Input Impedance Of Antenna Research Articles

FDTD based Analysis of Input Impedance of a Wearable Patch Antenna with Milli Bends

Wearable antennas are prone to bending and crumpling. This work presents the analysis of input impedance of a wearable patch antenna with milli bends using Finite Difference Time Domain (FDTD) method. FDTD method using stair case scheme is used to model milli bends at different places along the length of a wearable patch antenna and the effect of milli bends on input impedance of the wearable patch antenna is presented developing a FDTD code for it. It is shown that presence of milli bends in a wearable patch antenna significantly changed the input impedance of the antenna under consideration. It is observed that milli bends closer to the feeding strip line provided better input impedance matching. Results concluded that change in input impedance leads to change in resonant frequency and reflection coefficient of the antenna. General Terms: RF and microwave communication, Antenna design and analysis.

Impedance Matching Antenna-Integrated High-Efficiency Energy Harvesting Circuit.

This paper describes the design of a high-efficiency energy harvesting circuit with an integrated antenna. The circuit is composed of series resonance and boost rectifier circuits for converting radio frequency power into boosted direct current (DC) voltage. The measured output DC voltage is 5.67 V for an input of 100 mV at 900 MHz. Antenna input impedance matching is optimized for greater efficiency and miniaturization. The measured efficiency of this antenna-integrated energy harvester is 60% for −4.85 dBm input power and a load resistance equal to 20 kΩ at 905 MHz.

Fast frequency sweep of metallic antennas using frequency-independent reaction and enhanced gap source model

In this paper, the enhanced gap source model is combined with the frequency-independent reaction(FIR)-MoM to analyze the input impedance of the metallic antenna over a wide frequency band. The enhanced gap source model is a simple feeding model, which can overcome the instability of the gap source model. Thus, the stable results can be obtained. In the FIR-MoM, the exponential of the Green’s function is expanded as Taylor series in terms of the distance between the centers of the testing and source functions. The impedance matrix is expressed as the summation, in which each term is the multiplication of the frequency-independent matrix and the frequency-dependent phase term. The frequency-independent matrices are computed before the frequency sweep. Thus, the efficiency of the FIR is very high. The impedance matrix can be efficiently generated when the FIR is used for frequency sweep. The FIR-MoM is suitable for the whole frequency range. Moreover, the precision can be dynamically improved. By combining these two techniques, the stable results over a wide frequency range can be efficiently obtained. To test the accuracy and efficiency of the proposed method, two numerical examples are implemented. Numerical results validate the accuracy and the efficiency.

Calculation of the input impedance of a strip loop antenna located on a dielectric cylinder

A method for calculating the input impedance of a loop antenna (LA) located on a dielectric cylinder is developed. The integral equation (IE) for the unknown function of the current density distribution on the LA surface is obtained. A numerical method for the IE solution is described. An approximate formula for calculation of the LA input impedance is proposed. Plots of the input impedance on the LA length are given.

A single-sided meandered-dual-antenna structure for UHF RFID tags

A meandered-dual-antenna structure is proposed for UHF radiofrequency identification (RFID) tag. It is composed of two independent antennas printed on the one side of the substrate board. One of the antennas is exclusively used for receiving and harvesting sufficient energy to the tag chip having the complex conjugate impedance of the receiving antenna. The other is for backscatter to enhance maximum differential radar cross-section with purely real input impedance, to enhancement the read range. The receiving antenna is formed by a rectangular loop and a parasitic meandered line. The rectangular loop is used as a feeding element for the meandered line. The backscattering antenna is made using a meandered dipole along with a thin rectangular strip. The input impedance of the receiving antenna is designed to be conjugate matched to the chip impedance (13.5-j110 Ω), whereas the input impedance of backscattering antenna alternatively switched to open and short circuit for modulating the backscattered field. The input impedance of receiving and backscattering antennas is measured using differential probe technique. The simulated and measured results are found in good agreement. It is also demonstrated that the read range of UHF RFID system increased considerably by using the dual-antenna structure.

Low-Frequency Theoretical Analysis of a Source-Stirred Reverberation Chamber

Theoretical analysis of a reverberation chamber excited through the source-stirring technique at low frequency is presented. A complete analytical solution returns quickly the electromagnetic field inside the chamber thanks to an efficient computation technique for the slowly convergent triple series. The stirring function is accomplished by 62 monopole antennas, irregularly distributed on the chamber walls, and subsequently, fed two at a time for a total of 1891 combinations. At first, the empty chamber is characterized, as usually it is done for any test site and then a receiving loop is considered in the chamber working volume to simulate a device under test (DUT) and the induced voltage is studied in terms of amplitude and phase. The analytical approach allows to account for all mutual couplings between transmitting antennas and the receiving structure through the computation of all self and mutual impedances. We demonstrate that the contribution of chamber irrotational modes cannot be neglected in the computation of both antenna input impedance and DUT-induced voltage. The present study highlights the capability of source stirring to change the field amplitude and orientation in the undermoded condition. This advantage takes also benefit from the use of an out-of-phase feeding for each pair of active antennas, achieving a higher DUT-induced voltage.

Experimental Investigations of a Novel Lightweight Blade Antenna for UAV Applications [Antenna Applications Corner

Small, lightweight, and broadband antennas are a growing need for unmanned aerial vehicle (UAV) platforms due to the increasing demand of highly integrated systems for both electronic warfare (EW) and communication-oriented applications. In this article, an experimental characterization of a compact printed passive wide-band dipole antenna is presented. The main advantages of this design are the reduction of both the height and the weight of the antenna while keeping the structure as tough as possible from an environmental point of view with the only drawback being a tradeoff with respect to voltage-standing wave ratio (VSWR) performances. The simplicity (no lumped passive components are used to match the antenna input impedance to 50 X) and performance effectiveness of the proposed antenna is such that it avoids a complicated screening process among different phase-matched sets of radiating elements. The experimental characterization was performed through a near-field measurement setup, showing excellent agreement with numerical fullwave simulations.

Pole-zero analysis and wavelength scaling of carbon nanotube antennas

Carbon nanotube (CN) antennas have applications in the THz electromagnetic spectrum. Nanotubes have a highly dispersive and frequency dependent conductivity model. In this article, we compare the poles and zeros in the input impedance of CN antennas at different lengths. We used model-based parameter estimation to approximate the input impedance of the antenna with a rational function in the complex frequency domain. Despite dispersive conductivity of CN, the imaginary part of the poles and zeros are respectively the integer multiples and odd multiples of the imaginary part of the first pole and zero. However, the real part of poles is almost constant, while the pattern was not observed for the real part of zeros. We also show that CN dipoles operating between 43 and 53 GHz are well matched if the source impedance is much higher than 50 ohms, and even higher than 12.9 kΩ. The fundamental resonances (f0) of CN dipoles plotted versus their inverse-half-length (1/L) are linearly related, but the intercept of the fitted straight line is non-zero unlike that for perfect electric conductor (PEC) dipoles. This leads to non-linear variation in wavelength scaling of CN dipoles. The resonant CN antennas are relatively much shorter than PEC dipoles.

A Low-Profile Aperture-Coupled Microstrip Antenna With Enhanced Bandwidth Under Dual Resonance

A low-profile aperture-coupled microstrip patch antenna (MPA) using the TM10 and TM30 resonant modes to enhance the impedance bandwidth is proposed in this paper. Based on the cavity model for a square MPA, the TM10 and TM30 modes as well as both higher odd-order and even-order modes between them can be characterized. In order to combine the dual radiative resonant modes for a wide impedance bandwidth, a rectangular radiating patch with an aperture-coupled feeder is employed and theoretically investigated at first, aiming to demonstrate that all of the undesired modes between them can be removed effectively. After that, by loading the shorting pins properly underneath the patch, the resonant frequency of TM10 mode is shown to progressively turn up with slight effect on that of TM30 mode. As a result, these two radiative modes can be allocated in proximity to each other, resulting in a wide impedance bandwidth with a stable radiation pattern and the same far-field polarization. Moreover, the principal parameters of the MPA have been extensively studied in order to investigate the sensitivity in input impedance of the aperture-fed patch antenna. Finally, the proposed antenna is fabricated and measured. Simulated and measured results are found in good agreement with each other and illustrate that the antenna achieves a wide impedance bandwidth of about 15.2% in fraction or 2.32–2.70 GHz under $\vert \text{S}_{\mathrm {\mathbf {11}}}\vert dB, while keeping a low profile property with the height of 0.032 free-space wavelength. Besides, a stable gain varied from 3 to 6.8 dBi within the whole operating band is also obtained.

Evaluation of Efficient Closed-Form Green’s Function in a Cylindrically Stratified Medium

An efficient technique is proposed for the removal of the spatial singularity for a z-directed current source in a cylindrically layered structure by decomposing the spectral domain Green’s function into the particular and homogeneous parts. The particular part with the singularity is converted into the spatial domain using Sommerfeld’s identity, while the DCIM with general pencil of function technique is applied on the spectral domain homogeneous part. The use of the ratio form of the reflection/transmission matrices, the Debye approximation, and the subsequent envelope extraction for the treatment of the spatial domain singularity is thus avoided. In addition, a very significant reduction in the number of modes required for convergence is also achieved. The technique is applied for evaluating the input impedance of several antennas in the cylindrical multilayer structure together with a coupling configuration for the excitation of the multilayer structure.

Electromagnetic, complex image model of a large area RF resonant antenna as inductive plasma source

A large area antenna generates a plasma by both inductive and capacitive coupling; it is an electromagnetically coupled plasma source. In this work, experiments on a large area planar RF antenna source are interpreted in terms of a multi-conductor transmission line coupled to the plasma. This electromagnetic treatment includes mutual inductive coupling using the complex image method, and capacitive matrix coupling between all elements of the resonant network and the plasma. The model reproduces antenna input impedance measurements, with and without plasma, on a m2 antenna used for large area plasma processing. Analytic expressions are given, and results are obtained by computation of the matrix solution. This method could be used to design planar inductive sources in general, by applying the termination impedances appropriate to each antenna type.

Impedance Matching Techniques for Microstrip Patch Antenna

Objective: To present a concise and comprehensive summarization of various impedance matching techniques for microstrip patch antennas. Method: Designing an impedance matching network is a central issue for optimum performance in every part of RF systems like transceiver, amplifier and antenna to ensure maximum power transfer. Various design formulae to calculate the input impedance of patch antenna and techniques to design a matching network should be known to RF designer. Finding: In this paper various impedance matching techniques along with their design equations are presented that utilize quarter wave transformer, taper lines, open or short stubs and lumped elements etc. Methods to calculate the input impedance for various antenna structures like rectangular, circular and triangular patch antenna are described. Application: This paper concisely covers some of the existing techniques to design an impedance matching network that can be used to solve the impedance matching problem encountered during antenna design.

Small Antennas Remote Impedance Measurement Using Electrostatic Discharge

The conventional techniques to measure the input impedance of an antenna are based on comparing the reflected signal to the incident signal at the antenna port at a given frequency. A network analyzer connected to the antenna under test through a feeding cable is the most common impedance measurement setup. The induced current on the outer surface of the feeding cable in the electrically small antenna (ESA) case, however, significantly interferes with this process. Therefore, the traditional methods to measure the input impedance such as using a network analyzer may not be suitable for an ESA. A remote impedance measurement technique based on the impulse responses of an ESA terminated to three standard loads, short/open/matched load, is introduced. Two different types of antennas are designed and simulated using a full-wave simulator and then the impulse responses are computed using a transient simulator software. Thereafter, two different circuit models are introduced and the parameters of these models are computed using three impulse responses. The input impedance of both antennas are computed directly from the full-wave simulations as well as the remote measurement technique and the results are compared. Finally, both antennas are prototyped, measured, and the input impedances of the antennas are compared for the direct measurement using a network analyzer and the proposed technique. The presented results confirm the accuracy of the proposed method.

An Accurate Circuit Model for Input Impedance and Radiation Pattern of Two-Port Loop Antennas as E- and H-Probe

In this paper, a simple but very accurate circuit model for two-port (split) loop antennas typically used for direction finding in the HF-band is presented. The circuit model consists of lumped elements and can be used to accurately estimate the input impedances and received voltages of two-port loop antennas in receive mode. It is shown that the same circuit model can be reduced to predict the input impedance behavior of a single-port loop antenna with much more accuracy than the existing transmission line and lumped element models over a wide frequency range. The model is shown to be valid for frequencies up to a point where the loop circumference is about a wavelength. The same circuit model in receiving mode can provide the 3-D radiation pattern of the antenna.

A numerical–analytical method for calculation of curved dipole antennas

A numerical–analytical method for solution of equations and calculation of the input impedance of thin dipole antennas of arbitrary shape has been developed. High efficiency of the method has been demonstrated by examples.

Analysis of Plasma Monopole Antenna Using Numerical Method and an Equivalent Circuit

This study focuses on the effects of antenna radius and plasma frequency on the characteristics of a plasma monopole antenna in its first resonant region for reconfigurable applications. It is shown that the input impedance of a plasma monopole antenna can be modeled by a simple equivalent circuit. Variations of the equivalent lumped elements with the antenna radius and the plasma frequency are numerically approximated. The bandwidth of the plasma monopole antenna increases with increase in the antenna radius. This numerical investigation estimates the relation between the resonant length and the radius, as well as the plasma frequency of the plasma antenna. The resonant length decreases by increasing the radius of the antenna and decreasing the plasma frequency. The numerical modeling shows that there is a specific band of plasma frequency for which the rate of variations of input impedance versus plasma frequency is minimal. This frequency band is the most efficient region for the plasma frequency in the application of a plasma antenna as a reconfigurable radiating element.

Classification of Human Activities Using Variation in Impedance of Single On-Body Antenna

We propose to use the variation in input impedance of a single on-body antenna to classify human activities. The reflection coefficients of a single on-body antenna are shown to exhibit unique time-varying features for various daily activities. Employing seven human subjects, six different activities are measured by two antenna placements at three operating frequencies. A dynamic time-warping algorithm is introduced to classify the human activities. We find that the classification accuracy can be as high as 98% when the antenna is placed on the chest and operates at 433 MHz. The classification accuracy in terms of the time-window size is also presented and discussed.

Classification of Finger Movements Based on Reflection Coefficient Variations of a Body-Worn Electrically Small Antenna

We propose to classify finger movements based on the reflection coefficient variations of a wrist-worn electrically small antenna. Near field perturbed by finger movements results in unique variation of antenna input impedance, which can be used to identify finger-motion patterns. Two folded cylindrical helix (FCH) antennas, operating at 890 MHz and 2.43 GHz, are implemented in this study. First, four movements of the left-hand fingers are recorded by measuring S 11 of the antenna attached to the wrist at the left hand. Second, we measure four movements of the right-hand fingers above the antenna attached to the left wrist. To help classify the finger motions based on the S 11 variation with time, the dynamic time-warping technique was employed. We found that the average classification accuracies are higher than 97% for the first scenario and 100% for the second scenario using the FCH that operates at 890 MHz.

Design and Measurement of a Differential Printed Antenna for a Wireless Sensor Network Node

This letter presents the design and measurement of an antenna integrated into an agricultural sensor network node operating on the 2.4-GHz ISM band using the ZigBee protocol stack. The novel structure, coined bowtie-shaped folded dipole (BSFD) , has a differential 100- $\Omega$ input impedance matching a particular controller widely used in wireless sensor network (WSN) applications. This letter also presents a survey of existing WSN antennas to which the proposed design compares favorably. The challenges of testing a differential antenna using conventional single-ended equipment are addressed regarding antenna impedance and radiation pattern measurements. The differential antenna input impedance was confirmed with four different test setups using a conventional vector network analyzer. The radiation pattern of the proposed antenna was measured in-prototype resorting to a spectrum analyzer. The measured patterns show that the proposed antenna has a maximum received power over 4 dB compared to two early versions of the WSN node that used commercial off-the-shelf antenna solutions. The performance was also confirmed in a field test where a BSFD-equipped prototype achieved close to a 300-m range, tripling that typical for ZigBee applications.

On the sensitivity of matching network to load variation for passive RFID sensor

ABSTRACTA novel study on sensitivity of matching network to load variation is presented in this work. In passive RFID sensor tag, it is desired that a small change in input impedance of tag antenna should yield a significant response, which can be achieved by a matching network with high sensitivity. In this work, network sensitivity is defined and maximum sensitivity matching network is acquired for certain source and load impedances. Several LC and microstrip matching circuits are designed and fabricated to verify the network sensitivity theory. By inserting a sensitive matching network to a temperature sensor tag in literature, three times higher sensitivity is achieved, which proves the merit of the theory. © 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 59:232–236, 2017