R. Collin

Bio: R. Collin is an academic researcher from Case Western Reserve University. The author has contributed to research in topics: Aperture & Scattering. The author has an hindex of 10, co-authored 20 publications receiving 782 citations.

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.

Radiation of a Hertzian dipole immersed in a dissipative medium

Abstract: The general radiation formula for a Hertzian dipole immersed in an isotropic dissipative medium of infinite extent has been derived. As a boundary condition of the source, it is assumed that the dipole moment is a given quantity. When the conductivity of the medium is finite, the total radiating power is found to be infinite. Thus, in order to define a finite physically meaningful quantity, the dipole must be "insulated." The total radiating power is then a function of the thickness of the insulator and the constants of the media. When the radius of the spherical insulator is large compared to a wavelength, the reflection coefficient of the wave traveling from the dielectric to the dissipative medium with the dipole as a source reduces to that of a plane wave as derived from Fresnel's equations. The similarity between this and the problem by Weyl (1919) is discussed.

Some observations about the near zone electric field of a Hertzian dipole above a lossy earth

Abstract: In some recent publications, King (1990, 1992) and King and Sandler (1994) have provided formulas for the electromagnetic field radiated by an infinitesimal vertical Hertzian dipole above a lossy homogeneous half space (earth) and above a lossy half space coated by a thin dielectric layer. Several authors have questioned the accuracy of the King-Sandler formulas, particularly for field points close to the dipole (Yokoyama (1995), Wait (1996), Mahmoud (1999)). The authors have responded to these critiques by asserting that their formulas are accurate subject only to the restriction that the magnitude of the relative permittivity of the lossy half space is greater than 9. The principal results they obtain are that the near zone fields are independent of the electrical parameters of either the coating or the homogeneous half space. In this communication we provide some results obtained from a numerical evaluation of the exact integral for the near zone axial electric field along with numerical results obtained from an evaluation of the approximate integral used by King and Sandler. The King-Sandler formulas show that there is no contribution to the near zone axial electric field from a term they called the surface wave field. Our numerical evaluations show that this is not the case, in actual fact the surface wave term makes a significant contribution to the near zone axial electric field. We also present a quasistatic analysis that provides an alternative formula for calculating the near zone field. Our analysis supports the conclusions reached by Wait and Mahmoud.

Electromagnetic scattering from perfectly conducting rough surfaces (a new full wave method)

Abstract: A method that does not make use of the telegraphist's equations and takes into account the two-dimensional roughness of the surface from the start is developed. It is shown that the scattering coefficients obtained agree with those given in earlier work by E. Bahar (1973, 1987). The method is based on reducing the three-dimensional scattering problem to a two-dimensional problem by expanding each rectangular component of Maxwell's equations in terms of local basis functions along the perpendicular direction to the mean surface. The transformed two-dimensional field equations are solved using Fourier transforms. The full wave solutions are also compared with the first-order perturbation solutions, the Kirchhoff-type solutions, and integral equation results. >

Equivalent line current for cylindrical dipole antennas and its asymptotic behavior

Abstract: The concept of an equivalent line source for representing the current on a cylindrical dipole antenna is introduced. It is shown that the Fourier series coefficients of this equivalent line source are related to those of the actual antenna current by exponentially growing factors that grow rapidly for the higher order harmonics. This is used to explain why Hallen's integral equation using an approximate kernel does not have an exact solution. The asymptotic behavior of the Fourier coefficients is established. An explanation of why approximate solutions to the approximate integral equation often provide good results for the current and input impedance is also given.

A re-examination of the fundamental limits on the radiation Q of electrically small antennas

Abstract: An exact method, which is more straightforward than those previously published, is derived for the calculation of the minimum radiation Q of a general antenna. This expression agrees with the previously published and widely cited approximate expression in the extreme lower limit of electrical size. However, for the upper end of the range of electrical size which is considered electrically small, the exact expression given here is significantly different from the approximate expression. This result has implications on both the bandwidth and efficiency limitations of antennas which fall into this category.

Impedance, bandwidth, and Q of antennas

Abstract: To address the need for fundamental universally valid definitions of exact bandwidth and quality factor (Q) of tuned antennas, as well as the need for efficient accurate approximate formulas for computing this bandwidth and Q, exact and approximate expressions are found for the bandwidth and Q of a general single-feed (one-port) lossy or lossless linear antenna tuned to resonance or antiresonance. The approximate expression derived for the exact bandwidth of a tuned antenna differs from previous approximate expressions in that it is inversely proportional to the magnitude |Z'/sub 0/(/spl omega//sub 0/)| of the frequency derivative of the input impedance and, for not too large a bandwidth, it is nearly equal to the exact bandwidth of the tuned antenna at every frequency /spl omega//sub 0/, that is, throughout antiresonant as well as resonant frequency bands. It is also shown that an appropriately defined exact Q of a tuned lossy or lossless antenna is approximately proportional to |Z'/sub 0/(/spl omega//sub 0/)| and thus this Q is approximately inversely proportional to the bandwidth (for not too large a bandwidth) of a simply tuned antenna at all frequencies. The exact Q of a tuned antenna is defined in terms of average internal energies that emerge naturally from Maxwell's equations applied to the tuned antenna. These internal energies, which are similar but not identical to previously defined quality-factor energies, and the associated Q are proven to increase without bound as the size of an antenna is decreased. Numerical solutions to thin straight-wire and wire-loop lossy and lossless antennas, as well as to a Yagi antenna and a straight-wire antenna embedded in a lossy dispersive dielectric, confirm the accuracy of the approximate expressions and the inverse relationship between the defined bandwidth and the defined Q over frequency ranges that cover several resonant and antiresonant frequency bands.

Metamaterial-based efficient electrically small antennas

Abstract: A metamaterial paradigm for achieving an efficient, electrically small antenna is introduced Spherical shells of homogenous, isotropic negative permittivity (ENG) material are designed to create electrically small resonant systems for several antennas: an infinitesimal electric dipole, a very short center-fed cylindrical electric dipole, and a very short coaxially-fed electric monopole over an infinite ground plane Analytical and numerical models demonstrate that a properly designed ENG shell provides a distributed inductive element resonantly matched to these highly capacitive electrically small antennas, ie, an ENG shell can be designed to produce an electrically small system with a zero input reactance and an input resistance that is matched to a specified source resistance leading to overall efficiencies approaching unity Losses and dispersion characteristics of the ENG materials are also included in the analytical models Finite element numerical models of the various antenna-ENG shell systems are developed and used to predict their input impedances These electrically small antenna-ENG shell systems with idealized dispersionless ENG material properties are shown to be very efficient and to have fractional bandwidths above the values associated with the Chu limit for the quality factor without any degradation in the radiation patterns of the antennas Introducing dispersion and losses into the analytical models, the resulting bandwidths are shown to be reduced significantly, but remain slightly above (below) the corresponding Chu-based value for an energy-based limiting (Drude) dispersion model of the permittivity of the ENG shell

PCS antenna design: the challenge of miniaturization

Abstract: PCS (personal communication system) devices have become an important part of everyday life. The pressure to design small, lightweight, and user-friendly mobile-communication devices has increased accordingly, creating the need for optimal antennas for mobile applications. In this paper, we present some basic rules about electrically small antennas, give clues and guidelines about efficient antenna miniaturization, and, finally, show some examples of miniature antennas developed in our laboratory for practical applications.

The Koch monopole: a small fractal antenna

Abstract: Fractal objects have some unique geometrical properties. One of them is the possibility to enclose in a finite area an infinitely long curve. The resulting curve is highly convoluted being nowhere differentiable. One such curve is the Koch curve. In this paper, the behavior the Koch monopole is numerically and experimentally analyzed. The results show that as the number of iterations on the small fractal Koch monopole are increased, the Q of the antenna approaches the fundamental limit for small antennas.