The antenna in free space exhibits a voltage standing wave ratio (VSWR) of less than 2.6, and figure-of-eight radiation patterns with gain from 0.7 to 2.5 dBi in a frequency range from 470 to 950 MHz. The antenna attached to flat dielectric plates of materials that are typically used in automotive body parts, maintains a VSWR of less than 2.7 from 470 to 710 MHz for digital terrestrial TV reception.

Design of planar RFID tag antenna for metallic objects

Proposed is a novel RFID tag antenna consisting of an inner spiral dipole, an outer bent dipole and a double T-matching network that is suitable for attaching to metallic objects. For low-cost fabrication, the antenna is printed on a single-layer substrate without a ground plane or shorting pins. The detailed design parameters of the antenna were optimised using the Pareto genetic algorithm so that the antenna would operate adequately both in free space and on metallic objects. The antenna was fabricated, based on the optimised design, and the performance in free space and on a metallic surface were measured. The half-power matching bandwidth (VSWR&lt;5.8) and the reading range of the antenna in free space were 7.6% and 2.8 m, while on a metallic surface the corresponding values were 0.8% and 1.8 m. For non-metallic surfaces (i.e. low dielectric targets, 2&lt;ɛr&lt;4), the antenna exhibited the reading range of 2–3 m.

Edge diffraction of a narrow beam of millimeter waves in the shadow zone of an obstacle

A mathematical description of the edge diffraction of a narrow beam of millimeter waves in the deep geometric shadow of an obstacle is proposed. The description uses the Babinet principle, according to which the complex amplitude of the diffracted field is determined as the difference of the complex amplitude of the radiation generated by an antenna in free space and the complex amplitude corresponding to the virtual radiation of the shadowing silhouette (obstacle) considered an aperture. The results of field experiments on diffraction of a beam of millimeter waves performed on a Luch telecommunications system are presented. The obtained experimental data completely confirm the effect of diffraction-induced amplification of a beam of millimeter waves in the shadow zone of an obstacle relative to the field of an omnidirectional (spherical) wave.

Electric Field Simulations around a Car of the Tire Pressure Monitoring System

In order to support driving safety, TPMS (Tire Pressure Monitoring System) has been introduced in U.S.A. and Europe. In Japan, the AIRwatch system has been developed and commercialized. Some studies were made to clarify the electric field environment of this system. However, no detailed calculation of the electric field between the transmitter in the tire and the receiving antenna has been published. This paper clarifies the electric field environment of the Japanese system through electromagnetic simulations by a high performance MoM simulator that utilizes the MLFMM scheme. First of all, electric wave emissions from an antenna mounted in a tire are shown to be larger than that of the same antenna in free space. The tire rubber effects are also investigated. Next, electric field distributions on the windshield holding the receiving antenna are calculated. By comparing calculated electric field levels with those in the free space condition, car body interruptions are clarified. Because car body interruptions are not so severe, it is shown that the free space electric field levels can be used as rough design parameters. Moreover, electric field changes due to tire rotation are also clarified. Calculation accuracy is confirmed by the good agreement with measured data collected from a 1/5 scale car model. To permit estimations to be made in actual situations, the effects of the ground are also investigated. This simulation study introduces a lot of important data useful in TPMS system design.