Subpicosecond photoconducting dipole antennas

Photoconducting antennas have been demonstrated which are capable of generating and coherently detecting subpicosecond electrical pulses. These antennas, when illuminated with femtosecond optical pulses, radiate electrical pulses which have frequency spectra that extend from <100 GHz to >2 THz. Microscope dipoles measuring 50, 100, and 200 μm have been fabricated and tested. Integrated photoconductors of radiation-damaged silicon-on-sapphire were used both for impulsive current excitation of the transmitting antennas as well as for gating the receiving antennas     

Optimization of log-periodic dipole antennas

It is shown that a log-periodic dipole antenna can be optimized by varying the feed-line characteristic impedance while carrying out swept-frequency far-field and impedance measurements. The minimization of narrow-band anomalies in the measured quantities results in a well-defined optimum characteristic impedance.     

Travelling wave microstrip dipole antennas

While it is a common antenna, the unloaded conducting microstrip dipole suffers from a very limited bandwidth because it is a resonant structure supporting a standing wave of current. The authors present the characteristics of a microstrip dipole modified to behave like a travelling wave structure     

Backfire antennas with dipole elements

A method is set up for a theoretical investigation of arbitrary backfire antennas based upon dipole structures. The mutual impedance between the dipole elements of the antenna is taken into account, and the field radiated due to a surface wave reflector of finite extent is determined by calculating the surface current distribution on the reflector plate. Numerical results obtained for Yagi backfire antennas and short-backfire antennas using this theory are compared with experimental results.     

Broadband calculable dipole reference antennas

A broadband calculable standard dipole antenna has been developed, with an uncertainty in the antenna factor (AF) of better than ±0.15 dB at the resonant frequency, f_res, in the frequency range 30 to 500 MHz and ±0.2 dB in the range 600 MHz to 1 GHz. For broadband operation of the dipole resonant at 60 MHz the uncertainty is ±0.2 dB over a range 0.33 f_res to 1.83 f_res. These uncertainties have been validated by close agreement of the measured insertion loss between dipole antennas above a conducting ground plane, with the loss predicted by analytical and numerical methods. The AF measured by the two-antenna method also agrees with the calculated AF. The technique was applied to reference monopole antennas for which AF was determined to an uncertainty of ±0.2 dB over the frequency range 10-100 MHz. The key achievements are: the construction of a very large and flat ground plane, validation of numerical versus analytical calculations of impedance and effective length of resonant dipoles, excellent agreement between measurements and method-of-moments calculations of the coupling between resonant dipoles, good agreement over a broad bandwidth, careful design of antennas and supports, and precision measurements     

Moment-method solution of directed dipole antennas

A range of directed dipole antennas is analysed for interdipole spacing of 0.10?, 0.15?, 0.20? and 0.25?, using driven and parasitic conductors of any length between 0.40? and 0.58?, and of both equal and unequal radius.     

Asymmetry phenomenon of log-periodic dipole antennas

The existence of asymmetric resonant modes on standard log-periodic dipole antennas is established experimentally. These modes are characterized by sharply-resonant side radiation sometimes accompanied by reduction in front-lobe gain. The resonances occur at frequencies whose relationship is approximately log-periodic. In a single antenna the resonant modes are excited by any structural asymmetry, while in anE-plane array each individual antenna excites the others asymmetrically. The resonances can be eliminated by the addition of lossy material to appropriate parts of the antenna. Qualitatively the phenomenon is explained in terms of transmission-line resonances along the two-wire lines formed by adjacent dipoles. Automated swept-frequency far-field measurement techniques were employed throughout, and their worth is clearly established for broadband antenna research and development.     

Study of compressed log-periodic dipole antennas

The behavior of log-periodic dipole (LPD) antennas, which are compressed along the transmission-line axis, is studied both experimentally and theoretically. Compressed LPD antennas are found to be efficient, of low gain, and frequency independent. With compression, the radiation pattern approaches dipole-like behavior and the bandwidth increases slightly. It is also found that LPD antennas exhibit anomalous frequency-dependent behavior in narrow bands of frequencies (approx 1/8of a log period in width) when a reactive termination is used and when the scaling factortauis below about 0.92-0.93.     

DIRECTIVITY OPTIMIZATION OF CURVED SURFACE DIPOLE ANTENNAS

Based on the calculation of the characteristic parameters by moment method, the curved surface dipoles are optimized by an optimization method, the maximum directivities of some V-curved and Gauss-curved surface dipoles are given.     

Optimal design of multiply fed dipole antennas

The gain of a mulitply fed dipole antenna of lengthL, small radiusaand arbitrary locations of feed voltages along the antenna is computed using the well-known moment method. An optimization routine is then employed to study the possibility of maximizing the gain in a specified angular direction and minimizing it at other directions for any given number of excitations and antenna length in order to determine the optimum complex values and location of each source. The results are presented in tables and graphs for a wide range of antenna length and number of feeds. It is shown that both the gain and beamwidth are improved by this technique at the expense of appearance of new sidelobes and requirement to design a more complicated feed network. The Fourier series expansion method is extended in order to determine the gain of a multiply fed wire antenna, and the results for the radiation pattern show good agreement with those based on the moment method.     

Rigorous modal-expansion analysis of asymmetrical dipole antennas

This paper describes a rigorous analysis of asymmetrical cylindrical dipole antennas by the modal-expansion method. The analysis is facilitated by introducing two parallel conducting plates to enclose the dipole antenna. Field expressions for all the subregions of the resulting structure can be readily derived, and the expansion coefficients are found by enforcing the continuity conditions of tangential field components across the regional interfaces. Comparison of computed results with experimental data shows an excellent agreement. Numerical results are also presented for the input impedances, current distributions, and radiation patterns of various dipole antennas     

Transient response of coupled linear dipole antennas

Transient responses of coupled linear dipole antennas are measured for a basic understanding of coupling in the time domain. A very short pulse for excitation is used and it is shown that the measured data represent the quasi-impulse responses. For further investigation and comparison with the measured data, a simple formula of the impulse response of this configuration is derived by using the transmission line approximation. They suggest that the tips and feed points of each antenna have a significant role in the electromagnetic coupling.     

Photoconductive twin dipole antennas for THz transceiver

A THz transceiver using twin photoconductive dipole antennas fabricated on the same substrate chip has been demonstrated. With reduced photoconductive noise compared to that of a single photoconductive transceiver, signal-to-noise operation as good as that of a conventional separate emitter-receiver system was achieved. An interference effect due to the adjacent antennas was also observed     

Numerical analysis technique for diode-loaded dipole antennas

A circuit model for a double-loaded dipole electric field measurement probe that is valid for frequencies below 10 GHz is developed. A nonnegligible dipole radius and the nonlinear characteristics of the diode are taken into consideration. The dipole lumped-circuit elements and diode capacitances are determined by theoretical means, while the diode V-I characteristics and resistance are determined by a combined theoretical and empirical approach. Newton's iteration method is used to numerically solve the resulting nonlinear differential equation. Experimental measurements on a probe designed for operation between 1 and 1000 MHz are reported and compared with the theoretical results     

对数周期偶极天线扇形阵的分析

采用矩量法对对数周期偶极天线扇形阵进行理论分析，在计算广义阻抗矩阵元素时，利用矩阵的互易性、天线阵的结构对称性和对数周期天线的工作原理进行了算法简化，从而在保证计算精度的前提下，大大提高了计算速度。     

A simple formula of current in dipole antennas

A simple and quantitatively accurate representation of the current distribution in a dipole antenna is derived. Numerical data are given and are found to be in good agreement with the experiment whenh geq O.15lambda.     

A moment-method characterization of V-Koch fractal dipole antennas

This article compares input impedances and radiation characteristics of half wavelength Koch fractal V–electric dipoles having included angles 60°, 90° and 120°. The study considers three structures. In the 1st structure the Koch arms open into the V-region, in 2nd structure they open away from the V-region and in the third structure, one arm opens into and the other away from the V-region. A first iteration, structure 1 of V-Koch electric dipole antenna with included angle of 120° was fabricated and the experimental return loss was in good agreement with simulation. At their first resonances the antennas’ gain and input resistance decrease with decrease in included angles, an observation synonymous to Euclidian electric dipoles. In terms of gain, the first structure is found to give better performance than the other two. For this structure, the pattern distortion at the second resonance was also less compared to the other structures.     

Field simulation of dipole antennas for interstitial microwavehyperthermia

The electromagnetic field of dipole antennas for interstitial microwave hyperthermia is investigated using a finite integration algorithm program. The numerical method is applied to conventional, clinically used applicators and is also used for the improvement and optimization of sophisticated applicators, e.g., in a triaxial technique. Simulations of the frequency dependent impedance match, the E-field and the specific absorption rate (SAR) distribution of different applicators immersed in a muscle phantom are presented and compared with measurements. Moreover, results for arrays of two and four applicators are given. The field simulation allows one to study the effects at the various discontinuities of the applicator-catheter-tissue system and gives a better understanding of known phenomena     

Solution of Maxwell's equations for log-periodic dipole antennas

A theory of the log-periodic dipole antenna, which is a solution of the antenna boundary-value problem, is presented here. The theory is derived from Maxwell's equations by solving the wave equation in cylindrical coordinates and satisfying all boundary conditions. The theory is not limited to the log-periodic dipole antenna, but can be easily modified and applied to other antenna configurations using parallel linear elements. The radiation coupling between all antenna elements is taken into account; the calculated results show good agreement with the measurements. Current distributions, radiation patterns, and antenna input impedances are considered, and the application of this theory to the problem of optimal log-periodic dipole antennas is presented as well. Such an antenna obtained by numerical computation is discussed in detail.