Showing papers by "Per-Simon Kildal published in 2007"

Comparison of bandgaps of mushroom-type EBG surface and corrugated and strip-type soft surfaces

Abstract: Corrugated and strip-loaded soft surfaces are characterised in the same way the electromagnetic bandgap (EBG) surfaces are characterized, that is, in terms of the bandgaps in their dispersion diagrams. The strip-loaded surfaces are provided with metalised via holes, and both central and lateral locations of these are investigated. It is found that strip-loaded surfaces with vias have the same bandgaps as mushroom-type EBG surfaces of the same period. It is also found that the strip-loaded surface can be designed to have the bandgap appearing at lower frequency than the mushroom surface, by locating the vias at the edge rather than in the middle and by optimising the period of the vias along the strips.

Monopulse Tracking Performance of Multi-Port Eleven Antenna for Use in Satellite Communications Terminals

Abstract: The paper describes the design of a laboratory model of the multiport Eleven antenna and explains how the sum and difference patterns are generated in this model. Its tracking performance is investigated through both simulation and measurement. This paper also introduces a practical method for determining the scattering parameters of an antenna with multiple ports, when it is excited for both sum and difference beams, without using a feeding network with sum and difference ports. The monopulse tracking capability may open up interesting opportunities for using the Eleven antenna in satellite communications over one or more of the L-, S-, C-, X- and Ku-bands.

Tripling bandwidth of hat feed by genetic algorithm optimization

Abstract: Genetic algorithm optimization combined with V2D-FDTD simulations has been used to redesign the geometry of the Hat Feed. The new evolved feed has improved return loss properties while keeping the good overall efficiency of the original feed. The bandwidth of the new feed is about 33% (at 7.9 GHz), compared to about 11% for the original. In addition, the return loss level within the hand has dropped about 8 dB on average - it is better than 17.7 dB over the entire band and better than 20 dB in almost 90% of the hand. The new feed has been manufactured and return loss measurements confirm the simulated results.

Overview of 6 Years R&D on Characterizing Wireless Devices in Rayleigh Fading Using Reverberation Chambers

Abstract: The paper gives an overview of how measurements in reverberation chambers can be used to characterize the performance of small antennas and wireless terminals for mobile communications in environments with Rayleigh fading. The measurement capabilities include radiation efficiency, radiated power, diversity gain, MIMO system capacity and receiver sensitivity. In the latter case the reverberation chamber enable fast measurements of bit error rate (BER) and frame error rate (FER), and thereby total isotropic sensitivity (TIS). In addition, the terminals can be tested under realistic and continuous fading very similar to what is present in urban and indoor environments, referred to as average fading sensitivity (AFS). The chamber is the only known measurement instrument for directly measuring diversity gain and channel capacity; the alternative being to drive measurement instruments around in an actual urban environment. The measurements can fast and easily be performed for different talk positions relative to a head phantom.

MIMO performance of enhanced UMTS four-antenna structures for mobile phones in the presence of the user’s head

Abstract: In this paper, we have presented the reverberation chambers (RC) evaluation of the multiple-input-multiple-output (MIMO) system performance of several UMTS four-antenna systems with low and high planar inverted-F antennas (PIFAs) isolation. It has been measured that the neutralization technique is always improving the performances even with the presence of the user's head, due to the high isolation obtained between the antennas and their high total efficiencies.

Diversity characterization of optimized two-antenna systems for UMTS handsets

Abstract: This paper presents the evaluation of the diversity performance of several two-antenna systems for UMTS terminals. All the measurements are done in a reverberation chamber and in a Wheeler cap setup. First, a two-antenna system having poor isolation between its radiators is measured. Then, the performance of this structure is compared with two optimized structures having high isolation and high total efficiency, thanks to the implementation of a neutralization technique between the radiating elements. The key diversity parameters of all these systems are discussed, that is, the total efficiency of the antenna, the envelope correlation coefficient, the diversity gains, the mean effective gain (MEG), and the MEG ratio. The comparison of all these results is especially showing the benefit brought back by the neutralization technique.

Optimization of antenna diversity gain by combining full-wave and circuit simulations

Abstract: This paper presents a method for using data from full-wave EM simulations in combination with lumped circuit models for calculating antenna parameters such as radiation patterns, radiation efficiency, diversity gain etc. The method can be applied on multi-port antennas for arbitrary loading and excitation conditions. The paper also describes how the method is used in combination with a global optimization scheme to maximize the diversity gain for a given antenna and circuit layout. The goal function is maximized by optimizing the component values of the given circuit layout. Results for two parallel dipoles and a prototype mobile terminal, both cases with and without the optimized impedance network, are presented. (5 pages)

RF invisibility using metamaterials: Harry Potter’s Cloak or The Emperor’s New Clothes?

Abstract: Generally, it is very important when performing research that improvements are properly characterized and compared to known and simpler solutions in terms of well established performance measures. Otherwise, very misleading results could appear such as in another metamaterials-related paper commented upon by Kildal (2006). If the published results for the metamaterials-based invisibility cloaks cannot stand comparison with known technology, they may be better associated with the "Emperor's new clothes" (i.e. an invisible "nonworking" coat) than Harry Potter's invisibility cloak.

Evaluation of the performances of several four-antenna systems in a reverberation chamber

Abstract: This paper results from a short-term mission granted by the COST 284 where the antenna-design competences of the LEAT have been gathered with the measurement skills of Chalmers Institute of Technology. Three multi-antenna prototypes have been characterized in the Bluetest reverberation chamber in terms of total efficiency and diversity gain. Particularly the performances of an initial four-antenna system with poor isolation have been evaluated with and without the presence of a phantom head and then we characterized in the same manner two prototypes with highly decoupled antennas. The paper especially focused on demonstrating that the neutralization technique is still giving improvements in the presence of the user's head.

Fundamental Directivity and Efficiency Limitations of Single- and Multi-Port Antennas

Abstract: The paper considers the fundamental limitations on directivity and radiation efficiency of antennas with both single and multiple ports. The treatment is limited to antennas made of lossless materials. The maximum available directivity measured on a single port of both small antennas and large antennas is considered in terms of the diameter of the smallest sphere that can surround the antenna. The maximum available directivity of large antennas is given by the common aperture directivity formula, and that of small antennas is 4.67 dBi (same as for a Huygen source). These two expressions are combined in a heuristic manner to a continuous maximum directivity limitation. The theoretical limitation is plotted together with directivities of known antennas which are known to have large directivities. The paper discusses also fundamental limitations of dense arrays. Dense linear arrays can be used to provide superdirectivity, but their superdirectivity is lower than what can be obtained with normal antennas under the present definition of maximum available directivity. Dense arrays also suffer from severe gain reduction due to strong coupling to the ports of the neighbouring elements. This gain reduction can be characterized in terms of a coupling efficiency, also referred to as embedded element efficiency, and appears as a reduction of the total radiated power. The fundamental property of this coupling efficiency is known from classical papers, and it has recently been verified by measurements in a reverberation chamber. (6 pages)

Radiation Q and radiation efficiency of wideband small antennas and their relation to bandwidth and cut-off of spherical modes

Abstract: Resonators are traditionally characterized by their quality factor Q, which is inversely proportional to the relative bandwidth. Small antennas are often resonant, so they can be characterized by a Q, except for the fact that the correct quality measure of an antenna is rather the inverse Q. i.e. the relative bandwidth. Still, it has become common to study fundamental size limitations of small antennas in terms of a so-called radiation Q (or antenna Q). The paper explains how this intrinsic radiation Q relates to: i) the bandwidth- efficiency product of small single-resonance-type antennas, ii) the gradual cut-off of spherical waves for wideband gradual-transition-type small antennas, and iii) the number of resonances needed to cover a certain frequency band for wideband multiple-resonances-type small antennas. The paper also introduces one intrinsic radiation Q for basic single TE and TM spherical mode sources, and another for combined TE and TM sources.

Measurement of Embedded Element Efficiencies of Wideband Dense Arrays in Reverberation Chamber

Abstract: The paper describes how coupling efficiencies in dense arrays can be measured in a reverberation chamber. This is much faster than measuring S-parameters between all element ports and calculating the coupling efficiencies from these. Results are presented between 2 and 6 GHz for an array of 144 Vivaldi elements developed by ASTRON for use as a technology demonstrator for SKA (Square Kilometer Array). The results include coupling efficiencies for singly-excited elements (referred to as embedded element efficiencies), as well as for three different focal-plane-array excitations. The embedded element efficiencies are compared with an approximate formula derived from classical papers. The results show significant efficiency losses at low frequencies. (5 pages)

Improvement of Input Reflection Coefficient of Eleven Antenna - a Compact Wideband Feed for Reflector Antennas

Abstract: The paper presents the improvement of input reflection coefficient of the Eleven antenna .. This improvement plays an important role when it is used as a feed for a reflector in radio telescopes, because high reflection coefficient will increase system noise temperature. The reflection coefficients of the previous Eleven antennas had peaks up to -5 dB [3], whereas the goal of the present work is to keep it below -10dB. Therefore, an optimization on input reflection coefficient has been performed. The solver is a commercial moment method code – WIPL-D [6] and the optimization is done by running WILP-D from self-made Matlab program. We perform the optimization in connection with the development of an Eleven feed for the RATAN radio telescope with the goal to cover a frequency range from 0.5 to 3 GHz. The final measurements confirm a reflection coefficient close to -10 dB over 80% of the frequency band. Measured radiation patterns are also presented in the paper.

Bandgaps and cloaks with soft and hard surfaces

Abstract: Soft and hard surfaces for electromagnetic waves were defined in 1988. Here we will explain the similarity between the anisotropic soft surface, and the last decade's isotropic artificial magnetic conductor and in particular the so-called bandgap surface. It is summarized that a soft surface normally will have better performance than a bandgap surface when the purpose is to stop waves propagating along a surface for dual or circular polarization. We will also explain how the hard surface can be used to make cylindrical objects invisible, and that this was experimentally proven to have decent bandwidth in a journal article in 1996, ten years before the promised theoretical metamaterial cloak published last year. The invisibility is correctly and conveniently characterized in terms of an equivalent blockage width that characterizes the forward scattered field. There will be presented recent results in which the performance of hard surface cloaks and metamaterials cloaks are compared, showing the superiority of the hard surface cloak.

Measurements of Total Radiated Power of UMTS Phones in Reverberation Chamber

Abstract: The present paper contains description and validation of the setup used for measurements of total radiated power (TRP) in the reverberation chamber. Measurements of TRP for various mobile phone standards have been presented before, and this paper is specifically focused on characterizing UMTS handsets designed to use the W-CDMA air interface. The reverberation chamber is shown to be a fast and repeatable alternative for these types of measurements even when the correlation bandwidth is smaller than the UMTS channel bandwidth. Examples of measurement times and uncertainties are shown. The measurement time is reduced to 1 minute per channel by making use of continuous instead of stepwise mechanical mode stirring. (6 pages)

Software for Optimizing multi-port antennas based on precomputed embedded element patterns from commerical codes

Abstract: Software for optimizing multi-port antennas based on precomputed embedded element patterns from commerical codes

Modal solutions in dual-depth longitudinally corrugated hard waveguide

Abstract: A longitudinally corrugated circular waveguide with dielectric-filled grooves of alternating depths is considered. The dispersion equation for determining the waveguide eigenmode parameters is derived for the small corrugation period. The waveguide is shown to support two quasi-transversal electromagnetic (TEM) modes, associated with each of the two frequencies at which the TEM condition is realised. Some features of these quasi-TEM modes are revealed. The potential performance of a dual-band conical horn is presented, and some future studies are discussed here.

The Eleven antenna feed for monopulse tracking and combining L-, C-, X- and Ku- satellite bands

Abstract: The Eleven feed, featured as a low-profile, logarithmic-periodic and ultra wideband antenna, has during the past few years been developed at Chalmers [1]-[5]. It is referred to as the Eleven feed because its basic geometry is two parallel folded dipoles half-wavelength apart, and it can be used over more than a decade bandwidth with 11dBi directivity by extending the basic configuration logarithmically. The Eleven feed can be used in both linear and dual polarizations, and its phase centre changes so little with frequency that the corresponding phase efficiency is better than -0.1dB at all frequencies. The 10dB semi-beam width is 50-60 degrees over the entire frequency band, and a reflector aperture efficiency of 66% or better can be achieved when the subtended angle towards the reflector is about 53 degrees. Therefore, it is suitable as feed for reflector antennas, in particular when they are primary-fed. The Eleven feed was developed on projects for California Institute of Technology (Caltech) as a feed for the reflectors of the US SKA proposal (Square Kilometer Array) [2].

Comparison of different loading configurations and numerical models of reverberation chambers

Abstract: Reverberation chambers are often loaded with lossy objects. In this paper we compare two numerical models of the chamber losses; a concentrated load and a uniform volume loss. The statistics of the coupling between two dipoles in many random positions over a frequency band is used to compare the two cases. It is shown that the two cases gives similar statistics.