On the interpretation for radiation from simple currentdistributions

We have considered the radiation from two simple filamentary current distributions: traveling-wave and uniform. For an excitation that is a Gaussian pulse of characteristic time τ, the total energy radiated by the distributions, U_rad, was shown to behave as ln(τ_a/τ) for the traveling-wave distribution, and as τ_a/τ for the uniform distribution, where τ_a =h/c is the time for light to travel the length of the filament. An examination of numerical results shows that two physical interpretations can be used for the radiation. Radiation can be considered to arise at the two ends of the filament in the form of spherical wavefronts centered at the ends. The overlap of these wavefronts, which changes with the ratio τ_a/τ, is the cause of the observed dependencies for U_rad. An alternative explanation is that radiation occurs along the entire length of the filament. Destructive interference of the radiation from different points on the filament then causes the radiation to be insignificant except at the times corresponding to radiation from the ends of the filament     

On the interpretation for radiation from simple currentdistributions

It is indisputable that Maxwell's equations provide a unique electromagnetic field for any reasonable distribution of charge/current. However, the interpretation of the results from such calculations may not be unique; different physical models may be used to explain the same results. We examine the radiation from similar filamentary current distributions (traveling-wave and uniform) for pulse excitation. For pulse excitation, it is easier to establish the causal relationship between the elements of current and the radiated field. We also show, for these simple distributions, that the total energy radiated for pulse excitation depends on the length of the filament in the same way as for the time-harmonic case     

Ultrawide-band modeling of transient radiation from aperture antennas

A new method for the numerical calculation of transient field radiated through aperture-type antennas (slot, open-ended waveguide, and horn) is described. The finite-difference time-domain method is applied for the near-field prediction in the close surrounding of the antenna and a proper data-fitting procedure of the aperture field, involving interpolating functions with separation of space- and time dependence, permits: 1) to calculate "off-line" the radiated field without the need to store a great amount of data; 2) to avoid, in the case of far field, the numerical evaluation of radiation integral; and 3) to obtain approximate far field formulas which are still separable with regard to space and time. The method enables a full data reusability in calculation of field pattern over a wide angular range at a same time, or of the transient response at fixed observation points.     

Theoretical solutions of transient radiation from traveling-wavelinear antennas

Analytic expressions based on time-domain integral equations for the electric and magnetic field intensities are developed for linear antennas of finite length. With the help of Fourier transformation and successive partial integrations, explicit closed-form expressions for the transient radiation are obtained. There are no restrictions on the waveform of the current along the antennas, the length of antennas, and the position of observations. The method can also be adopted to solve for the transient radiations due to reflected traveling-wave currents along the antennas, with similar results     

An analysis of the transient fields of linear antennas

Theoretical solutions for transient radiation from traveling-wave linear antennas were published recently by Chen (see ibid., vol.30, p.80-3, 1988), but the results have some mistakes, and the methods are tedious. A solution in which each term has obvious physical meaning is obtained here in a simple and direct way. The characteristics of the solutions are discussed and compared with strict numerical results. In the time window, the solutions are of a simple form, and their natures are independent of distance. The transient far-field conditions are obtained directly in the time domain     

Accurate computation of the radiation from simple antennas usingthe finite-difference time-domain method

Two antennas are considered, a cylindrical monopole and a conical monopole. Both are driven through an image plane from a coaxial transmission line. Each of these antennas corresponds to a well-posed theoretical electromagnetic boundary value problem and a realizable experimental model. These antennas are analyzed by a straightforward application of the finite-difference-time-domain (FD-TD) method. The computed results for these antennas are shown to be in excellent agreement with accurate experimental measurements for both the time domain and the frequency domain. The graphical displays presented for the transient near-zone and far-zone radiation from these antennas provide physical insight into the radiation process     

Broad-band microwave measurements with transient radiation fromoptoelectronically pulsed antennas

A broadband microwave measurement technique based on picosecond transient radiation from optoelectronically pulsed antennas is described. It is performed with exponentially tapered coplanar stripline antennas which are integrated with the photoconductive devices used for ultrafast pulse generation and sampling. The signal analysis required for deriving the desired physical properties from the measured time-domain waveforms is discussed. This is a coherent technique that independently determines both the real and the imaginary parts of the dielectric constants of materials, from 10 to 130 GHz, in a single experiment. Some representative results are presented     

A simple FDTD model for transient excitation of antennas bytransmission lines

A simple FDTD model is developed for use with antennas that are fed from transmission lines. The model is especially designed for use with transient excitations, where the incident and reflected waveforms within the transmission line are of interest, and the latter is determined directly in the FDTD calculation. The model is verified for both transmission and reception of transient waveforms by comparison with measured results for a cylindrical monopole antenna with a plane reflector     

Analytical solution for early-time transient radiation frompulse-excited parabolic reflector antennas

A closed-form analytical solution is developed for predicting the early-time transient electromagnetic fields which are generated by a perfectly conducting parabolic reflector antenna when it is illuminated by a transient step spherical wave due to an elemental Huygen's source located at the focus. This closed-form time-domain solution, which is valid both near and far from the reflector (and anywhere in the forward region) can be used via the convolution theorem to efficiently obtain the early-time transient fields generated by the same parabolic reflector antenna when it is illuminated by a realistic finite-energy pulse which emanates as a spherical wave from the focus. The transient solution is developed here by analytically inverting, in closed form, the corresponding frequency-domain solution in terms of a radiation integral that employs an asymptotic high-frequency geometrical optics (GO)-based approximation for the fields in the aperture. Numerical results are presented for the transient fields both near and far from the reflector. The fields on boresight exhibit an impulse-like behavior similar to that of the impulse radiating antenna (IRA) introduced by Baum et al. (1989, 1993)     

Improvements to nonnumerical methods for calculating the transient behaviour of linear and aperture antennas

The time-domain response of linear and aperture antennas is analyzed. The results are based on models which have proven to be satisfactory for sinusoidal steady-state analysis. The calculated results correlate the physical parameters of both transmitting and receiving structures with the time-domain characterization of their response. The methods presented herein are useful to provide simple yet powerful alternatives to numerical techniques for the solution of transient radiation problems. The time-domain impulse response is calculated for thin cylindrical dipoles and the aperture formed by two semi-infinite parallel planes. Comparison between experiment and theory is provided. Analytical expressions are derived which characterize the response to excitations that are bandlimited in a manner dictated by the models, e.g., thin wire approximation and TEM mode excitation limitations.     

Broadside radiation from periodic leaky-wave antennas

A double-strip grating leaky-wave antenna consisting of two strips per unit cell is analyzed. The stopband behavior exhibited at broadside scan in the single-strip grating antenna is characteristic of all periodic leaky-wave antennas having a single strip per unit cell, and results in a drastic increase in the attenuation rate of the leaky wave as the beam is scanned to broadside. By nearly eliminating this stopband behavior, the double-strip leaky-wave antenna can scan from backward end fire to forward end fire without any large frequency regions of high attenuation. An approximate design rule for the double-strip antenna is discussed, and results are presented to show how the antenna may be further optimized to achieve the minimum possible variation in attenuation as the beam is scanned through broadside. Although the stopband behavior is never completely eliminated with the addition of the extra strip, the optimum design shows an almost negligible region of rapidly varying attenuation near broadside     

On the radiation patterns of parasitic loop counterpoise antennas

The radiation properties of parasitic loop counterpoise antennas are investigated in detail. It is found that the parasitic loop concept can be used advantageously to reduce considerably the undesirable effects produced in loop counterpoise antenna patterns due to the counterpoise edge diffraction effects. Results of extensive numerical and experimental investigation of various aspects of the parasitic loop counterpoise antenna patterns are reported. The method of design of such antennas is also discussed. Three such new antennas are proposed which appear to be capable of bringing out superior performance from the present-day VOR system.     

On the radiation fields of rhombic antennas buried in snow

The purpose of this letter is to describe a method of approximately determining the radiation fields of rhombic antennas buried in snow. This method is based on reciprocity arguments in conjunction with an assumed current distribution in the rhombic.     

Local oscillator radiation from active integrated antennas

Active receiving antennas with an integrated filter, amplifier, mixer and local oscillator have significant radiation at the local oscillator frequency. This radiation may be out of band and could be a problem in cluttered environments. In this Letter, the problem is evaluated through measurements and simulation on integrated microstrip patch antennas     

Forward radiation from axially symmetric reflector antennas

The fields radiated from focus-fed symmetric reflector antennas in the vicinity of forward axial direction are determined by a series representation of the physical optics integral. The considered feeds have cosine-tapered patterns with different tapers inE- andH-planes. The influence of feed pattern asymmetry, subreflector blockage, feed taper, focal distance to diameter (f/D) ratio and the reflector size on the performance parameters of paraboloidal, Cassegrain, near-field Cassegrain, and corresponding Gregorian antennas is investigated. Design curves are presented to predict the performance parameters of the considered antennas.     

Far-field performance of linear antennas determined from near-fielddata

The main plane far-field radiation pattern of an antenna under test from the corresponding main plane near-field data, using a circular-line acquisition, is presented. The method is based on the reconstruction of equivalent magnetic currents (EMCs) using decoupled integral equations and one-dimensional source components. The resultant fast procedure is applicable to linear and quasilinear array antennas. Experimental data results and comparison with complete spherical acquisition and center-line acquisition are presented     

Analysis of transient responses between coupled dipole antennas by using a simple equivalent circuit

Transient responses between two dipole antennas were analyzed by using a simple equivalent circuit of a dipole antenna. This equivalent circuit is composed of a lossless transmission line and two shunt resistances representing electrical discontinuities at a feed point and tips of the dipole antenna. A closed-form formula for the induced load voltage in the time domain was derived. Since the damping factor due to radiation from antennas is included in this expression, this formulation gives better results than those given by the conventional transmission-line approximation. Measured time histories support the validity of theoretical results.     

On the application of parametric models to the transient analysisof resonant and multiband antennas

The transient responses of a resonant antenna and a fractal multiband antenna, both formed by conducting surfaces, are calculated. The surfaces are modeled by planar triangular patches and the study is carried out by solving the time-domain electric-field integral equation (TD-EFIE). Linear parametric modeling techniques are applied to considerably reduce the computation time. Numerical results are compared with experimental measurements     

Radiation from linear antennas in a dissipative half-space

A formulation for the electromagnetic fields of finite linear antennas of arbitrary length immersed in a dissipative halfspace (such as the ocean or the earth) is presented. The electric fields in either medium can be readily evaluated once the current distribution is assumed or prescribed. The fields are given in each medium for each of the three major subdivisions of the horizontal range, the near-field range, the intermediate range, and the asymptotic range. Antenna patterns obtained from computer results using formulas derived in this paper are presented for some typical submerged linear antennas. These computer results are compared with experimental measurements performed with linear antennas submerged in the Atlantic Ocean.