Brad A. Kramer

Size Reduction of a Low-Profile Spiral Antenna Using Inductive and Dielectric Loading

This letter discusses a miniature low profile ultrawideband (UWB) spiral. The antenna is miniaturized using a combination of dielectric and inductive loading. In addition, a ferrite coated ground plane is adopted in place of the traditional metallic ground plane for profile reduction. Using full-wave simulations and measurements, it is shown that the miniaturized spiral can achieve similar performance to a traditional planar spiral twice its size.

A Novel Reflective Surface for an UHF Spiral Antenna

As is well known, metallic cavity backings reduce antenna gain particularly for broadband configurations at low frequencies. This letter presents a novel reflective surface consisting of a ferrite tile with variable conductive coating as a backing for a conformal UHF spiral antenna. The ferrite material is used in the outer region to preserve the free-space antenna gain at low frequencies. Toward the center, the backing tapers to a conductive coating making it suitable for high frequency operation. It is shown that this hybrid surface (ferrite to metallic) improves the antenna performance significantly at lower frequencies without compromising the high frequency gain.

Fundamental Limits and Design Guidelines for Miniaturizing Ultra-Wideband Antennas

Fundamental physical limitations restrict an antennas performance based on its electrical size. These fundamental limits are of the utmost importance, since the minimum size needed to achieve a particular figure of merit can be determined from them. In this paper, the physical limitations of antennas are reviewed in general, with particular emphasis on impedance matching as it relates to ultra-wideband (UWB) antennas (high-pass response). Additionally, the use of antenna miniaturization techniques to approach these limits is discussed. Using a spiral antenna as an example, guidelines are presented for miniaturizing UWB antennas.

Antenna Miniaturization Using Magnetic-Photonic and Degenerate Band-Edge Crystals

Engineered materials, such as new composites and electromagnetic bandgap and periodic structures have been of strong interest in recent years, due to their extraordinary and unique electromagnetic behaviors. This paper discusses how modified materials, inductive/capacitive lumped loads, and magnetic materials/crystals are impacting antenna miniaturization and performance improvements (e.g., bandwidth and gain reduction, multi-functionality, etc.). Dielectric design and texturing for impedance matching has led to significant size reduction and higher-bandwidth low-frequency antennas, for example. The recently introduced magnetic-photonic crystals (MPCs) and double band-edge (DBE) materials, displaying spectral nonreciprocity, are also discussed. Studies of these crystals demonstrated that magnetic-photonic crystals exhibit the interesting phenomena of (a) drastic slowing down of the incoming wave, coupled with (b) significant amplitude growth, while (c) maintaining minimal reflection at the interface with free space. The phenomena are associated with diverging frozen modes that occur around the stationary inflection points within the band diagram. Taking advantage of the frozen-mode phenomena, we demonstrate that individual antenna elements and linear or volumetric arrays embedded within the magnetic-photonic crystal and double band-edge structures allow for antenna sensitivity and gain enhancements

Miniature UWB Conformal Aperture with Volumetric Inductive Loading

This paper introduces a new concept for a conformal aperture antenna that operates from 3000 MHz down to VHF frequencies. The intended size is less than 15 in. diameter, corresponding to lambda/22 at 30 MHz, and having a gain better than -15 dBi at the lowest operational frequency with a nominal gain of greater than 0 dB at the higher frequencies. A key aspect of this paper is the use of inductive loading within the entire substrate/superstrate volume rather than modifying the metallization only on the surface of the substrate. The proposed antenna is based on a spiral configuration which has been miniaturized using a combination of dielectric and inductive loading. A significant effort is also made to maintain a thin cavity-backed configuration. This is done by using a ferro-metallic ground plane that is optimally designed and integrated with the dielectric substrate along with volumetric coiling for best performance. Overall, it is shown that a 6 in. aperture can operate down to 130 MHz (lambda/15 aperture) using a combination of dielectric and volumetric inductive loading

Miniaturization methods for narrowband and ultrawideband antennas

A strong interest exists in the commercial and military sectors for small ultra-wide band antennas, such as the spiral. In this paper, we consider novel methods for miniaturizing spirals and other antennas using dielectric loading, artificial lumped loads, textured dielectrics and other approaches within the context of metamaterials. Our aim is to achieve miniaturization without compromising gain and by retaining as much bandwidth as possible. The spiral antenna, operating in the slot or wire mode, is a good choice to start with.

UWB miniature antenna limitations and design issues

A small conformal ultra-wideband (UWB) antenna has many important commercial and military applications. This stems from a multitude of factors such as cost reduction, space, payload, and, most importantly, the number of antenna systems. A common approach to reducing the number of antennas on a platform is to use a single broadband element, such as a spiral antenna. However, the issue is complicated by the fact that a single antenna element may need to operate over more than 10:1 bandwidth, such as 20 MHz to 2000 MHz. To cover such a vast frequency range with a single antenna using conventional designs would require a large physical size to achieve a desirable gain. Therefore, miniaturization techniques, such as the use of dielectric materials or reactive loading, must be used in an attempt to increase the electrical size of the antenna without increasing its physical size. We focus on antenna miniaturization using dielectrics and discuss some of the design issues related to the actual implementation of a miniature antenna.

Considerations on size reduction of UWB antennas

To successfully reduce the size of UWB antennas requires a good understanding of radiation mechanisms, impedance behaviour, acceptable trade-off between gain and bandwidth and effective methods to reduce phase near-field velocity. The fact that theoretical limitation of a small antenna is determined by the antennas size in terms of free-space wavelength regardless of the geometry or composition the antenna should also be acknowledged. Our current finding seems to indicate, inductive loading is more advantageous over capacitive loading due to the increase of antenna impedance associated with this treatment.

A miniature conformal spiral antenna using inductive and dielectric loading

In this paper we introduce a new concept for a miniature conformal antenna combines inductive and dielectric material loading for size reduction.

Some basic guidelines for miniaturizing UWB antennas

The purpose of the paper is to discuss broadband antenna miniaturization in general. The goal is to illustrate some simple guidelines that can used for miniaturizing a broadband antenna. To demonstrate these guidelines, we use a wire log-spiral antenna which is miniaturized via inductive loading. In this case, the inductive loading is achieved without modifying the antenna structure by increasing the self inductance of the wire segments from which form the spiral. This is readily accomplished using a method of moments code such as NEC which allows the user to assign a distributed or lumped impedance to each wire segment. Therefore, the phase velocity can be arbitrarily reduced depending upon the inductance per unit length. Using this approach, we discuss two important guidelines for miniaturizing an UWB antenna such as a spiral. The first is in determining how much the antenna should be miniaturized. Secondly, we examine how the loading profile impacts the performance of a spiral antenna.