Liang-Chi Shen

Resonant frequency of a circular disc, printed-circuit antenna

The resonant frequency is obtained in analytical form for a planar, circular disc antenna which is etched on a printed-circuit board so that the low-profile antenna is separated from the ground plane only by a thin layer of dielectric material. The formula is found to have an error of less than 2.5 percent when compared with experimental data.

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 when h \geq O.15\lambda .

The dipole antenna with eccentric coating in a relatively dense medium

In a medium of high permittivity or conductivity, such as sea water, lake water, or wet earth, a dielectric coated antenna can produce a directional radiation pattern when the coating is eccentric. The current distribution on a finite or infinite dipole with such a coating is obtained. Formulas for the directivity and the radiation pattern of an infinitely long dipole with eccentric coating are also derived.

Remote probing of atmosphere and wind velocity by millimeter waves

A technique is developed to probe the atmospheric turbulence strength C_{n}^{2} and the wind velocity along a path using millimeter waves as a tool. Data obtained in a line-of-sight millimeter-wave propagation experiment are processed and used as the source of information. The averaged C_{n}^{2} and wind velocity together with their gradients along the propagation path are calculated by inverting a set of integral equations. A numerical method is used to yield the least-square-error solutions. Comparison is made between the theoretically calculated wind velocity over a 33-hour period and that measured by a conventional anemometer.

A Comprehensive Study of Subsurface Propagation from Horizontal Electric Dipoles

The general exact integrals for the electromagnetic field of an electric dipole in the earth near its boundary with the air are evaluated numerically over the full range of conductivities and permittivities that characterize different parts of the earths surface. The following ranges are included: permittivities from 2 to 80, conductivities from 4 x 10-6 to 4 S/m, frequencies from 10 to 109 Hz, radial distances from 0.1 to 100 km, and depths of the source and point of observation from 0.15 to 15 m. The generally useful component of the electric field Ep is represented in graphs and contour diagrams that display the characteristics of propagation in their dependence on the frequency, the radial distance, the depth, and the electrical parameters of the earth. They are interpreted in terms of approximate formulas of Banos which are evidences of the lateral-wave nature of the field.

Directivity and bandwidth of single-band and double-band yagi arrays

The purpose of the communication is to obtain the optimum design of a Yagi array under constraints on bandwidth, directivity, or the size of the array. It is shown that the array configuration is determined if any two of these constraints are specified. The present study also shows that a properly designed Yagi array can be operated at two frequency bands with the frequency ratio approximately equal to 3.5.

The horizontal-wire antenna over a dissipative half-space: Generalized formula and measurements

It is shown and verified by experiment that a horizontal resonant or traveling wave antenna placed in air close to a dense halfspace with the properties of lake or sea water or earth behaves like a terminated lossy transmission line. The terminal impedance is related to the radiation of the antenna into the air and the complex wavenumber and characteristic impedance are those of an infinitely long line. The complex wavenumber takes account of both dissipation in and radiation into the dense half-space.

Measured field of a directional antenna submerged in a lake

Field patterns of a VHF directional antenna submerged in a lake are shown with the receiver both under and over the surface of the water. The measured relative strength of the electric field as a function of the radial distance from the source is also shown. The rate of decrease with distance of the radial component of the field in the water agrees with theoretical predictions.

Field of an antenna submerged in a dissipative dielectric medium

The field components due to a horizontal dipole antenna submerged in a lake are represented in integral forms. The integrals are then evaluated using three techniques: 1) numerical integration along branch cuts where the integrands are smoothly varying as a function of the integration variable, 2) integration along the real axis with the fast Fourier transform technique, and 3) analytical integration with the modified saddle point method. Both the transmitter and the receiver are placed very near the interface. While the analytical result is good only at the larger distances from the transmitter, the numerical integration methods provide savings in computer time for small distances from the transmitter. Thus the analytical and numerical methods are good complements of each other. All three approaches yield essentially identical results over the distance ranges of interest. The theoretical results are compared with recent experimental measurements.

Numerical analysis of wave propagation on a periodic linear array

The propagating mode on a periodic array of wires or strips is studied. Numerical solutions of the current distribution on each element, the propagation constant, and the cutoff frequencies of the propagating mode are found. It is discovered that this periodic structure in some cases can support propagating waves in two or more widely separated frequency bands.