Near-field Radiation Patterns Research Articles

Open-ended rectangular waveguides as antennas for medical diagnostics.

Open-ended rectangular waveguides, potentially useful as microwave antennas for medical diagnostics are described. Reflection coefficients have been measured for the waveguides fitted with standard flanges in contact with water and skin. Antenna matching to the tissue is discussed. Near-field radiation patterns in phantom solutions were measured to obtain operature and depth distribution patterns.

Measurement of acoustic fields using schlieren and holographic techniques

Two methods, schlieren imaging and acoustical holography, are shown to be useful for quantitatively measuring detailed nearfield and farfield acoustic radiation patterns from a submerged cylindrical target. The two imaging methods are discussed so as to illustrate the fundamental mechanisms of imaging involved. The advantages and limitations of each are made clear, and examples of typical laboratory acoustic field measurements are shown for a cylindrical aluminum target. The farfield of the target is computed from an emerging theory for nonrigid cylinders, and the experimental farfield data obtained holographically is shown to agree with that theory. The nearfield of the same target is plotted directly from the photo intensity of the schlieren image.

A relatively short cylindrical broadband antenna with tapered resistive loading for picosecond pulse measurements

A relatively short cylindrical antenna with continuously tapered resistive loading has been studied for the purpose of picosecond pulse measurements. The antenna considered is a nonconducting cylinder with continuously deposited varying-conductivity resistive loading. The current distributions on the antenna were numerically calculated using the method of moments. Using these current distributions, other quantities such as input admittance, near-field and farfield radiation patterns, and radiation efficiency, were also numerically calculated and compared with the results using the Wu-King's approximate current distribution. Agreement is relatively good except at high frequencies <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">kh &gt; \pi/2</tex> where the method of moments appears to give better results. To verify the theoretical results, several resistively loaded antennas were fabricated, and their picosecond pulse receiving characteristics were analyzed for the frequency range between 5 kHz and 5 GHz. The experimental results indicate excellent linear amplitude and phase response over the frequency range. This provides the unique capability of this antenna to measure fast time-varying electromagnetic fields with minimal pulse-shape distortion due to nonlinear amplitude or phase characteristics.

Partially cladded triangular-cored glass optical fibers and lasers*

Optical fibers that possess the characteristics of both conventional cladded fibers and of a new type of fiber have been drawn and evaluated. The core has an equilateral triangular cross section with slightly bulging curved sides. The rounded vertices are in contact with the inside of the cladding tube of lower refractive index. Most of ther periphery of the core is surrounded by air. The cavity-like waveguide modes of this new type of fiber can be described as due to skew rays being reflected systematically from the glass–to–air interfaces as they propagate along the fiber. Near-field and far-field radiation patterns have been photographed and are presented. A new type of laser based on these cavity-like waveguide modes is proposed, in which the triangular core is made of a material (e.g., neodymium-doped glass, YAG, etc.) that can be optically pumped. The normal mode of operation of the laser consists of six diverging, spatially separated output beams; it can be optically pumped axially from both ends simultaneously, while being pumped from the sides.

Acoustic radiation from a long solid rod into a semiinfinite liquid medium

An approximate numerical technique is presented whereby the pressure-radiation patterns in a liquid half-space produced by radiation from the end face of a circular cylindrical, semiinfinite, solid rod set flush in an infinite rigid baffle at the boundary of the half-space can be calculated as a function of the elastic properties of the solid rod. The method is restricted to the low-frequency range such that the lowest numbered mode in the solid rod is the only propagating mode. Axisymmetric longitudinal modes are the only ones assumed to exist in the rod. The velocity distribution of the rod end face does not have to be known or assumed. The radiation impedance of the rod end is calculated and compared with that of the rigid piston, as are the pressure radiation fields. The work here presents farfield radiation only, though the method can easily be extended to study the nearfield radiation patterns in the liquid half-space.

The estimation of aerial radiation patterns from limited near-field measurements

A series expression for the near-field radiation pattern of an aerial obtained with a directive measuring aerial in a two-dimensional system is derived from a cylindrical-mode expansion of the radiated field. Patterns calculated from this series show better agreement with experimental results at shorter ranges than is provided by previous series evaluations. An inversion of the first few terms of this series suggests a convenient technique for the determination of the radiation pattern from near-field measurements. Calculations based on this method of inversion indicate that the approximate shape of the pattern may be obtained with relatively little effort from the near-field pattern yielded by a small measuring aerial at a range of a2/λ, where a is the aperture length of the transmitting aerial. At ranges considerably shorter than a2/λ, a more directive measuring aerial, which provides a closer initial approximation to the far-field pattern, has the advantage of reducing the amount of calculation required.

An investigation of near-field radiatin patterns measured with large antennas

The plane-wave spectrum concept is used to derive an expression for the radiation pattern of an antenna in a two-dimensional system and an investigation is made of the effect of the aperture size of the measuring antenna on the pattern obtained. It is shown that measured sidelobe levels either above or below the far-field values may be obtained. Measurements of near-field antenna radiation patterns are carried out in a parallel-plate pattern range, and the predicted effects of measuring antennas of finite size on the beamwidth and sidelobe levels of the measured patterns are confirmed experimentally.