Benedikt A. Munk

Reflection properties of periodic surfaces of loaded dipoles

Two-dimensional periodic arrays of dipoles or slots act as reflecting or transmitting surfaces, respectively, which have bandpass filter characteristics. The resonant frequency and the bandwidth may be controlled by varying the length, spacing, and load impedance of the dipoles (slots). A theoretical and experimental investigation of the scattering by a two-dimensional array of loaded dipoles is described. The scattering through the resonance region shows that a unit reflection coefficient is achieved. The effect of grating-lobe radiation is included. The scattering properties as a function of the angle of incidence are given for both loaded and unloaded dipoles. The loaded dipole array described in this paper produces a narrower bandwidth than the array of unloaded dipoles, and the resonant frequency is much less dependent on the angle of incidence. The resonant frequency of the array as well as the bandwidth depends strongly on the resonant frequency of the dipole element as would be expected; however, it is also substantially influenced by the interelement spacing and the angle of incidence.

On Designing Jaumann and Circuit Analog Absorbers (CA Absorbers) for Oblique Angle of Incidence

Most investigations of Jaumann and circuit analog absorbers (CA absorbers) consider normal angle of incidence only. This paper expands our investigation to also include oblique angle of incidence as well as arbitrary polarization. Essentially two problems are encountered for oblique angle of incidence: An upward shift of the center frequency; and a mismatch that, in general, will vary with angle of incidence of the incident field. The new contribution given in this paper is that it considers ways to combat both of these dilemmas. Finally, comparison will be made with similar designs obtained by the genetic algorithm (GA) approach. It will be observed that the analytic approach, as used in this paper, in general leads to designs that are not only superior to the GA designs but also simpler, at least in the present case

The effect of perturbating a frequency-selective surface and its relation to the design of a dual-band surface

A single-layer dual-band frequency selective surface (FSS) created by the perturbation of a single-band dipole array is studied. A simple algebraic formulation is derived to analyze both element and spacing perturbations. Guidelines are then proposed to allow dual-band FSSs to be designed with no anomalous behavior occurring in the two desired bandstop responses. Examples of unsuccessful and successful dual-band FSSs are calculated.

Transmission through a two-layer array of loaded slots

The mutual admittance method is used to calculate the transmission through a two layer array of slots. The slot arrays are thin and parallel. The plane wave illumination is at an arbitrary angle in either the E -plane or H -plane. The mutual admittance between two slot antennas, either in the same or different planes, is derived. In this derivation it is assumed that the slots are small enough so that when short-circuited at their terminals the electric field scattered from the slot is negligible. The evaluation of the mutual admittance sums is simplified by the extension to the slot case of certain impedance relationships established previously for a two-layer dipole array. For many applications the two-layer slot array is more desirable than a single-layer array since a wider passband and sharper skirts can be obtained with proper design. The analysis shows, however, that for high incidence angles the dip in the center of the passband may be deeper than for the similar two-layer dipole array.

Mid-infrared filters using conducting elements

This paper presents measured data on mid-infrared filters using conducting elements based on a simple first-order design philosophy. To the best of our knowledge, the filter center frequencies are at least a factor of 10 higher than any previously reported. The use of conducting elements adds an additional degree of freedom, which may be exploited to obtain filter characteristics superior to simple multilayer dielectric filters.

Edge effects of truncated periodic surfaces of thin wire elements

The effects of truncating a periodic structure of thin wire elements is examined. The structure is assumed to be infinite in extent along a single axis, enabling the analysis to be simplified via Floquets currents on the infinite axis for plane wave incidence. The analysis of an infinite linear array of elements is thereby reduced to that of a single element. Scattered fields are presented for several truncated planar geometries and are compared to three approximate solutions that use unperturbed two-dimensionally infinite Floquet currents, diffraction from a strip, and a physical optics solution for a strip, respectively. >

Reflection properties of two-layer dipole arrays

The mutual impedance method for determining the reflection from a loaded dipole array is extended to the case of two parallel planar dipole arrays. The dipole arrays are illuminated by a plane wave with arbitrary incidence angle in either the E - or H -plane. The specular reflection coefficient obtained is shown, with proper design, to have a more narrow stop band with steeper skirts and a flatter top than that for a single array. The near-field coupling is included in the analysis, and the criteria for the validity of a simple transmission line solution are given. For close spacings the near fields may cause the reflection coefficient curve to: 1) not reach unity reflection; 2) attain unity reflection at one frequency; or 3) attain unity reflection at two frequencies, with a shallow dip between. The type of resonance curve obtained depends on the spatial arrangement of the two arrays. The evaluation of the mutual impedance sums is greatly simplified by certain impedance relationships that are presented. Calculated and measured reflection curves for various array separations are included.

The broad-band scattering response of periodic arrays

A technique for computing the broad-band scattering from periodic arrays of linear antennas is presented. This technique is based upon the expansion of the current distribution on each element in the array into a set of resonant modes. With the proper set of modes this technique is capable of interpreting the frequency response of many periodic arrays. This implies thai resonances can be attributed to certain modes on the elements while antiresonances are caused by the interaction between two or more modes. The scope of this paper will be limited to arrays of dipoles and complementary arrays of slots. It will be shown that this technique is capable of giving good agreement between measured and computed results of the scattering in both amplitude and phase of large two-dimensional dipole or slot arrays.

Matching properties of arbitrarily large dielectric covered phased arrays

A method is presented for approximating the impedance or admittance properties of arbitrarily large but finite dielectric covered phased arrays which use a plane wave expansion of the fields radiated by the elements. The method is general in its application to dipole, slot, or waveguide geometries even though results from dipoles near resonance are used in this paper. Results calculated using the method are compared to a well-established procedure for an array in free space. Further results for a dipole array in a dielectric slab are presented, and a problem which occurs due to a surface wave phenomenon is dealt with in an approximate manner using the insights possible from the physical interpretations of the method. The method is also shown to be valuable in its predictions of edge effects and behavior of impedances across the extent of large finite arrays which methods that work with infinite arrays or small finite arrays cannot accomplish.

On Stabilization of the Bandwidth of a Dichroic Surface by Use of Dielectric Slabs

ABSTRACT Dichroic surfaces, also called Frequency Selectric Surfaces (FSS), are typically made of periodic surfaces of either dipole or slot like elements. In either case, a significant change of bandwidth is generally experienced as a function of Incidence angle and polarization. In this paper it is shown that using dielectric slabs with the surface can greatly reduce this bandwidth variation. The dielectric slabs must have the proper dielectric constant (usually less than 2) and proper thickness (approximately a quarter wavelength, measured In the slab). An example of such a dichroic surface is given. This surface is reflective in the range 27.5 to 30.0 GHz and transparent in the range 17.7 to 20.2 GHz. The surface operates for arbitrary polarization and incidence angles from 0° (normal) to 75°. It is comprised of two dipole like periodic structures sandwiched between three dielectric slabs.