Far-field patterns of spaceborne antennas from plane-polar near-field measurements

Certain unique features of a recently constructed plane-polar near-field measurement facility for determining the far-field patterns of large and fragile spaceborne antennas are described. In this facility, the horizontally positioned antenna rotates about its axis while the measuring probe is advanced incrementally in a fixed radial direction. The near-field measured data is then processed using a Jacobi-Bessel expansion to obtain the antenna far fields. A summary of the measurement and computational steps is given. Comparisons between the outdoor far-field measurements and the constructed far-field patterns from the near-field measured data are provided for different antenna sizes and frequencies. Application of the substitution method for the absolute gain measurement is discussed. In particular, results are shown for the 4.8-m mesh-deployable high-gain antenna of the Galileo spacecraft which has the mission of orbiting Jupiter in 1988.     

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.     

Far-field pattern analysis of 94 GHz extended hemispherical lens monopulse antennas

In this paper, the far-field patterns are investigated for double-slot antenna array placed on extended hemispherical lenses. The radiation patterns of the extended hemispherical lens fed by single double-slot antenna or double-slot antenna array are computed using ray-tracing inside the dielectric lens and electric and magnetic field integration on the spherical dielectric surface. The computation results show that the sharp null of the difference pattern is below -37dB.     

Analysis of linear coaxial antennas

Two types of linear coaxial antennas, coaxial-colinear antennas, and slotted coaxial antennas are studied to check the possibility of using them as the base-station antenna in personal communication systems. The slot voltages and input impedance of linear coaxial antennas are obtained by using a transmission-line analysis where the radiation effect is accounted by a shunt and a serial admittance, respectively. The current distribution is obtained by solving an integral equation using the method of moments. The radiation pattern and directivity are then obtained from the current distribution and the reflection coefficient inside the coaxial cable. Factors analyzed include frequency, coaxial filling permittivity and segment number     

FDTD analysis of linear antennas driven from a discrete impulseexcitation

The finite-difference time-domain (FDTD) technique has been employed to analyze a large number of electromagnetic scattering problems. However, only a limited number of problems involving antenna radiation has been investigated. Furthermore, the excitation source of the radiating systems has been mostly a Gaussian type pulse. The concept of a discrete impulse excitation is developed and used to feed linear antennas and other radiating systems. Different cases of antenna analysis are considered to illustrate the applicability of the impulse excitation. Furthermore, a comparison with the method of moments as well as the FDTD technique with Gaussian type pulse excitations is made to validate the presented technique     

Transient response of linear antennas driven from a coaxial line

The initial transient response of straight wires connected to coaxial lines is studied theoretically for the case where a pulse is applied to the coaxial line. The wave form of the return pulse is first found approximately for the case of a pulse of zero rise time. Since this does not correspond to any feasible experimental situation, the effect of a finite rise time is considered in detail. Numerical results are obtained for several special cases.     

Solution of currents on linear antennas

A method for the solution of currents on linear antennas is proposed. The need to apply boundary conditions to an integral operator is eliminated by the use of an intermediate differential equation. It is shown the method has certain advantages from the point of view of numerical analysis.     

Coupled linear antennas with skew orientation

A theory is developed and numerical results are given for some planar and nonplanar configurations     

Crosspolarisation characteristics of linear comb-line microstrip antennas

Microstrip antennas are a subject of increasing interest in the microwave antenna field, and the comb-line antenna is particularly attractive for medium to low sidelobe level operation. However, the crosspolarisation properties of such structures have not been discussed previously, although these are of importance where the suppression of sidelobes in all polarisation planes is required. A mainly experimental investigation is described into the crosspolarisation characteristics of comb-line antennas, and criteria for crosspolarisation suppression are derived for particular board characteristics at 4.3 GHz.     

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.     

On the transient radiation of energy from simple currentdistributions and linear antennas

Smith (1998) examined the radiation from two simple filamentary current distributions: traveling-wave and uniform. The radiated or far-zone electric field was computed for an excitation that was a Gaussian pulse in time. Two interpretations for the origin of the radiation were presented, based on the far-field results. The present article continues this investigation; however, the emphasis is on an examination of the near field and the related transport of energy away from the current filament. We examine traveling-wave and standing-wave current distributions, because these distributions are frequently used to model practical antennas. Exact analytical expressions are presented for the electric and magnetic fields of the assumed, filamentary current distributions when the excitation is a general function of time. For the filamentary distributions, the current and charge are confined to a line (a line source). There is no radius associated with the filament. The expressions for the fields apply in both the near and far zones, and are used to determine the Poynting vector. For an excitation that is a Gaussian pulse in time, exact analytical expressions are obtained for the energy leaving the filament per unit time per unit length, the total energy leaving the filament per unit length, and the total energy radiated. Graphical results based on these expressions are used to study the energy transport from the filamentary current distributions. The results for the standing-wave current distribution are compared with those from an accurate analysis of a pulse-excited, cylindrical monopole antenna, performed using the FDTD method     

Improving the performance of microstrip-patch antennas

This paper describes a cavity-backed patch-antenna geometry, which features multiple dielectric layers and shorting posts. These features are exploited to design antennas which retain many of the desirable characteristics of conventional microstrip antennas, yet overcome some of their inherent disadvantages     

Self- and mutual impedances of traveling-wave linear antennas

A traveling-wave distribution of current can be produced on a conventional linear antenna, such as a dipole, through resistive loading. The self- and mutual impedances of parallel traveling-wave linear antennas are measured and compared with those of the corresponding conventional linear antennas. The effects of coupling on the input impedance of an array element is discussed, and it is shown that these effects are much less pronounced for traveling-wave antennas, which may be advantageous for some array applications     

Constrained beam scanning for linear arrays of antennas

A method to scan the radiation pattern of a linear array, simultaneously reducing the pattern amplitude in constant directions, is described. The scanning process is obtained by phase control of the array excitations and, if necessary, by amplitude perturbations not exceeding a small threshold.     

Inverted-F antennas with wideband match performance

Novel inverted-F antennas, double-wire (DW-IFA) and double-strip (DS-IFA) were developed for f₀ = 2.45 GHz, and a 0.8λ ₀ in size square ground plate. The antennas are distinctive for their wideband match performance, 12% (numerical) for DW-IFA and 18% (numerical)/17% (experimental) for DS-IFA, and radiation characteristics suitable for the mobile environment scenario     

Standard linear antennas, 30 to 1000 MHz

It is demonstrated that the insertion loss between pairs of thin, linear antennas may be calculated using fairly simple equations that are generally considered to be good engineering approximations. Although the insertion loss calculation does not involve antenna gain directly (some measurements are actually made in the near-field where gain is not defined), the result is precisely the quantity obtained using the antenna gains in Friis's transmission formula, assuming the mismatch losses are zero. Therefore, the antenna gain product is implicit in the more general insertion loss equations. The particular measurement of insertion loss used here yields a quantity called site attenuation by electromagnetic compatibility engineers. A close agreement between measured and calculated data provides confidence in the site attenuation calculations when the site is essentially perfect, and provides confidence in the gain product of the antenna pair calculated using basically the same equations as those used for insertion loss. It is assumed that one-half of the mean value of the difference between the calculated and measured data is a good estimate of individual antenna performance. For the antennas described here, this measure of performance is typicallylE0.05dB and on the outside,iE0.42dB.     

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     

Design and performance of small printed antennas

Electrically small microstrip patches incorporating shorting posts are thoroughly investigated. These antennas are suitable for mobile communications handsets where limited antenna size is a premium. Techniques to enhance the bandwidth of these antennas are presented and performance trends are established. From these trends, valuable insight to the optimum design, namely broad bandwidth, small size, and ease of manufacturing, is given     

Radiation patterns and correlation of closely spaced linear antennas

A simple point source analysis is used to prove that, in theory, completely decorrelated reception can be achieved from two linear antennas with an arbitrarily small spacing. The conditions necessary to achieve this are consistent with two high gain (superdirective) beams in opposite directions. It is shown that the horizontal radiation patterns and correlation coefficient of arrays of vertically orientated linear antennas can be found via an exact relation to simple, point-source theory that includes the effects of mutual coupling. This theory leads to practically achievable optimum diversity designs at closer spacings than previously thought possible. The theory is illustrated for a dual antenna configuration and can be extended to multiple antennas.