A new method of analysis of the near and far fields of paraboloidal reflectors

A simple derivation of the basic design equation for offset dual reflector antennas with rotational symmetry and zero cross polarization

Abstract: A simple geometric derivation is given of the equation for designing an offset dual reflector antenna with perfect rotational symmetry and zero cross polarization.

Modal behavior of spherical waves from a source of EM radiation with application to spherical scanning

Abstract: An exact analysis for deriving closed-form expressions for the coefficients of free-space spherical modes radiated by typical linear, array, and aperture type of antennas is presented. This analysis is employed to determine the number of spherical modes required to reconstruct the near/ far fields (NF/FF) with a prescribed accuracy. The analysis developed together with the theory of probe-compensated spherical scanning is also employed to arrive at a criterion for the minimum distance of separation between the probe and test antenna which will enable accurate nonprobe corrected spherical scanning possible.

The SNFGTD method and its accuracy

Abstract: The spherical near-field geometrical theory of diffraction (SNFGTD) method is an extended aperture method by which the near field from an antenna is computed on a spherical surface enclosing the antenna using the geometrical theory of diffraction. The far field is subsequently found by means of a spherical near-field to far-field transformation based on a spherical wave expansion of the near field. Due to the properties of the SNF-transformation, the total far field may be obtained as a sum of transformed contributions which facilitates analysis of collimated beams. It is demonstrated that the method possesses some advantages Over traditional methods of pattern prediction, but also that the accuracy of the method is determined by the quasioptical methods used to calculate the near field.

Radiation characteristics of spherical triangular array antenna

Abstract: Radiation characteristics of a type of spherical triangular array composed of turnstile elements are numerically analyzed and discussed. It is found that the spherical array proposed here is able to scan its beam up to large angle without variation of the directivity and the circular polarization property.

Front-to-back ratio of paraboloidal reflectors

Abstract: An analysis is presented which uses the uniform geometrical theory of diffraction for determining the near fields diffracted by a paraboloid either with or without a conical flange attached to its circular rim when an axially propagated plane wave is incident on the concave or convex portion of the paraboloidal reflector. The field correlation theorem is used to determine the power coupled by a prime focus feed associated with the paraboloid which is being illuminated by a uniform plane wave. Based on this analysis, the front-to-back ratio of unflanged and flanged paraboloids is computed. Computed results show satisfactory agreement with the available measured as well as computed results based on alternative procedures. The variation in the on-axis gain on a prime-focus reflector when the feed is displaced from the focus is studied. Typical computed results are presented and compared with the available measured data. Computed results on the front-to-back ratio of paraboloids (flanged or unflanged) illuminated by a PFF whose radiated field exhibits phase variation over a constant radius are also presented. >

Evaluation of antenna Q

Abstract: A method is presented for evaluating the Q of an antenna, where the Q is defined as in conventional network theory. The method consists of subtracting the energy density associated with the power flow from the total energy density, thus enabling the magnetic and electric reactive energy to be computed. Specific application of the method is made to the evaluation of the Q of spherical and cylindrical modes. It is verified that the Q becomes very large when the order of the mode exceeds k_{0}a where a is the radius of the sphere or cylinder on which the sources are located.

Multiple scattering of EM waves by spheres part I--Multipole expansion and ray-optical solutions

Abstract: Solution to the multiple scattering of electromagnetic (EM) waves by two arbitrary spheres has been pursued first by the multipole expansion method. Previous attempts at numerical solution have been thwarted by the complexity of the translational addition theorem. A new recursion relation is derived which reduces the computation effort by several orders of magnitude so that a quantitative analysis for spheres as large as 10\lambda in radius at a spacing as small as two spheres in contact becomes feasible. Simplification and approximation for various cases are also given. With the availability of exact solution, the usefulness of various approximate solutions can be determined quantitatively. For high frequencies, the ray-optical solution is given for two conducting spheres. In addition to the geometric and creeping wave rays pertaining to each sphere alone, there are rays that undergo multiple reflections, multiple creeps, and combinations of both, called the hybrid rays. Numerical results show that the ray-optical solution can be accurate for spheres as small as \lambda/4 in radius is some cases. Despite some shortcomings, this approach provides much physical insight into the multiple scattering phenomena.

Transmission through a Conducting Screen Perforated Periodically with Apertures

Abstract: A general solution to the problem of determining first the aperture field distribution and then the transmission and reflection coefficients of an infinite planar conducting sheet perforated periodically with apertures has been formulated. The excitation is considered to be a plane wave incident at any arbitrary angle. The aperture dimensions and array element spacings were assumed to be comparable with the wavelength of the incident electromagnetic field. The solution given can include the effect of a dielectric slab used to support the thin conducting sheet. The solution is obtained by matching the tangential field components at the surface of the screen. The resulting integral equation is solved by the method of moments which reduces the integral equation to a system of linear algebraic equations that can be solved with the use of a digital computer. Accurate results for both the magnitude and phase of the aperture field distribution and the transmission coefficients for the propagating modes are determined explicitly for a specific example of slots arranged in an equilateral triangular lattice. The balance of power flow between the reflected and the transmitted waves has been checked with satisfactory results. The solution can be applied to the problem of scattering from a conducting screen with periodic apertures and to the complementary problem of scattering from a set of conducting plates by the use of Babinet's principle.

Determination of far-field antenna patterns from near-field measurements

Abstract: In many cases, it is impractical or impossible to make antenna pattern measurements on a conventional far-field range; the distance to the radiating far field may be too long, it may be impractical to move the antenna from its operating environment to an antenna range, or the desired amount of pattern data may require too much time on a far-field range. For these and other reasons, it is often desirable or necessary to determine far-field antenna patterns from measurements made in the radiating near-field region; three basic techniques for accomplishing this have proven to be successful. In the first technique, the aperture phase and amplitude distributions are sampled by a scanning field probe, and then the measured distributions are transformed to the far field. In the second technique, a plane wave that is approximately uniform in amplitude is created by a feed and large reflector in the immediate vicinity of the test antenna. And in the third technique, the test antenna is focused within the radiating near-field region, patterns are measured at the reduced range, and then the antenna is refocused to infinity. Each of these techniques is discussed, and the various advantages and limitations of each technique are presented.

A Survey of the Theory of Wire Grids

Abstract: The paper gives a survey of the literature concerning the electromagnetic properties of wire grids. As an introduction to the literature survey, a short description of the properties and applications of wire grids is given. Finally some particular grid configurations are mentioned.