Karu P. Esselle

A planar resonator antenna based on a woodpile EBG material

A resonator antenna made from a complex artificial surface and a metallic ground plane is described. The complex surface is realized using a woodpile electromagnetic bandgap (EBG) material, which is shown to have a frequency dependent reflection plane location. A highly directive radiation pattern is created due to the angle-dependent attenuation of the resonator antenna coupling to free space. The antenna has the advantages of low height, low loss, and low sidelobes. It is shown that the directivity can be varied over a fixed range by changing the aperture size of the device, with the maximum directivity determined by both the feed element and EBG material properties. The complete bandgap for the woodpile EBG material is confirmed from a band diagram, and its properties as a complex surface are investigated through transmission calculation and measurement. The design of the antenna is described, and two means of exciting the resonator, a microstrip patch and a double slot, are investigated. Theoretical results for these two antennas are calculated the using finite-difference time-domain and are shown to be in good agreement with measured results.

E-shaped patch antennas for high-speed wireless networks

Thin, broad-band, E-shaped microstrip patch antennas (ESPAs), operating in the 5-6 GHz frequency range, are presented. They are intended for high-speed (IEEE 802.11a, 54 Mb/s) wireless computer local area networks (WLAN) and other wireless communication systems. They are suitable for WLAN adaptor cards in the PCMCIA (also known as PC) format, allowing users of current notebook computers to upgrade to this high-speed wireless standard at a low cost. Importantly, our antennas are thin enough to be accommodated in a PCMCIA card of standard 5-mm thickness, without making the antenna end thicker than the card itself. Two different closely spaced antenna pairs are also presented for diversity. A new ESPA configuration with a microstrip feed is presented for easy integration with microwave transceivers. In all cases, within the two IEEE 802.11a WLAN bands (5.15-5.35 GHz and 5.725-5.825 GHz), the reflection coefficient at the antenna input is <-10 dB and in both antenna pairs, mutual coupling between the two antennas is <-20 dB.

Prediction of the Notch Frequency of Slot Loaded Printed UWB Antennas

To prevent interference between ultrawideband (UWB) systems and existing wireless systems, creating notches in the UWB radiated field spectrum has been proposed. This filtering effect can be achieved by integrating a slot resonator to a UWB antenna. However, in printed monopole or dipole type UWB antennas, the notch frequency of the embedded slot depends on substrate and slot parameters. In this paper, we present a method to calculate, very accurately and efficiently, the notch frequency of such antennas. This method is validated by comparing theoretical and experimental results for 19 different slot width, slot length and dielectric constant combinations. The average difference is about 2%. We also assess the filtering action of slots from two aspects: the mismatch loss at the input and the radiation level in the far field. The sensitivity of the notch frequency to antenna parameters is also investigated.

The Use of Simple Thin Partially Reflective Surfaces With Positive Reflection Phase Gradients to Design Wideband, Low-Profile EBG Resonator Antennas

Partially reflecting surfaces (PRS) with positive reflection phase gradients are investigated for the design of wideband, low-profile electromagnetic band gap (EBG) resonator antennas. Thin single-dielectric-slab PRSs with printed patterns on both sides are proposed to minimize the PRS thickness and to simplify fabrication. Three such surfaces, each with printed dipoles on both sides, have been designed to obtain different positive reflection phase gradients and reflection magnitude levels in the operating frequency bands. These surfaces, and the EBG resonator antennas formed from them, are analyzed theoretically and experimentally to highlight the design compromises involved and to reveal the relationships between the antenna peak gain, gain bandwidth, the reflection profile (i.e., positive phase gradient and magnitude) of the surface and the relative dimensions of dipoles. A small feed antenna, designed to operate in the cavity field environment, provides good impedance matching (|S11| <; -10 dB) across the operating frequency bands of all three EBG resonator antennas. Experimental results confirm the wideband performance of a simple, low-profile EBG resonator antenna. Its PRS thickness is only 1.6 mm, effective bandwidth is 12.6%, measured peak gain is 16.2 dBi at 11.5 GHz and 3 dB gain bandwidth is 15.7%.

Neural stimulation with magnetic fields: analysis of induced electric fields

Spatial distribution of the derivative of the electric field induced in a planar semi-infinite tissue model by various current-carrying coils and their utility in neural stimulation are evaluated. Analytical expressions are obtained for the electric field and its spatial derivatives produced by an infinitely short current element. Fields and their derivatives for an arbitrarily shaped coil are then obtained by numerical summation of contributions from all the elements forming the coil. The simplicity of the solution and a very short computation time make this method particularly attractive for gaining a physical insight into the spatial behavior of the stimulating parameter and for the optimization of coils. Such analysis is useful as the first step before undertaking a more complex numerical analysis of a model more closely representing the tissue geometry and heterogeneity.<<ETX>>

A hybrid-resonator antenna: experimental results

Experimental results are presented for a hybrid-resonator antenna, consisting of a microstrip patch resonator coupled to a dielectric resonator. They demonstrate a 10 dB return-loss bandwidth of 5.14-6.51 GHz (23.5%) and a radiation pattern similar to that of a conventional microstrip patch or dielectric-resonator antenna. The peak cross-polarization level in the upper hemisphere is at least 18 dB below the peak co-polarization level.

Wideband Circularly Polarized Stacked Microstrip Antennas

A simple technique is developed to improve the axial ratio (AR)-bandwidth and quality of circularly polarized stacked microstrip antennas (CPSMAs) using a new C-type single feed. The proposed antenna has been optimized and fabricated, and the computed results agree very well with measurements. The antenna has a 3 dB AR bandwidth of 13.5%, gain is more than 7.5 dBi over the 3 dB AR bandwidth and the 10 dB return-loss bandwidth is 21%. The proposed feed optimization technique is useful for rapid design of circular polarized stacked microstrip antennas.

A novel absorb/transmit FSS for secure indoor wireless networks with reduced multipath fading

A novel absorb/transmit frequency selective surface (FSS) is presented for 5-GHz wireless local area network (WLAN) applications. The novelty of the design is that it is capable of absorbing, as opposed to reflecting, WLAN signals while passing mobile signals. The FSS consists of two layers, one with conventional conducting cross dipoles and the other with resistive cross dipoles. The absorption of the WLAN signal is important to reduce additional multipaths and resultant fading otherwise caused by the FSS. The structure has good transmission characteristics for 900/1800/1900-MHz mobile bands and performs well for both horizontal and vertical polarizations. The distance between the two layers is less than a quarter free-space wavelengths. Theoretical and experimental results are presented.

Oblique Incidence Performance of a Novel Frequency Selective Surface Absorber

Oblique incidence performance of a novel two-layer absorb/transmit frequency selective surface (FSS) is investigated. The FSS has good frequency stability for both horizontally and vertically polarized waves incident normally or at oblique angles. Due to its transmission for 900/1800/1900 MHz mobile bands and good absorption for 5 GHz waves, it has the potential as a security wall or isolator for 5 GHz WLAN systems. The absorption in the stop band helps reduce additional WLAN multipath fading caused by conventional reflecting FSS designs. The first layer of the FSS consists of conventional conducting cross dipoles having a circular aperture in the centre, while the second layer uses resistive cross dipoles. Moreover, the conducting cross dipoles have been sandwiched between two dielectric sheets to achieve a stable response for different angles of incidence. The periodicity of both FSS layers is the same while the distance between the two layers is reduced to one eighth of the free-space wavelength. This reduction leads to a more compact design compared to the conventional Salisbury screen, while still achieving acceptable absorption in the stopband. Both theoretical and experimental results are presented to confirm the performance of the absorb/transmit FSS.

Dielectric Loaded Impedance Matching for Wideband Implanted Antennas

In implanted biomedical devices, due to the presence of surrounding dissipative biological tissue, the antenna suffers poor impedance matching. This causes degradation in the performance of a wideband or ultra-wideband (UWB) implanted device. Moreover, the electrical properties of tissue change from organ to organ, and possibly from time to time. In this paper, it is shown that loading of antennas with suitable insulators can deliver broadband matching across a range of dissipative medium properties. An impedance-matched UWB antenna designed to operate inside a lossy medium, which has varying electromagnetic properties within the range expected in biological tissues, is presented. The operating bandwidth of the proposed design is 3.5-4.5 GHz, which is an interference-free subset of the unlicensed UWB band in the US. It is demonstrated that once the dielectric loading is applied, the conventional procedure for antenna design in free space can be followed. The proposed implantable small capsule-shaped slot antenna has been characterized using numerical simulations. Details of a proof-of-concept experiment are presented.