The Koch monopole: a small fractal antenna

Fractal objects have some unique geometrical properties. One of them is the possibility to enclose in a finite area an infinitely long curve. The resulting curve is highly convoluted being nowhere differentiable. One such curve is the Koch curve. In this paper, the behavior the Koch monopole is numerically and experimentally analyzed. The results show that as the number of iterations on the small fractal Koch monopole are increased, the Q of the antenna approaches the fundamental limit for small antennas     

A monopole antenna loaded by a hemispherical dielectric resonator

The input conductance, radiation pattern, and radar cross section (RCS) of a monopole loaded by a hemispherical dielectric resonator (DR) are studied. The effects of the DR on the monopole characteristics are investigated. Measurements were carried out to verify the theory. It is found that the input conductance, radiation patterns, and RCS are strongly affected by the DR.     

Bandwidth enhancement of the monopole antenna by using a parasitic normal-mode helix

An innovative monopole antenna coupled with a parasitic normal-mode helix has been designed and studied both experimentally and theoretically. The effects of various dimensions of the monopole and the parasitic helix were investigated extensively. This simple combination can significantly enhance the antenna bandwidth over that of the conventional monopole antenna. It is found that, with suitably chosen antenna dimensions, the VSWR bandwidth (VSWR ≤ 2) of the antenna can be increased to 36%, almost double that of the monopole antenna alone. The far-field pattern of the combined antenna is basically the same as that of the monopole antenna. This antenna is designed to operate at 1.8GHz. The substantially enhanced bandwidth of this simple and small antenna is a useful feature for future mobile communications.     

The radiation pattern of a monopole antenna attached to aconducting box

A system investigation was undertaken to determine the radiation patterns of a monopole antenna mounted on a cubical conducting box over a ground plane. The effects of the location of the monopole and the electrical size of the box were noted as compared to similar patterns measured over a flat ground plane. Experimental results were also compared with numerically predicted values obtained from a method of moments patch code. It was concluded that the location at which a monopole antenna is mounted on a conducting box and the electrical size of the box clearly affect the overall radiation pattern of a monopole antenna. In general, mounting the antenna away from the center of the box will increase the depth of the nulls     

The input impedance of a monopole antenna mounted on a cubical conducting box

An experimental investigation has been performed to determine the input admittance characteristics of a monopole antenna mounted on a conducting cubical box over a ground plane. Input admittances of monopoles from 2 to 6 cm long mounted on a 10 cm box were considered in the investigation so that effects of changes in the electrical size of the box could be evaluated. The monopoles were placed at various points to determine the functional dependence of input admittance on the position of the monopole antenna. A numerical analysis of the radiating structure was performed using the method of moments to compare the experimental data with the computed input admittance. This comparison and the observed empirical behavior of the input admittance were then utilized to predict the effects of the conducting box on the overall admittance of the radiator.     

Transient field produced by a traveling-wave wire antenna

The electromagnetic fields of a transient traveling wave current on a straight wire segment are calculated directly in the time domain. Retarded potential theory is used to establish an impulse response valid in both far and near zones. A closed-form expression for the field due to a rectangular current pulse is obtained, and corresponding plots are given. Numerical results from this permit the investigation of the differences between near versus far zones and coated versus uncoated wires. Experimental findings for both coated and uncoated wires show good agreement with theory. A formula for far-zone radiation from an arbitrarily shaped wire is developed; results for a proposed directive antenna geometry are presented     

The monopole slot as a two-port diversity antenna for UHF land-mobile radio systems

Urban land-mobile communication systems designed to operate over the upper portion of the UHF band are very susceptible to frequent wide signal variations. Deep fades arise due to the presence of many obstructions in the propagation path between the transmitter and receiver. One way to combat the effects of Rayleigh fading due to multipath propagation is to provide a diversity combiner in the mobile receiving system with two signals, each containing the same information, which fade independently of one another. Such a system can virtually eliminate deep fades from the receiver output. Described herein is a method of obtaining two diversity branches, with independent signals if so desired, from a single two-port antenna which is both well matched to the transponder and efficient over the land-mobile frequency band from 820- 890 MHz. A directive pattern is associated with each port. When operating with a direct wave, signal gain over an omnidirectional antenna is therefore possible.     

Seven-element ground skirt monopole ESPAR antenna design from a genetic algorithm and the finite element method

The design of an optimized electronically steerable passive array radiator (ESPAR) antenna is presented. A genetic algorithm using a finite element based cost function optimized the antenna's structure and loading conditions for maximal main lobe gain in a single azimuth direction. Simulated gain results of 7.3 dBi at 2.4 GHz were attained along the antenna's elemental axis. The optimized antenna was fabricated and tested with the corresponding experimental gain better than 8 dBi. The 0.7 dB error between simulated and measured gain was constant for numerous structures and therefore did not affect the optimization. The optimized antenna reduced average main lobe elevation by 15.3/spl deg/ to just 9.7/spl deg/ above the horizontal.     

Quasi-near-zone field of a monopole antenna and the current distribution of an antenna on a finite conductive earth

A mathematical expression is derived for the quasi-near zone of a monopole antenna on a finite conductive earth. Measured data are used to show the effect of finite conductivity of earth on the current distribution of dipole antennas.     

Impedance of a monopole antenna over a ground plane and immersed in a magnetoplasma

A treatment of the input impedance of a monopole antenna over a ground plane covered with a magnetoplasma with any arbitrary direction of the static magnetic field is presented. The analysis is restricted to a cold plasma with uniaxial and quasi-static approximations. It has been found that for parallel and perpendicular directions of the static magnetic field with respect to the ground plane perfect mirror reflections can be obtained. For other directions of the static magnetic field, the reflection is birefringent so that the monopole impedance becomes modified over a dipole impedance. These modifications can be significant under hyperbolic plasma conditions. A short and a long monopole were considered. From laboratory measurements of a long monopole impedance, the resonance cone and the resonances corresponding to the antenna length have been observed.     

Input impedance of a monopole antenna at the center of a finite ground plane

A technique for computing the input impedance of a monopole on a finite ground plane is provided. The case of an infinite ground plane is first examined, and then the concept of using a magnetic edge current as an equivalent source of diffraction on the limited size ground plane is introduced. The technique used is serf-consistent in that the circular ground plane considered is the limit of a polygonal ground plane as the number of sides tends to infinity. Good agreement has been demonstrated when the nearest edge of the ground plane is at least 0.3 wavelength from the base of the monopole.     

Diversity gain from a single-port adaptive antenna using switchedparasitic elements illustrated with a wire and monopole prototype

A new concept in single-port adaptive antennas using parasitic elements with switched terminating impedances is presented including results from a concept prototype. Each parasitic element can be effectively terminated in three impedance values. The antenna concept provides multiple radiation patterns with a single RF signal port without the need for RF switches or phase shifters in the direct RF signal path. Impedance variations in the active antenna element are minimized by use of only rotationally symmetric configurations. Measured patterns are used to demonstrate the performance improvement expected using switched diversity combining in a simulated uniform scattering scenario. The concept prototype having one active element and four parasitic elements, is shown to have equivalent diversity performance to between three and four uncorrelated branches     

On the radial component of the electric field for a monopole phasedarray antenna focused in the near zone

The near-zone radial and principal spherical components of the electric field for a linear phased array antenna that is focused at one to two aperture diameters in the near zone are investigated. The phased array antenna consists of thin monopole elements, which are analyzed by using the method of moments. A theoretical formulation is described and computer simulation results are presented. The results show that the radial component is negligible in the focused near-field region, which allows the principal component to be accurately computed from knowledge of only the tangential electric field on the focal plane     

爆炸产生的烟雾对激光传输影响研究

本文研究爆炸产生的烟雾对激光传输衰减的影响。根据爆炸烟雾扩散模式，给出透过率计算公式中烟雾浓度的计算方法，通过该方法建立了投弹量与烟雾透过率的关系。仿真结果显示：假设投弹量转化为烟雾剂量的比率为1％、投弹10kgTNT、复折射率为n＝1．87＋0．22i、入射波为1．06μm且烟雾厚度为10m时的透过率极大为0．9左右，而投弹量增大一个数量级时烟雾透过率小于0．5。根据仿真结果可知：炮弹爆炸释放烟雾对激光通信影响较大，且衰减变化随着弹量的增加呈指数降低。     

On the electromagnetic pulse produced by nuclear explosions

The electromagnetic pulse (EMP) produced by the gamma rays from nuclear explosions is discussed. The gamma rays produce a current of Compton recoil electrons, and these electrons produce further ionization so that the air becomes conducting. The Compton current leads to the generation of electromagnetic fields according to Maxwell's equations. The conductivity tends to limit the magnitude of the fields. Approximate methods of solving the equations are described by considering time regimes in which various terms in the equations are negligible, e.g., either the conduction current or the displacement current can be dropped. Further advantage is obtained by replacing the transverse fields by outgoing and ingoing waves; outgoing waves are dominant at early times. Features of the solutions are described for nuclear bursts at the ground surface and at high altitude. The history of EMP is reviewed briefly.     

Microwave diagnostics of the plasma sheath produced by slot antenna breakdown

The plasma sheath caused by pulsed RF breakdown in helium outside the aperture of a waveguide radiator has been probed. The highest measurable phase shifts and attenuation changes caused by the plasma were compared with those calculated on the basis of a slab model. The results are consistent with an effective thickness of the plasma sheath of about 1 cm, and electron densities in the active sheath being considerably in excess of the critical value.     

Radiation from a small monopole in a uniaxially anisotropic plasma

The radiation field of a small monopole over a ground plane covered with a uniaxially anisotropic plasma with an arbitrary inclination of the optical axis is studied. A steepest descent evaluation of the field integrals has been performed under the elliptic condition, and some calculated power patterns have been studied in comparison with those for a small dipole in an unbounded uniaxially anisotropic plasma. For an arbitrary inclination of the optic axis, considerable differences between the power patterns of the two configurations have been observed. Lateral waves and concentrated field regions are found to occur from mode coupling at the surface of the ground plane.     

核信号检测与控制系统中的调制解调信号LabVIEW仿真研究

在核信号检测与控制系统中,从传感器输出的信号,一般都很微弱,为了避免信号在远距离传输过程中衰减、严重失真,这就需要对信号进行调制解调。在Win7系统下,使用LabVIEW2010设计了一个信号调制与解调系统。并且在该系统中分别采用Butterworth滤波器、Chebyeshev(I型)滤波器、椭圆滤波器对信号进行滤波了处理,比较结果发现,使用Butterworth滤波器得到的解调信号较后两种滤波器的好。     

On the admittance of a monopole driven from a coaxial line

A simple formula is described for the apparent admittanceY(ka, b/a)of a monopole of fixed lengthhand radiusawhen driven from a coaxial line (conductor radiibanda). IfY(ka, b/a)is known atnvalues ofkawithb/afixed at(b/a)_{2}and at m values ofb/awithkafixed at(ka)_{1}, it can be evaluated from the given formula for all combinations ofkaandb/ain the two ranges ofnandm. It is assumed thatkb ll 1andh gg (b-a).