Balancing networks for symmetric antennas .I. Classification and fundamental operation

Emphasis has been placed on the quality of a metrology antenna system that allows it to be represented accurately using analytical or numerical models. Central to this capability is the efficacy of the balancing networks, sometimes referred to as balanced to unbalanced transformers (baluns). In this paper, we classify three fundamental types of baluns and show that all three can be derived from the 180/spl deg/, four-port hybrid network. Balanced antennas driven from coaxial feed lines and operated in the presence of an asymmetric scatterer present an intrinsically unbalanced load to the balun. We show that in such situations the current balun is the only appropriate balun to employ. For numerous antenna metrology applications, in particular, site attenuation measurements, 180/spl deg/, four-port hybrid networks have been employed as baluns. Network relations are derived relating the so-called voltage and current baluns to the 180/spl deg/, four-port hybrid network. It is shown that, in addition to acting as a 0/spl deg/ or 180/spl deg/ power divider, the hybrid network can serve as either a voltage or a current balun depending on the termination at the isolated port. In contrast to the traditional approach, it is shown that when using a 180/spl deg/ hybrid as a balun, the isolated port should be terminated in such a way that the hybrid behaves as a current balun as opposed to a 180/spl deg/ power divider. This will yield experimental results that can be more readily reconciled with models.     

Methods for modeling wire antennas loaded with shielded networks

Wire antennas are often loaded with lumped circuits in order to control or alter one or more of the antenna's characteristics. Shielding these circuits forces the load to manifest itself to the antenna from a small aperture. This ensures that the laws of circuit theory may be used in an analysis to characterize the load, rather than Maxwell's equations, simplifying the computations. To this end, four configurations of lumped loads are used in place of the antenna's shielded networks in the analysis of the loaded wire. Two of the lumped load configurations are specific to an aperture-coupled shielded network presented in this paper. Two other configurations considered, however, are physically representative of a large class of wire antenna loads. Data obtained from analyses based upon the models introduced and from measurements are presented for comparison.     

Decoupling and descattering networks for antennas

The possibilities of connecting a lossless network between input ports and antenna ports such that there is no coupling and scattering between the antennas are discussed. A necessary condition for complete decoupling and descattering is power orthogonality between the patterns of the individual antennas. Numerical and experimental results are presented for monopole antennas.     

Low-loss offset feeds for electrically large symmetric dual-reflector antennas

Circularly symmetric, dual-reflector, high-gain antenna systems often require feeds placed off the system's axis because of the need for multiple feeds to use the reflector antenna. Also, the constraint requiring the hyperboloid or shaped subreflector to remain circularly symmetric is sometimes added. In a Cassegrainian system, the subreflector and feed may be rotated off-axis around the paraboloid focus and retain main reflector focusing. However, substantial spillover results in considerable noise in a high-gain/low-noise temperature system. In a shaped system, the tilt of the shaped subreflector and feed together results in substantial defocusing as well as spillover noise. If the subreflector is tilted approximately one-half the angle of the feed tilt in either the Cassegrainian or the dual-shaped reflector antenna, it is found that spillover and noise are substantially reduced with tolerable defocusing. An extensive numerical analysis of these effects was conducted to determine the characteristics of a 70-meter, dual-shaped reflector versus Cassegrainian antenna and to gain some understanding of the cause of the observed effects.     

Optimum networks for simultaneous multiple beam antennas

The design of passive microwave circuits for the formation of simultaneous multiple beams with arbitrary but specified shapes is considered. The maximum possible efficiency is derived from energy conservation and is determined from a Hermitian matrix whose elements are the correlation coefficients between all beam pairs. The eigenfunctions of the correlation matrix are the basis of a synthesis procedure for a practical network that will achieve the maximum efficiency. Several practical examples are given where unavoidable losses are typically 1 dB or more     

Smart antennas for broadband wireless access networks

This article is an overview of smart antenna applications in fixed broadband wireless access networks. Different smart antenna techniques are described including advances such as “spatial multiplexing” that can dramatically increase the performance of BWA networks. The impact of SA techniques on capacity and throughput of BWA networks is discussed     

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.     

Simple lossless feed networks for array antennas

For a given array, describable as an n-port network for feed considerations, it is shown that, with a class of very simple lossless feed networks, it is possible to realise an arbitrarily specified voltage or current excitation at the array ports.     

Angular MAC: a framework for directional antennas in wireless mesh networks

Capacity of wireless mesh networks can be enhanced through the use of smart directional antennas, which not only enable nodes to have high quality links but also increase network throughput by allowing spatial reuse. This paper proposes a new MAC protocol and framework, called Angular MAC (ANMAC) that enables directional antennas in wireless mesh networks. The protocols and algorithms of the ANMAC framework fit well with the requirements of mesh networks such as neighbor discovery and self-configuration, while providing significant throughput enhancements. The throughput enhancements are proven by comprehensive simulations with realistic antenna patterns, including performance comparisons of ANMAC with directional schemes using a similar node architecture and omni 802.11. Also, the effect of contention window size is analyzed and a dynamic contention window adaptation algorithm is proposed to maximize the throughput of the self-configuring mesh network, by taking instantaneous traffic conditions into account.     

Broad-band characteristics of rotationally symmetric antennas and thin wire constructs

The rotationally symmetric monopole antenna is given by a set of simultaneous equations on the rotationally symmetric surface current of a conductor, separated with respect to the variable of the envelope length on the surface of the rotationally symmetric antenna and the variable in the direction of revolution. The structure is analyzed in detail by means of an extended moment method based on the present theory. As an application, this analysis is applied to a conical monopole antenna. Studies are performed to find body shapes providing broadband characteristics. It is found that the antennas with lengthwise cross sections approximated by straight lines, ellipses, and circles have an impedance of50 Omegaover a wide frequency band. When these shapes are replaced with wire elements for analysis, 16 elements are found sufficient to approximate the characteristics of the rotationally symmetric antenna. Theoretical calculations and experimental results are compared. Agreement is excellent and the method is found useful.     

Traffic modeling for telecommunications networks

As new communications services evolve, professionals must create better models to predict system performance. The article provides an overview of computer simulation modelling for communication networks, as well as some important related modelling issues. It gives an overview of discrete event simulation and singles out two important modelling issues that are germane to extant and emerging networks: traffic modelling and rare event simulation. Monte Carlo computer simulation is used as a performance prediction tool and Markov models are considered     

Efficient implementation of multiple-beam sampler for continuously scanned array antennas

A multiple-beam sampler is described for continuously scanned array antennas using time modulation or frequency modulation. It is shown that, for the special case of an array antenna with a number of elements equal to an integer power of two, the multiple-beam sampler can be implemented using a minimum number of phase shifters and adders.     

Planar circularly symmetric EBG structures for reducing surface waves in printed antennas

This paper discusses the design and analysis of planar circularly symmetric (PCS) electromagnetic band gap (EBG) structures for reducing the surface waves excited by printed antennas on dense dielectric substrates. The key advantage of the circularly symmetric geometries is that a surface wave generated by a source located at its center experiences the same band gap effect in all radial directions. To obtain simple design rules of the PCS-EBGs for the optimization of the bandwidth, an equivalence is established between 2-D-EBGs and PCS-EBGs. Integrated planar printed antennas with bandwidths up to 20% are designed, manufactured and tested.     

Power-aware ad hoc networks with directional antennas: Models and analysis

Directional antennas offer numerous benefits for wireless ad hoc networks, such as extended communication range, better spatial reuse, improved capacity and suppressed interference. In this work, we study analytically the benefits of transmission power control on throughput and energy consumption in a uniformly distributed power-aware ad hoc network where nodes are equipped with directional antennas. We construct an interference model for directional antenna based on a honey grid model to calculate the maximum interference. We further derive a directional collision avoidance model and based on the integrated interference/collision model and signal to interference requirements (SIR), we present the maximum end-to-end throughput under the maximum interference. We further investigate the effect of collision on the energy consumption and propose an energy consumption model that utilizes all aspects of energy wastage.     

The near field of dipole antennas, part I: Theory

The theoretical and experimental evaluation of the electromagnetic fields in the immediate vicinity of resonant dipole antennas is presented. This type of antenna is widely used with portable and mobile radio transmitters. The work presented herein has been motivated by the concern that future Radio Frequency Protection Guides with respect to human exposure to nonionizing electromagnetic radiation might be expressed strictly in terms of the intensity squared of the electric or magnetic fields. It is shown in the results that it is possible to detect relatively high intensity electromagnetic (EM) fields in close proximity to resonant dipoles even for very low levels of radiated power (1 mW and less). The paper is divided into a theoretical section and an experimental section because its goals are twofold. First, the formulas for the correct evaluation of the EM fields in the close proximity to dipole antennas are established. Second, it is shown that such EM fields, which can be theoretically predicted and experimentally verified with satisfactory accuracy, are indeed strong enough to violate proposed Radio Frequency Protection Guides even for very low levels of radiated power. Thus portable radios are rendered virtually useless, although the same guides permit exposures to much higher levels of power in the far field. Part I of the paper is essentially theoretical and expresses the fields near dipole antennas in terms of cylindrical waves, which lend themselves to closed form integration. The asymptotic expressions of some components of the field are particularly simple for close distances (in terms of wavelength) from the antenna. The correctness of the solution is checked by evaluating how closely boundary conditions are satisfied. Results have shown that previously used formulas for evaluating field intensity very near dipole antennas can give incorrect values.     

Two-layer modeling for local area networks

A two-layer performance model of a local-area network (LAN), consisting of a media-access control (MAC) layer submodel and transport layer submodels, is presented. A multiple-queue cyclic-service model of the token-passing MAC layer is used along with a closed-queueing network model for the transport layer. To deal with acknowledgement traffic with priority over data messages, a mean-value analysis (MVA) priority approximation is devised for the case where the priority can change at each queue. An iterative solution algorithm is proposed for this two-layer performance model. The method is applied to five models of communication systems: a symmetric-load, piggybacked acknowledgement model: a symmetric-load, explicit-acknowledgement model; full-duplex communication models (without and with priorities); and a client/server model. System performances measures such as throughput and mean-message delay, computed from the analysis, are compared to the simulation results over a wide range of parameters. In most numerical examples, the approximate analysis is shown to be within 90% confidence intervals in the mean-message delay, and within a few percent error in the throughput     

Inter-cell interference modeling for cellular networks

We develop and analyze several inter-cell interference modeling methods for various fading scenarios in wireless systems. The models can analyze multiple interfering signals under different fading scenarios. Incoherent addition of multiple interfering signals is assumed. In this paper, we present two approaches, i.e., the approximate and exact analysis methods to calculate the probability density function (PDF) of the power of the interference signals. We propose the use of the generalized Gram-Charlier Series to analyze the error of the reference model. Although the computational complexity of exact analysis is high, it can be used to ensure the accuracy of the approximate analysis method. Hermite polynomials are used to simplify the integration operation into the summation operation, thus reduces the computational complexity of the exact method dramatically. The approximate method is simple although it may lead to increased errors. The methods proposed are useful in designing and analyzing practical systems.     

Compact thermal networks for modeling packages

In this paper, thermal networks for modeling packages are rigorously introduced. A multipoint moment matching method for state space reduction of these discretized thermal networks is formulated. In this manner reduced thermal networks are derived that can be used as boundary condition independent compact thermal models of packages. This algorithm is successfully applied to the detailed analysis of an idealized ball grid array package.