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Most sensing networks rely on punctual/local sensors; they thus lack the ability to spatially resolve the quantity to be monitored (e.g. a temperature or humidity profile) without relying on the deployment of numerous inline sensors. Currently, most quasi-distributed or distributed sensing technologies rely on the use of optical fibre systems. However, these are generally expensive, which limits their large-scale adoption. Recently, elongated sensing elements have been successfully used with time-domain reflectometry (TDR) to implement diffused monitoring solutions. The advantage of TDR is that it is a relatively low-cost technology, with adequate measurement accuracy and the potential to be customised to suit the specific needs of different application contexts in the 4.0 era. Based on these considerations, this paper addresses the design, implementation and experimental validation of a novel generation of elongated sensing element networks, which can be permanently installed in the systems that need to be monitored and used for obtaining the diffused profile of the quantity to be monitored. Three applications are considered as case studies: monitoring the irrigation process in agriculture, leak detection in underground pipes and the monitoring of building structures.

In this work, an innovative system for structural health monitoring of metallic pipes is presented. The proposed system relies on exploiting the pipeline as a waveguide for the propagation of an electromagnetic (EM) signal. By analyzing the reflected signal, it is possible to assess the possible presence of anomalies or damage in the pipe.The innovative aspect of the proposed monitoring system is that the EM test signal is injected in the pipeline/waveguide through a coaxial/waveguide transition that is made on the surface of the pipe. In practical applications, this strategy would allow to connect more easily to operating, buried pipes.To characterize the propagation of the EM signal in such conditions and to verify the changes on the reflected signal provoked by variation of the conditions of the pipe, experimental tests were carried out and a dedicated processing strategy was developed. The obtained results, although preliminary, confirmed that the proposed method holds unexplored potential to be used as a structural health monitoring solution.

In this paper, a time domain reflectometry (TDR)-based system for measuring water content of raw construction materials is presented. The proposed system relies on the fact that the presence of water leads to an increase of the dielectric permittivity of materials; therefore, from TDR-based permittivity measurements, it is possible to infer the water content value. In practical applications, the proposed system could be used for assessing the intrinsic water content of construction materials before they are poured into the concrete mixture. Knowing the intrinsic water content of the raw materials, in fact, would allow to evaluate the optimal amount of water that should be added to the mixture in order to achieve the desired waterto- cement ratio. This, in turn, would permit to fine-tune and control the mechanical properties of the final concrete structures. For assessing the feasibility of using the proposed system for the intended purpose, water content measurements were carried out on three construction materials, namely, sand, gray cement, and white portland cement. For each of these materials, a calibration curve relating water content to the apparent dielectric permittivity was derived; additionally, through repeated measurements, also a confidence interval was associated with the calibration curves.

Dielectric permittivity-based measurement techniques are establishing themselves as attractive solutions for assessing moisture content of historic masonry materials. The relative simplicity of the measurement principle and the inherent adaptability to diverse operating conditions are two of the most notable features of these techniques. In spite of these specific advantages, however, there are still some aspects that hinder the widespread use of permittivity-based moisture content measurement systems, and make their standardization difficult. In particular, the density of the sample under test may affect the estimation of permittivity, thus possibly leading to inaccurate moisture content measurements. As a result, the measurement system should be re-calibrated even when the same type of material is being investigated (e.g., two samples of the same type of stone, but extracted from different quarries).To circumvent this problem and to fully exploit the potential of permittivity-based moisture content measurements, in this work, a strategy for compensating for the effect of density is addressed. In order to verify the suitability of this strategy, moisture content measurements were carried out on samples of two type of stones typical of historic masonry, namely gentile stone and red-clay brick.

In this work, the assessment of water content of building materials, to be used for moisture content monitoring of Cultural Heritage structures is addressed. To this purpose, different methods and probes were comparatively used to infer, noninvasively, the relationship between the water content of stone materials and the reflection properties at microwave frequency of the material. In particular, three types of probes were used: an open-ended coaxial probe; a patch resonator; and an open-ended waveguide. In addition, two different measurement instruments were compared; namely, a vector network analyzer and a time-domain reflectometer. Experimental tests were carried out on two types of stone materials ( gentile and leccese stone), which are typically found in Cultural Heritage structures of Southern Italy. For each of the considered measurement systems, the experimental results and the related uncertainty evaluation are reported.

A transmission line simulator for applications to TDR measurements and TDR-based sensors is presented. The proposed simulator, fully developed in MATLAB environment, is designed to be a fast, efficient and easily customizable tool, and its performance are compared with commercial simulation software packages. The simulator is intended for measuring arbitrary profiles of primary parameters (R, C, G, L) of a transmission line, by matching simulated TDR waveform with experimental TDR data. In the paper, this use is demonstrated by measuring the parameter profiles of actual coaxial cables. The simulator can be used in any kind of TDR measurement, e.g. leak detection in pipes, distributed moisture measurements in soil, health monitoring in building concrete, etc.

In this paper, the most recent advances in the time-domain reflectometry (TDR)-based system for leak-localization in underground pipes are described in detail. More specifically, a new design of sensing element and the use of a new connection modality are proposed. Thanks to these new features, the practical implementation of the system becomes much quicker and its use more effective. Additionally, the present work also describes all the practical aspects and technical details (from installation to functional tests), related to the practical implementation of the system. Finally, to assess the possibility of further increasing the cost-effectiveness of the TDR-based leak localization system, experimental tests were carried out by comparatively using two TDR instruments, differing in specifications and costs, to identify the position of a leak.

In this work, a time domain reflectometry (TDR)-based system for measuring water content of construction materials is presented. The proposed system relies on the fact that the presence of water leads to an increase of the effective dielectric permittivity of materials (being the relative dielectric permittivity of construction materials in the order of 3–7, and that of water approximately 78). Therefore, through TDR measurements of the apparent permittivity of the material, it is possible to obtain calibration curves that allow retrieving the water content. Starting from these considerations, in this paper, the results related to moisture content measurements of three construction materials (namely, plaster mortar, portland cement and pozzolan cement) are reported. The advantage of the proposed system is that it can allow to monitor in real time the water content of construction materials inline, so as to assess their quality throughout the production process.

In this work, the validation and the practical implementation of a methodology exploiting a time domain reflectometry (TDR)-system for monitoring rising damp in building structures are described in detail. The proposed system employs wire-like, passive, diffused sensing elements (SEs) that are embedded, at the time of construction or renovation, inside the walls of the building to be monitored. The SEs remain permanently inside the wall, ready to be interrogated when necessary. Experimental and simulation results are reported, which demonstrate the possibility of practical implementation and the associated performance in terms of sensitivity.

In this work, time domain reflectometry (TDR) is used in conjunction with an innovative type of low-cost, wirelike diffused sensor for diffused soil water content monitoring in agriculture. Thanks to its wire-like configuration, these passive, maintenance-free sensors can be rolled out and buried along the perimeter/path to be monitored (for example, along a row of plants/trees). A single sensor can be long up to 150 meters, and several sensors can be simultaneously installed in a large area, thus creating a diffused sensor network which can be controlled through a single measurement instrument. To assess the practical feasibility of the proposed TDR-based system, a practical experiment was carried out by employing a bifilar, wire-like sensing element (SE) for monitoring soil water content in a watermelon cultivation. For comparative purposes, also in view of continuous remote monitoring, measurements were carried out through three TDR instruments with different specifications and costs.

The design and the experimental characterization of a waveguide system for complex permittivity measurements on both solid and granular materials are presented. The proposed system is intended for the dielectric characterization of asphalt concrete and of its components at frequencies around 2.45 GHz. Therefore, the system provides measurements on granular materials, such as mineral aggregates found in the asphalt concrete. A series of experimental tests on reference materials shows, via comparison with a standard measurement system, errors within 1%. A theoretical analysis of uncertainty contributions confirms a predicted expanded uncertainty lower than 2%. Furthermore, to test the proposed system for dielectric spectroscopy on materials typical of the asphalt industry, measurements were also performed on calcareous and basaltic materials, which are typically used for the production of asphalt. Finally, three different dielectric models (namely, $\alpha $ model, Ansoult’s model, and Topp’s equation) were comparatively assessed to identify the most suitable ones in describing the water content and dielectric permittivity relationship for the considered materials.

Olive oil production represents a big part of the Mediterranean economy, and as such it must be protected from frauds. For this reason, it is necessary to develop alternative low-cost techniques, applicable on large scale, for checking the quality of the product and for detecting adulteration. On such bases, the present work deals with the possibility of adopting microwave reflectometry for obtaining a 'spectral signature' of vegetable oils. For this purpose, time domain reflectometry (TDR) measurements, in combination with specific data processing, are first used for the dielectric characterization of several oil types. Successively, the acquired data are processed through the principal component analysis (for identifying clusters of oil types that exhibit common features) and through the partial least square analysis (for identifying a predictive model for detecting oil adulteration). Results confirm that the proposed procedure holds considerable potential for quality and anti-adulteration control purposes, especially in view of practical applications. © 2012 Elsevier Ltd. All rights reserved.

The monitoring of moisture levels and the investigation of the dielectric properties of materials are tasks that can be suitably performed through Time Domain Reflectometry (TDR) techniques. In particular, moisture monitoring of granular agro-foods is a major interest for several industrial applications. Despite the extensive literature regarding the use of TDR technology, the state of the art is lacking in a rigorous metrological characterization of the results achieved, especially for the agro-foods area. To fill this gap, in this paper the Authors carry out a comparative analysis between moisture content measurements obtained through two different TDR instruments on various granular materials. First, the major error contributions affecting the overall performance of the TDR-based method are discussed; in particular, the error contributions that are intrinsic in the method are discriminated from those related to the specific experimental conditions. Successively, systematic errors effects are minimized through a preliminary calibration procedure of the experimental set-up. Finally, results are compared and analyzed through a statistical approach, providing both an assessment of the repeatability of measurements and a rigorous metrological characterization. This ultimately leads to a performance characterization, particularly useful for practical industrial applications. © 2008 Elsevier Ltd. All rights reserved.

The design and preliminary metrological characterization of a waveguide system for complex permittivity measurement on both compact and granular materials is presented. The proposed system is intended for the dielectric characterization of asphalt concrete and of its components at frequencies around 2.45 GHz. Therefore, the system easily allows performing measurements on granular materials, such as mineral aggregates found in the asphalt concrete. A series of experimental tests on reference materials shows, via comparison with a standard measurement system, errors within 1%. A theoretical analysis of uncertainty contributions confirms a predicted expanded uncertainty lower than 5%.

In this work, an accuracy analysis of the estimation of the time of flight (ToF) for time domain reflectometry (TDR) signals is carried out. For this purpose, three different criteria (referred to as ‘maximum derivative’, ‘zero derivative’, and ‘tangents crossing’) were comparatively applied for the evaluation of the ToF of a TDR signal propagating along a set of RG-58 coaxial cables (with different known length and with known electric parameters). Successively, as a further experimental test, the same criteria were applied on bi-wire cables with unknown electric parameters. Results show that, among the tested criteria, the ‘zero derivative’ criterion provided the best accuracy in the estimation of the ToF.

In this paper, a performance analysis, in terms of accuracy, linearity, and repeatability, of three criteria to estimate the time of flight in time-domain reflectometry (TDR) signals is carried out. In a first set of experiments, the three criteria [referred to as maximum derivative (MD), zero derivative, and tangent crossing (TC)] are applied to TDR signals propagating along a set of coaxial cables, with different known lengths and known electrical parameters. In a second set of experiments, the same criteria are applied to biwire cables in air, with different known lengths and unknown electrical parameters. Finally, in the last set of experiments, the criteria are applied in a more complex situation, i.e., on a biwire used as a sensing element for water-level measurement. The results show that, among the tested criteria, TC appears to provide a very good performance in terms of systematic errors and superior performance in terms of repeatability. The popular MD criterion appears to be more prone to random errors due to noise and TDR artifacts. The results of this paper are relevant to many practical applications of TDR, ranging from fault location in cables to media interface sensing.

In this paper, a time-domain reflectometry (TDR) based system for monitoring liquid level inside tanks is presented. The proposed system resorts to a flexible, two-wire probe, which can be attached to the walls of containers and adapted to their shape. The proposed system is particularly useful for industrial applications; in fact, after a preliminary calibration, it can measure automatically and in real time the level of liquids inside containers (even tens of meters tall). The proposed system was tested on metallic and on nonmetallic containers. In the latter case, as the probe is positioned on the outside wall of the container, a totally noninvasive monitoring of liquids is achieved. The obtained results show that the system has considerable potential for being easily and effectively employed in practical, industrial applications. Finally, simulations were carried out, thus providing useful information for optimizing the system performance and for predicting its limitations.

This work aims at assessing the practical applicability of an innovative time domain reflectometry (TDR)-based system for leak localization in underground pipes. For this purpose, two laboratory experiments were carried out. The first experiment (E1) allowed assessing the reliability of the TDR system in presence of two simultaneous leakage-like conditions. The second experiment (E2), which was also carried out in presence of two leaks, allowed assessing the possible effect of the type of soil on the performance of the leak-detection system. Results show that, differently from traditional leak detection methods, the TDR-based system can correctly localize the presence of two simultaneous, closely-spaced leaks and that the type of soil has very little influence on the performance of the system. Finally, in this work, the practical implementation of the system in urban areas is also addressed, thus highlighting the steps to maximize the effectiveness of the system. © 2013 Elsevier Ltd. All rights reserved.

In this work, three different techniques, namely time domain reflectometry (TDR), ground penetrating radar (GPR) and electrical resistivity tomography (ERT) were experimentally tested for water leak detection in underground pipes. Each technique was employed in three experimental conditions (one laboratory or two field experiments), thus covering a limited but significant set of possible practical scenarios. Results show that each of these techniques may represent a useful alternative/addition to the others. Starting from considerations on the obtained experimental results, a thorough analysis on the advantages and drawbacks of the possible adoption of these techniques for leak detection in underground pipes is provided. © 2013 Elsevier Ltd.

In this work, an innovative cellulose-based superabsorbent polymer (SAP) was experimentally assessed as an environmentally friendly alternative to acrylate-based SAPs, for the optimization of water consumption in agriculture. The cellulose-based SAP was synthesized and tested for its swelling capability in different aqueous media. The effectiveness of the SAP in agricultural applications was then evaluated by analyzing its performance after several absorption/desorption cycles, over a period of approximately 80 days, upon addition to different types of soil, i.e., white and red soil, for the cultivation of two varieties of plants typical of the Mediterranean area (tomatoes and chicory). The results confirmed that SAP-amended soil can store a considerable amount of water and can release it gradually to the plant roots when needed. The adoption of the proposed SAP in cultivations could thus represent a promising solution for the rationalization of water resources, especially in desert areas.

In the present work, an alternative probe configuration to be used for time domain reflectometry (TDR)-based monitoring of liquid level inside tanks is presented. The proposed probe, which resorts to a bifilar transmission line configuration, is flexible and adaptable to the surface of the containers to be monitored. This feature, along with the other advantageous characteristics of TDR, makes this probe configuration particularly attractive for industrial applications. In this paper, to test the applicability of the method, two different scenarios are considered: monitoring of liquids contained in metallic tanks and non-invasive monitoring of liquids in non-metallic tanks. Results show that, in both cases, the proposed system has considerable potential for being easily and effectively employed in practical, industrial applications.

In this paper, different customized systems for microwave permittivity measurements on liquid samples, based on reflectometric measurements, are presented and analyzed. Their performance is compared against the one deriving from the most widely adopted commercial measurement setup. The systems are designed with the aim of providing less expensive solutions without compromising measurement accuracy. The purpose of the first proposed solution is to replace the commercial measurement software exploiting a reformulation of the classical theory. Based on this alternative formulation, a 'homemade' probe is built by properly modifying an N-type coaxial connector, thus providing a system requiring a lower quantity of liquid under test. Moreover, a different experimental approach which uses time-domain reflectometry (TDR) instrumentation is presented. Such solution is by far the least expensive, as it allows avoiding the use of costly instrumentation (such as a vector network analyzer). In order to metrologically characterize the proposed solutions, a series of repeated measurements is performed on a set of well-referenced liquids. After extracting the Cole-Cole parameters through each of the considered measurement methods, the resulting type A uncertainty is evaluated. Finally, comparison with literature data allows the estimation of measurement bias. The analysis evidences that custom solutions generally exhibit an accuracy comparable to the one of the commercial solution, with a slight degradation of performance for the TDR-based setup, which, however, compensates for this drawback with its appealing low cost. © 1963-2012 IEEE.

In this paper, a time domain reflectometry (TOR)based system for the non-invasive localization of leaks in underground pipes is described and experimentally validated. The proposed system simply relies on TOR measurements performed on a two-wire cable (biwire) that is rolled out on the underground pipe at the time of installation of the pipe (but made accessible from the road surface through an inspection well); the biwire on the pipe acts as a permanent, distributed sensing element and can be connected to the TOR instrument whenever it is necessary to perform a check. Two of the major advantages of the proposed system are that it can be employed with pipes made of any material and that it strongly reduces the inspection time typically required by traditional leak detection methods. To validate the system and to assess its performance, a new pipe was installed and a leak condition was intentionally provoked at a reference position. The TOR system was then used to evaluate the position of the leak as if had been unknown. The results of this experimental test are reported and thoroughly discussed.

In this paper, a time domain reflectometry (TDR)-based solution for detecting water leaks in underground pipes is described and experimentally validated. The proposed method strongly reduces the inspection time that is typically required by traditional leak detection methods. In fact, it can successfully inspect pipes that are hundreds of meters long. In this work, two different configurations of the system are described: one can be used for detecting leaks in 'already-installed' metal pipes; whereas the other is suitable to be employed for detecting leaks in 'newly-installed' pipes made of any material. The performance of both these system configurations was assessed through an extensive, on-site experimental campaign, carried out with the collaboration of one of the largest European water operators, Acquedotto Pugliese S.p.A. (AQP). Representative results, related to both configurations, of three experimental cases (ECs) are reported and commented on. Finally, some practical considerations on the applicability of the system are provided. © IWA Publishing 2013.

In most water distribution systems, a fairly sizable amount of water is lost because of leaks and faults in pipes. For this reason, the individuation of leaks is extremely important for the optimization and rationalization of water resources. However, the techniques and methodologies that are currently used for the individuation of leaks, despite being universally accepted, are extremely time-consuming and require highly-experienced personnel. Additionally, such techniques become unreliable and ineffective when the measurements are not performed in specific operating conditions of the pipe (e.g., high water pressure). On this basis, in this paper, a time domain reflectometry (TDR)-based system for the non-invasive detection of leaks in underground metal pipes is presented. Not only does the adoption of the developed system leads to accurately pinpoint the leak, but it also allows to dramatically reduce the required inspection times. The TDR-based system for leak detection is described in detail (with particular attention to the measurement principle behind the method and to the methodology). Furthermore, a strategy for enhancing the accuracy in pinpointing the leak is addressed. The proposed system is validated through experimental campaign that consisted in carrying out a leak-detection survey through the traditional methods and through the proposed method. © 2001-2012 IEEE.

In this work, a dielectric permittivity measurement system, to be used for quality control of edible oils, is presented. The proposed system relies both on low-frequency and high-frequency measurements (performed through and LCR meter and through a time-domain reflectometer, respectively), thus allowing a "broadband" investigation of the dielectric behavior of edible oils. The adoption of a single, specifically-designed probe (used for all the measurements), along with the possibility of using portable instrumentation, makes the proposed system an appealing candidate for the realization of a continuous, on-site system for quality monitoring of edible oils. The proposed system was first validated on materials with well-known dielectric characteristics. Successively, it was used to investigate the dielectric behavior of some autochthonous varieties of olive oils and of seed oils. The ultimate goal of this work is to pave the way for the development of an alternative method of analysis that could be able to distinguish among different kinds of oils, to certify oil quality and origin, and to detect the possible presence of adulterants. © 2011 IEEE.

In this chapter, broadband microwave reflectometry (BMR)-based methods for measuring simultaneously the levels and the dielectric characteristics of liquid materials are presented. First, an approach based solely on time domain reflectometry (TDR) measurements and on the analysis of TDR waveforms is described. Secondly, a further enhancement of this method is accomplished by resorting to the combination of TDR with frequency domain reflectometry (FDR). Also this method provides, in one shot, the level and the dielectric permittivity of the considered liquids (this time, the complete spectral signature is obtained in terms of Cole-Cole parameters). This approach is considered the optimal solution especially when dealing with dispersive media. In this second measurement method, the accuracy is enhanced also through the adoption of a transmission line (TL) modeling of the measurement cell and through the realization of a custom-made fixture that allows performing short-open-load (SOL) calibration measurements. Finally, BMR-based measurements of the dielectric characteristics are extended to edible liquids: in particular, vegetable oils are considered. Reported results show that all the proposed methods have strong potential for possible practical implementation in the field of industrial monitoring. It will be evident how, in all the considered approaches, the design of the probe is crucial for enhancing the final measurement accuracy. © 2011 Springer-Verlag Berlin Heidelberg.

Quality control of vegetable oils is becoming more stringent, and strict laws are being enforced, especially for avoiding adulteration. The public bodies that are responsible for the prevention of the adulteration of foodstuffs necessitate methods of analysis that could facilitate large-scale in situ controls. Similarly, oil producers constantly strive to speed up internal quality control. As a direct consequence, there is an increasing demand for innovative methods of analysis that could guarantee real-time in situ monitoring and provide adequate accuracy. On such bases, the present work addresses the possibility of monitoring qualitative characteristics of vegetable oils through microwave dielectric spectroscopy. To this purpose, the Cole-Cole dielectric parameters of different vegetable oils are evaluated through an innovative automatable procedure that suitably combines traditional TDR measurements, SOL calibration, frequency domain processing, TL modelling and, finally, a minimization routine. The proposed procedure is carried out first on different "pure" oils and, secondly, on some oil mixtures. The obtained results confirm that different dielectric characteristics are associated with different oils, thus confirming the considerable potential of dielectric spectroscopy for quality and anti-adulteration control purposes, especially in view of practical applications. © 2010 Elsevier Ltd. All rights reserved.

Antenna-characterization measurements are traditionally performed in the frequency domain (FD) through a vector network analyzer (VNA) in an anechoic chamber. Nevertheless, the high cost of the required setup strongly limits the possibility of using this approach. Starting from these considerations, a time-domain (TD)-based approach for characterizing antennas without using an anechoic chamber is assessed. As a matter of fact, instruments operating in TD are usually less expensive than VNAs; nevertheless, with appropriate data processing, they provide as much information. Particularly, it is demonstrated that the selection of an optimal time windowing is the main factor that guarantees a high accuracy level in the corresponding FD. The proposed approach leads to the accurate evaluation of the reflection scattering parameter S 11 (f) from time-domain reflectometry (TDR) data. The experimental validation is tested on a commercial radio-frequency identification (RFID) reader antenna, and the results are compared with reference VNA measurements performed in an anechoic chamber. The ultimate goal of this paper is to demonstrate that, through calibrated TDR measurements, along with an optimal time windowing, an accurate antenna characterization can be achieved. © 2008 IEEE.

Monitoring soil moisture is a major interest for several practical applications, and this task can be suitably performed through various methods. Although extensive literature deals with the adoptions of standard systems (many of which are commercially available), the state of the art is lacking in a rigorous validation of specific systems, such as those based on the seismic wave propagation (SWP) method. Such methods have attracted a growing interest thanks to the simultaneous combination of reliability, ease of use, low cost and, most importantly, customization possibility. In this regard, in this paper the Authors propose an experimental comparative approach for fully validating a SWP-based system through reference time domain reflectometry (TDR) measurements. To this purpose, reference TDR measurements are performed for different moisture levels, and the corresponding reference propagation velocities are compared with those directly measured through the SWP-based system. This approach provides a comprehensive assessment of the SWP-based method, from both an experimental and a metrological point of view. The ultimate goal is to provide an exhaustive performance characterization of the system, in view of practical implementation for soil moisture evaluation. © 2009 IEEE.

Traditionally, antenna characterization measurements are performed in the frequency-domain through a Vector Network Analyzer (VNA) in anechoic chamber. Nevertheless, the expensiveness of this set-up prevents itfrom becoming widespread in laboratories. On such bases, in this paper a time-domain-based approach is comparatively investigated; such a method virtually makes the anechoic chamber unnecessary, thus dramatically reducing the related costs. The proposed approach, based on the evaluation of the scattering parameter (S 11) from Time Domain Reflectometry (TDR) data, has been tested for the characterization of a commercial Radio Frequency Identification (RFID) reader antenna. Data acquired in the time-domain are processed in the frequency-domain through a dedicated algorithm, thus allowing the S11 extrapolation. Furthermore, results are compared to VNA measurements performed in an anechoic chamber. In order to assess the investigated method, from a metrological point of view, the root mean square error between the results is reported as a figure of merit. This way, it is demonstrated that calibrated time-domain measurements represent an alternative, inexpensive and accurate method for characterizing antennas. © 2008 IEEE.