Recently, wireless communication systems for use in ITS(Intelligent Transport System) are increasing rapidly. When the system is used in a car, the most important problem is that there is few space for these antennas to be installed, because many communication antennas have already been equipped in it. To solve the problem, one solution is to install one multi-band antenna in a car. Several techniques have been reported for the multi-band antennas. However, the most articles are multi-layer, multi-feed multi-band antennas. In this paper, we propose single-layer, single-feed multi-band antenna which is available for GPS(Global Positioning system, 1.57542GHz), VICS(Vehicle Information and Communication System, 2.4997GHz), and DSRC(Dedicated Short-Range Communication system, 5.81GHz-band).

Generally the model airplanes, ships, vehicles and robots are controlled by using radio frequency of 27MHz, 40MHz and 72MHz. So the portable radio controller is required to have triple operating frequency band. Furthermore the antenna is pointed at the object due to human behavior, so the antenna is required to have radiation of axial direction. In this research, a triple-band small antenna using closely wound loop antenna is proposed.

The configuration of the antenna is shown in Fig.1. The antenna is constructed with three loop antennas, each loop is closely wound and they are wound alternately because taking into account of mutual coupling between loops. In this paper, in order to simplify the measurement and the analysis, the operating frequencies are chosen ten times of the desired frequencies. The dimensions of the antenna are shown in table 1. The size of the ground plate is 40*40mm, it is very small compared with the wave length. The return loss characteristics and the calculated radiation patterns are shown in Fig.2 and 3 respectively. The parameters have not been optimized, however the results show that the proposed antenna works on the triple-band and has radiation of axial direction. The other results give us the following information: the height (b) of the loop from the ground plate contributes to the impedance characteristics; the length of the loop contributes to the resonant frequency; and so on.

The antenna has a small size, triple-band and no radiation null point.

Several approaches has been developed to design multiband patch antennas. They can be generally divided according to the way of patch topology generation. The standard and oldest one is the concept of radiating patch resonator completed by direct or capacitively coupled or superstrate resonator(s) so that they create coplanar or stacked arrangement. Among them planar inverted F antennas (PIFA) are especially popular as several active and/or passive resonant part can be coupled to design compact electrically small multiband antennas. Common feature of this approach is that predominantly basic modes of coupled parts are excited. Further recently still popular multiband approach exploiting on the contrary higher order modes is prefractal antenna concept that utilize mathematical procedures to generate patch antennas with fractal boundary. This concept take advantages of geometrical self-similarity in several scale levels that are projected into the electrical properties of fractal antennas. Another approach is finding less intuitive shapes of multiband planar radiators. It is oftenly based on genetic algorithm (GA) control random generation of radiation motifs composed of more or less compact grid of conductive cells. Implementation of mapping the patch topology into the genetic chromosome, and particular shapes of designed radiators are results of individual authors invention.

Here presented approach is inspired by the last mentioned GA based random patch topology generation. The objective of the paper is to present an implementation a priory knowledge of multiband patch antennas operation based on multiresonator behavior into the GA controlled design of nonintuitive shaped grid-based patches. Mentioned apriory information is represented here by random generation of so called perturbation elements (PE) in the grid forming patch topology, exploitation of radiator symmetry, and the idea of prefractal antenna concept by introducing of periodic, multistage PEs. Design loop exploites random generation of patch topology with deterministic features, GA optimization procedures. For the structure analysis calling electromagnetic simulator IE3D is used. Resulting patch radiators are investigated through description of vector surface current distribution of operational modes. Several designed patch topologies and corresponding antenna properties will be demonstrated.

The work has been performed under the framework of ACE Activity 2.2 Small antennas and 2.3 Broadband and multi-band antennas.

Recently, fractal tree antennas have been studied with particular attention, as they provide miniaturization and multiband operation [1]. In this paper, a 3D fractal-tree dipole antenna was realized using electrochemical deposition technique. Return loss Measurements over the band 1-20 GHz and radiation patterns at resonating bands are presented. The multiband electromagnetic behavior of the antenna was studied and compared to previously reported 2D fractal-tree antenna studied by C. Puente and al [2]. It is shown that both the size and the impedance bandwidth of the 3D structure are improved compared to the 2D structure.

I. Antenna Design

The electrodeposition of the 3D-fractal shape was performed in an electroforming bath containing electrolyte solution prepared by analytical reagent copper sulfates and ultrapure water. The concentration of this electrolyte solution is 0.05 M. The metal deposit was grown on an aluminum-cathode, which turns with a speed of 1 turn per minute around its vertical axis, under a voltage of 12 V between electrodes. Finally, the fractal antenna was realized by connecting one like-tree copper deposit to the central axis of a coaxial connector as shown in Fig. 1

II. Results Fig. 2 shows the obtained results for the input return loss of the tree-dipole antenna with three ground planes of different dimensions ( 20'20, 50'50 and 150'150 mm).

The measured gain peaks at 3.73, 7.04, 9.55, 11.94, 14.56 and 17.41 GHz, are 4.72, 4.5, 5.7, 5.6, 5.85 and 5.1 dB, respectively. The measured far-field radiation patterns of E(theta) component for the (x-z) plane and (y-z) plane, at the resonance frequencies, are shown in Fig. 3.The radiation patterns for the E(theta) component present a certain degree of similarity. These patterns are characterized by a two-lobe structure although the patterns at 11.94, 14.56 and 17.41 GHz, show several dips. At higher frequencies, the number of lobes is not significantly increased, and the patterns presents several ripples, especially at f = 17.41 GHz, which can be explained by the diffraction at the edges of the ground plane.

III. Conclusion

The radiation properties of a random; 3D-fractal tree dipole generated by electrochemical deposition has been studied experimentally in a monopole configuration. The fractal morphology of the structure explain the multifrequency behavior observed in the return loss evolution over the band 1-20 GHz. The radiation patterns seem to be similar through the bands, although that some patterns present several dips. The measured bandwidths are clearly better than those of the 2D-fractal tree antenna [2].

References

1. J. Petko and al., IEEE Trans. on Ant. Propag., vol. 52, No. 8, pp. 1945-1956, Aug. 2004.
2. C. Puente and al., IEE Electron. Lett., vol. 32, no. 25, pp. 2298-2299, Dec 1996.

A coaxial-fed two-step rectangular dielectric resonator antenna (DRA) with wideband characteristics is studied theoretically and experimentally. The fundamental TE₁₁₁ mode of the rectangular DRA features some attractive properties, such as a low radiation Q factor and broadside radiation patterns. In this paper, a two-step rectangular DRA is excited by means of a probe placed in the center of the facet of the first (larger) step. The lower resonant frequency is approximately equal to the theoretical value for a single slab of the same dimensions as the larger step. The second step (smaller slab) is responsible for the excitation of a degenerate mode at a higher frequency, which is not directly linked to the higher-order modes of the larger slab. Therefore, dual-band or wideband operation can be achieved, where the polarization, the radiation patterns and the gain remain stable.

Fig. 1. Front and back view of the coaxial-fed two-step rectangular DRA.

The proposed antenna geometry is shown in Fig. 1. It comprises a two-step rectangular DRA of dielectric permittivity ε_rd = 9.8 that is placed on top of a ground plane. A probe of length l_p = 8 mm is feeding the larger slab, which has length a₁ = 10 mm, width d₁ = 10 mm and height b₁ = 20 mm. For these dimensions, the theoretical resonant frequency of the slab is 5.7 GHz. The smaller slab has dimensions a₂ = 5 mm, d₂ = 6 mm and height b₂ = 10 mm. The simulated return loss of this structure, together with the radiation patterns at 7.4GHz and 6.1 GHz in the x-z and y-z planes, are displayed in Fig. 2 a), b) and c) respectively. The TE^y₁₁₁ mode is at 5.8 GHz, a higher-order mode at 7 GHz and the mode excited by the smaller slab at 7.4 GHz. The structure achieved a bandwidth of 48% with stable radiation characteristics and low cross-polarization.

Fig. 2. Return loss and radiation patterns at 7.4GHz and 6.1GHz in the x-z and y-z planes.

Introduction. The current development in wireless telecommunication industry brings a necessity of using antennas for different frequency bands. The common multi-band antenna realizations often have low gain [1]. Its increasing by the antenna structure is complicated by the request of multi-band operation itself. Construction of arrays is disabled especially by achievement of suitable feeding and sufficient impedance matching. The use of passive ways of directivity increasing (reflectors, directors) is again with common techniques complicated.

Methods Used. Problems with using of common reflectors can be eliminated by implementation of frequency selective surfaces (FSS) into reflector structure [2]. They embody reflection properties only or especially in designed band while out of it they do not. Their cascade setup in adequate spacing enables operation in all frequency bands of design. With this constellation, planar antennas (with continuous ground plane) can not be used. Printed dipole and its modifications were used for concrete design [3].

Original Results. The constellation with frequency selective surfaces, symmetrical dipole antenna and planar reflectors for multi-band operation is the principal original idea presented in the paper. Several types of FSS are compared in the paper to achieve ideal properties (reflection or transmission). Several modifications of printed dipole antenna also with fractal elements were investigated. The feeding and impedance matching is outlined for combination of frequency bands NMT (CDMA) 450 MHz, GSM 900/1800/1900, ISM (Bluetooth) 2.4 GHz and ISM 5.3/5.7 GHz.

Verification. The design is made by method of moments in frequency domain (Zeland IE3D) with verification by finite differences time domain method (CST Microwave Studio) [4]. The design will be followed by manufacture and measurement with comparison of results.

Conclusion. The actual results exhibit the availability of using the FSS as cascaded reflectors. The directivity of approximately 10dBi can be reached with the transversal dimensions equal to the symmetrical wire dipole arm length. The antenna is compact and easy to manufacture. The suitable way of setting the antenna into array can further increase the antenna gain.

References
[1] GARG R., BHARTIA P., BAHL I., ITTIPIBOON A. Microstrip Antenna Design Handbook. Artech House: Boston&London, 2002.
[2] MUNK B. A. Frequency Selective Surfaces - Theory and Design. Wiley: New York, 2000.
[3] HERTL I. On Multiband Properties of Printed Dipole Antenna. In Proceedings of 15th International Czech - Slovak Scientific Conference Radioelektronika 2005. VUT: Brno, 2005.
[4] CERNOHORSKY D., RAIDA Z., SKVOR Z., NOVACEK Z. Analýza a optimalizace mikrovlnných struktur. VUT: Brno, 1999.

Dielectric resonator antennas (DRAs) have been increasingly investigated in recent years for microwaves and millimeter-wave communications applications. DRAs can provide low losses, high radiation efficiency, and high density integration and reduced size. In the same perspective, we proposed a new microstrip fed low profile broadband antenna. This antenna was numerically designed using Ansoft HFSS simulation software package. The investigated antenna uses a dielectric resonator, a microstrip fed patch and an intermediate to widen the bandwidth. A parametric study using an electromagnetic simulator was performed, and based on the optimised design; a prototype was fabricated and measured. Simulations and experiment measurements were carried out, and the comparison between them gives a good agreement. Using this approach, a fractional bandwidth of 41 % at the center frequency of 10.05GHz was achieved. This proposed antenna can be used for broadband wireless communications systems. In this paper, the proposed antenna will be described, then, the design procedure will be presented. Finally, simulated and experimental results will be discussed and compared.

A very wideband patch antenna is proposed. By employing a wide meandering strip to feed a patch, a planar antenna with an impedance bandwidth of 51 % for SWR<1.5 and a gain of 9dBi is presented. The proposed antenna has a stable broadside radiation pattern and low cross polarization across its operating bandwidth. The impedance bandwidth can be further enhanced to 64 percent by increasing the substrate thickness and the width of the strip if the requirements of the cross polarization and back lobe levels can be released. The antenna is simple in structure and low in cost. It can potentially be used as a base station antenna for multibands operation. A novel bandwidth enhancement technique has been proposed for increasing the impedance bandwidth of a patch antenna. The antenna is suitable for applications in GSM1800 , CDMA1900, UMTS, 3G IMT 2000, bluetooth and Wireless LANs. The antenna has a simple structure and is low cost. It only consists of a rectangular patch, a wide meandering strip and a ground plane, and no other parasitic elements are required.

Self-similar antennas such a bow-tie antenna and a bi-conical antenna have been proposed for broadband wireless systems. These antennas have ultra wideband characteristics under the condition of infinite structures. Therefore, in case of finite structures, the relative bandwidth of antennas may be narrower than the case of infinite structures.
This paper describes the maximum relative bandwidth of a conventional bow-tie antenna under the condition of finite structures in order to apply bow-tie antennas to elements of an antenna array, and proposes the optimum shape of a new type bow-tie antenna with wider bandwidth rather than a conventional type.
First, we consider a conventional bow-tie antenna as shown in Fig.1. When the structure of bow-tie antenna is infinite and the angle ƒÆ is 90 degrees, the antenna has constant impedance as �gPrinciple of Self-complementarity�h. Figure 2 shows the relative bandwidth variation against the width w of element and the angle ƒÆ. As shown in Fig.2, it is clarified that the maximum relative bandwidth is 160% at the condition of 0.7 wavelength width and the angle ƒÆ of 80-90 degrees.
Next, in order to obtain the relative bandwidth greater than one of a conventional bow-tie antenna, the shape of bow-tie antenna can be changed from trapezoidal shape as shown in Fig.1 to other shapes as shown in Fig.3. By applying the elliptical shape, 170% maximum relative bandwidth is obtained.
Based on the above-mentioned results, we consider an antenna array with bow-tie antennas as elements of array. In case of a conventional bow-tie antenna, the simulation results show that the variation of relative bandwidth in case of the angle Į of 80-90 degrees is less than the case of other angles. And, it is verified that the maximum relative bandwidth is obtained by applying the elliptical shape in case of 0.5 wavelength spacing of array.
The above theoretical results verify that the bow-tie antenna with the elliptical shape has the maximum relative bandwidth for applying to antenna arrays.

Abstract:

In this paper electromagnetic genetic algorithm (GA) optimization for patch antenna design, to achieve multiband characteristics will be demonstrated. Method of moment (MoM) solution used in simulation relies on RWG (Rao-Wilton-Glisson) edge elements. The Application will demonstrate GA/MoM integration in manipulating antenna parameters to achieve optimization goals.

Key words:Multiband, wideband, UWB antennas

1. Introduction

The demand for multi mode antenna for personnel communication network (PCN) has been increasing in recent days, and patch antennas are probably the most widely used class of antennas in PCN. Antenna design for multi band purpose presents a unique challenge as such antennas should be as simple as possible, light weight and low cost while at the same time satisfying particular electrical requirements. GA optimization has been successful in different disciplines of engineering and its integration with MoM forms one of the best solutions in finding simple and best configuration of antenna which fulfill electrical parameters according to the system requirements.

2. Design process

Probe feed microstrip patch antenna will be examined in this paper. Approximate antenna dimensions are determined using the following common formula for half wavelength patch:

c- speed of light, fo – designed resonant frequency, εr – dielectric constant

GA optimization in this paper will be demonstrated in two stages. First stage involves finding of best optimal feeding position and antenna height. In this stage GA creates individuals by randomly selecting feeding position and height simultaneously then followed by MoM analysis. Second stage deals with optimization of patch radiator. The Best model for first stage serves as mother model in this stage. GA creates individuals by randomly removing rectangles from radiator to create different shapes, and then MoM analysis follows as above. If Fitness function is well programmed, good results can be achieved as it serves as the connector between physical problem being optimized and GA. In both stages the best individual is the one with best S11 magnitude and bandwidth in specified resonates.

3. Demonstration

The following patch antenna has been demonstrated for trial and the results are presented before and after optimizations. Design goal: to achieve triband characteristics at 2 GHz, 3GHz and 4 GHz.

(a) (b)

(c)

Fig 1. a) Antenna model before optimization b) after second stage optimization c) simulation results ".." before optimization, "continuous line" after first stage, "--" after second stage optimization

4. Conclusion

Above trial demonstrates the power of GA/MoM in designing of simple and integrated antenna. In the full paper well worked out results will be presented and discussed. Comparison of simulation results of the best model from different numerical code will also be presented.

Broadband high gain antenna design, due to the large frequency bands required by standards, is one of the most challenging issues of electromagnetic compatibility testing. During the time, different kinds of broadband antennas such as biconical and log-periodic antennas have been used by standards, but in recent years the approach has been towards double ridged guide horn antennas (DRGH). Due to the special features of these broadband antennas such as high gain and directivity performance, low VSWR and wide half power beamwidth over the entire frequency band, they are one of the favorable choices for EMC and standard antenna measurement.

Although they have been used over a half of century, recently it was shown that the traditional ones have a major problem in their radiation pattern and gain especially in higher frequencies. This deficiency confronted the use of traditional DRGH in EMC testing with a question. To find a solution for this problem, a constructed 1-18 DRGH with known typical data was exactly modeled. This model was simulated with two different softwares (HFSS and CST). The simulations results were in good agreement with each other and with measurement data. A complete sensitivity analysis was performed on the antenna parameters. This information contributed and led to design of a new DRGH antenna step by step (will be explained in full text). In addition to its considerably smaller size this design of DRGH antenna not only removes the deterioration of radiation pattern and gain of the antenna in higher frequencies, but also improves the performance of the antenna. Figures 1, 2 and 3 show the process of modification in sequence from the traditional design towards the new design. Figures 4 and 5 depict the major superiority of the new design against the traditional one in improving the radiation pattern in higher frequencies (here f=18 GHz). This design has also better behavior in VSWR, broadband gain, half power beamwidth and side lobe level (SLL).

Fig. 1

Fig. 2

Fig. 3

Fig. 4

Fig. 5

The international radio astronomy community is making detailed plans for the development of a new radio telescope, the Square Kilometer Array (SKA). For frequencies below 2 GHz aperture arrays are considered. Cost effectiveness is very important for the feasibility of such large aperture arrays. To this end, a new dual-polarized demonstrator tile of wideband, closely packed Vivaldi antennas named VALARRY (Vivaldi ALuminium ARRaY) has been developed. The cost of this square-meter tile (see figure below) is significantly reduced with respect to THEA, the previous SKA demonstrator. The size of the dielectric substrate has been limited to the feed region only, thereby reducing the losses, corresponding noise contribution and cost. For similar reasons, the antenna has been made of relatively low-cost aluminium. Supporting metal posts are used to electrically connect the array elements, rather than soldering them, and to enable us to conveniently mount every antenna element separately. Although the cost has been reduced with respect to THEA, we do not comprise the performance characteristics. The Microstrip feed with radial stub has been designed first order using a back-to-back configuration, for which the measured S-parameters are shown below for 50-Ohms. The antenna and feed has been further optimized for its impedance characteristics over scan angles up to 50 degrees and for a frequency band of 0.5GHz -- 1.5GHz. For this purpose, we utilized the infinite array simulation capabilities of HFSS. Measurement and simulation results of the array performance will be presented for the 8x7x2 Vivaldi array, for which the preliminary results obtained by the back-to-back measurements as well as the array simulations are very promising.

Modern aircraft are equipped with many airborne antennas for communications, radar, etc. The performance of these antennas may be degraded due to the aircraft structure interferences. In these situations, a significant front to back radiation ratio is desired in order to minimize the influence of the aircraft on the antenna input impedance and radiation characteristics. In airborne applications, broadband and low profile antennas are required. Spiral antennas are widely used to fill the above-mentioned specifications. In classical spiral antennas, a unidirectional beam is performed with an appropriate backing cavity, a conducting reflector plane, an electromagnetic wave absorber. The presence of a ground plane requires the radiating element to be located at a quarter of wavelength above it, hence increasing the dimensions of the antenna and limiting its bandwidth. When the radiator is placed above an electromagnetic absorber, a low efficiency results due to power dissipation, and according to the frequency band the antenna may become heavy due to the quantity of absorbing material.

In airborne applications, the dimensions and weight of antennas have to be reduced in order to integrate new systems into aircrafts. In this paper, we propose a novel Loaded Electromagnetic Band Gap (LEBG) substrate as reflector for an Archimedean spiral antenna. The benefits from using such a substrate are related to the suppression of surface waves excited in the substrate by the radiating element. Suppression or reduction of surface waves is expected to improve the antenna's efficiency and to bring a reduction of side lobe level. As a result of the surface waves suppression, an antenna above a high impedance ground plane produces a smoother radiation profile than a similar antenna above a conventional metal ground plane. The proposed antenna is composed of two parts: An Archimedean spiral as radiating element and an LEBG substrate. The top surface of the LEBG substrate is a capacitive frequency selective surface (FSS), composed of a planar array of square patches. Each patch is connected to adjacent patches by a resistor, the bottom surface of the LEBG substrate is metallized to avoid backward radiation. The frequency band gap is defined by the patch size, the gap between patches, the height of the LEBG structure, its permittivity and the resistor value. A complete method based on a transmission line model is presented to design the LEBG substrate. Simulation and measurements are also presented.

A -10dB impedance matching is achieved over a 10:1 frequency range. The simulated and measured broadside gain will be also presented. The height and weight of this novel antenna is greatly reduced and their performances are better compared to a conventional spiral antenna with unidirectional beam. The antenna has attractive features such as a very thin thickness of a hundredth of wavelength at the lowest frequency, a good impedance matching as well as circularly polarized patterns over a 10:1 bandwidth. The novel LEBG substrate can be used as reflector for any kind of planar antennas using ground plane in order to enhance their radiation characteristics and input impedance matching.

An ultra wideband hybrid dielectric resonator antenna (DRA) has recently been investigated and patented in [1] and [2], respectively wherein a simple combination of a monopole and a dielectric ring resonator was employed (Fig. 1) to achieve nearly 3:1 impedance bandwidth (S11<-10 dB). Omtimized performance based on the simulatuion and experimental results were presented [1]. From those studies [1] it is apparent that the ultra wide bandwidth of the new antenna is caused by the multiple resosnaces as shown in Fig. 2. But for practical application, it is essential to pursue further investigations to identify the resonant modes which may lead to some well defined guidelines for designing the antenna.

In this paper, we have investigated the resonant modes in the composite structure and a better insight into resonance phenomena is developed. Anfoft's HFSS fullwave solver has been used to study the input impedance and the electric current and field distributions in the composite structure. The various existing resonance modes as well as the interactions between the monopole and fields in the DRA are identified. The gained physical insight should lead to solving many new problems associated with designing the antenna for different frequency bands.

References
[1] M. Lapierre, Y. M. M. Antar, A Ittipiboon and A. Petosa, " Ultra wideband monopole/dielectric resonator antenna," IEEE Microwave Wireless Comp. Lett. Vol. 15, No. 1, pp. 7-9, Jan. 2005
[2] A Ittipiboon, A. Petosa, S. Thirakoune, D. Lee, M. Lapierre and Y. M. M. Antar, " Ultra wideband antenna," US Patent No. 6,940,463 B2, Sept. 2005.

Bourtoutian, R.¹; Delaveaud, C.¹; Toutain, S.^2
1CEA/LETI Grenoble, FRANCE;
²IREENA Nantes, FRANCE

Introduction

Ultra-wideband omnidirectional antennas are subject to numerous studies for applications in high data rate transmission. However UWB systems interfere with the WLAN systems at the frequency band ranging from 5 to 6 GHz. To create a filtering function at this frequencies thin half wavelength spurlines, slots and notches have been inserted in the antenna structures.
Classical heights of spherical, conical, planar and other type of UWB antennas range from 1/2 to 1/4 wavelength. Shorted, top loaded and biconical wideband antennas show heights as low as 0.18 wavelength. These antennas use trapezoidal feed plates, biconical feed, and four shorting wires, which make the insertion of slots, in order to realize a frequency notch function, become an extremely hard operation.

In this paper we present a more compact antenna structure, fed by an elliptical plate, and connected to a reduced size ground plane by only two shorting wires. The reduced number of shorting wires and the elliptical form of the feed plate made it possible both to reduce the antenna height to a value of 0.08 wavelength at the frequency of 3.1 GHz and to insert slots in the antenna hat in order to realize band rejection filtering at the WLAN frequencies which interfere with the UWB frequencies.

Classical UWB filtering antennas provide gentle rejection edges which partially fail to suppress the undesired signals. To improve the filtering function, we used an arrangement of slots with two different lengths that create two near resonances which are coupled together to form a wider rejection bandwidth with sharp rejection edges.

A prototype of the proposed antenna have been realized and measured. The antenna is matched at -10 dB over all the UWB frequencies (3.1 to 10.6 GHz), with a sharp edge frequency band notch at the WLAN frequencies (5-6 GHz).

Antenna Design

The geometry of the proposed antenna is shown in the Figure 1. It consists of a circular hat realized on a low loss substrate (ε_r = 3.38), connected to the circular ground plane by two shorting wires. The antenna hat is fed in the middle by an elliptical form plate connected at its base to a coaxial cable probe. The height of the hat is 7.5 mm which represents 0.08 wavelength at the lowest operating frequency. The radius of the hat is 13.6 mm, and the radius of the reduced size metallic ground plane is 37.5 mm.

Four arc shaped symmetrical notches, etched in the circular hat, assume the band-rejection function. These slots create two resonances at 5.3 GHz and 5.9 GHz. The coupling between these two resonances creates a notched frequency band of 1 GHz around 5.5 GHz.

Results

A parametric study has been carried out using CST Microwave Studio, in order to optimize the antenna and the band notch characteristics. Two prototypes for the notched and the reference antenna have been realized. Figure 2 shows the measured return loss of the proposed antenna compared to the reference antenna without slots. At the center frequency the return loss is equal to 1.4 dB. The band rejection function has a measured lower slope of 16dB/GHz and an upper one of 80dB/GHz.

In the final paper the filtering behaviour will be illustrated using the radiation characteristics of the proposed antenna obtained experimentally from both frequency and time domain measurements.

In recent years, antenna design for Ultra-Wideband (UWB) systems has attracted increasing interest due to the appealing properties of this new communication standard. Planar monopole antennas have been found to be excellent candidates to operate in UWB systems, since they can cover the system's required frequency range (from 3.1 GHz to 10.6 GHz) and they present a very compact low-cost structure. Several planar geometries have been experimentally investigated to provide UWB operation capabilities, using either a perpendicular or a coplanar ground plane configuration.

Over the UWB frequency band there exist other wireless systems operating bands, such as the 5.2 GHz (5150 GHz-5350 MHz) and 5.8 GHz (5725-5825 MHz), which might cause interference with the UWB system. As a consequence, a step forward in UWB antenna design has been to provide a frequency-notched feature in the antenna itself. This interference filtering is easily accomplished by inserting a narrow-band resonant slot in the antenna planar geometry and by properly adjusting the slot length to make it resonant at the filtering frequency.

The objective of this paper is to further improve the antenna filtering capability by making this band-notched behaviour switchable. Depending on whether interference from other systems is present or not, the antenna filtering feature can be activated. Fig. 1 shows the geometry of the switchable band-notched antenna. It consists of a bevelled square planar monopole antenna with an embedded rectangular slot ring. A switch device is then inserted in the slot ring, so that the filtering behaviour is achieved when the slot ring is fed. Fig. 2 shows the VSWR response of the proposed UWB antenna both when the switch is activated or not. As observed, when the slot ring is fed a band-reject response is obtained, which is centred at the frequency where the slot ring resonates.

The required switching operation can be implemented by either a PIN-diode or a MEMS device with appropriate performance at the operating frequencies. An early prototype of the proposed antenna has been fabricated using a commercially available PIN-diode. Comparison of simulations and measurements for the two switching states will be provided and some details of the prototype design will be discussed. Results regarding frequency response will be completed with investigations in time domain, in order to guarantee the suitability of the proposed antenna for being employed in UWB systems.

The Ultra Wide-Band (UWB) technology used with indoor high data rate wireless communication systems is now emerging. But, the design of antennas which are able to work from 3.1 to 10.6 GHz with very short pulses to convey information, still restrains its deployment. Nowadays, more and more structures are presented: they are compact, low cost, omnidirectional, and give the required impedance matching. However, their radiation patterns are not stable in the band or their transient responses are not always well-suited for UWB applications. In addition, although no authoritative regulation has been yet established in Europe, the frequency band from 5 to 6 GHz should be filtered to avoid interference with other sensitive technologies.
In this paper, we present a design drifted from a wide slot antenna (X. Qing, M. Y. W. Chia and X. Wu, IEEE APS URSI, July 2003): the stripline wide slot antenna. This structure offers a better radiation pattern bi-directionality and a wider bandwidth of 132%. Moreover, it can be easily modified to filter from 5 GHz to 6 GHz.

The stripline slot antenna proposed is a square structure with a side of 42 mm that is 0.9 of the wavelength at the central frequency 6.5 GHz. A 2.2 permittivity substrate is used. Two wide rectangular slots have been realized on the two ground planes in the front and in the back of the stripline structure. The feed is fork-like, figure 1. In measure, the antenna covers the UWB band: 2.8 to 13.6 GHz that is a 132% fractional bandwidth. Then, the use of 2D diagrams (gain versus frequency) allows efficiently characterising the antenna radiation. It gives the normalized gain curve at a given frequency (vertical reading) and the stability in terms of radiation over the whole band in a particular direction (horizontal reading). An example is shown in E-plane, figure 2. This structure offers stability versus frequency with a radiation bandwidth from 3 to 10 GHz.

The time domain antenna behavior has been analyzed too. First, the phase linearity versus frequency indicates the low pulse distortion direction. The use of 2D diagrams (phase in a cut plane versus frequency) points out that the antenna is low dispersive from 3.5 GHz to 10 GHz. This result is validated by measuring the pulse received after a UWB link realized with two stripline antennas. This waveform is compared with the pulse shape feeding the transmitting antenna. Measurements are done in time domain in several directions in E-plane. Figure 3 shows an example in line of sight. The pulse main beam does not change and its magnitude stays higher than the ripple magnitudes in line of sight. So, this antenna can be used with short pulses and is a good candidate for UWB applications. Moreover by modifying the feed shape, the band 5 to 6 GHz can be filtered. More details and results will be given in the final paper.

For some UWB applications, it is crucial to know the transient responses of antennas. With the presence of applied impulse, we are able to evaluate the distortion or fidelity of radiated as well as received impulses. From UWB point of view, the transmitting and receiving processes are non-symmetrical.

When evaluating the performance of UWB antennas, some papers tend to take into account only the marching, whereas the paper takes also into consideration a distortion of the radiated impulses.

In the abstract, the optimization of the following UWB antennas is discussed: UWB dipoles and Vivaldi antennas. Optimization process searches for the dipole shape, which accomplishes the required parameters - i.e. good matching and minimal distortion.

The optimized UWB antenna should be matched in the required frequency band. Moreover, it should have the required frequency bandwidth (for UWB antennas minimally 500MHz or FBW > 0.2) and should minimally distort the applied impulse (signal). The dipoles have derivative characteristics. The distortion is evaluated from the derivative of the excitation impulse with the radiated impulse.

The particle swarm optimization (PSO) method was used for optimization. This method was implemented in MATLAB®, where the antenna structure is generated. The antenna is subsequently simulated in the CST Microwave Studio®.

The optimized UWB dipole is perfectly matched and minimally distorts the applied signal. In case of the second derivative of Gaussian impulse feeding, the fidelity of radiated impulse with the third derivative approximately equals 93.3 %, see Fig. 1.

The final paper will discuss more extensively obtained results using the PSO optimization method.

This research work is a part of the activities of the CTU in Prague in a frame of the Antenna Centre of Excellence network (ACE2).

Fig. 1. Transient response of excitation and radiated impulse

In this paper we present a specific type of monopole antenna designed in order to obtain the lowest return losses over the extremely high bandwidth defined by the frequency band between 3 and 20 GHz. This frequency band has been chosen to cover the definition of Ultra Wide-Band (UWB), which is placed between 3.1 and 10.6 GHz in its major part but uses frequencies over 11 GHz like the 16.2 to 17.7 GHz band. This massive frequency band allows the use of a great amount of other technologies. In order to obtain the best results, the profile of the designed antenna has been optimized.

Although the analysis of radiating structures with full wave numerical methods involves some problems due to the fact that they are time and resources consuming methods, the use of these tools are mandatory when we face the analysis of devices with arbitrary geometrical shape that do not allow analytic methods. The efficiency of the method is then essential if we want to use it in an optimization loop for synthesis. In this case the computational effort is reduced because of the axial symmetry of the device. Is this axial symmetry what allows the problem to be analyzed in two dimensions (2-D) with the obvious reduction in the computational effort that allows the whole optimization.

The objective of this design is to perform a blind optimization of the profile of the antenna under study. Due to this, to avoid local minimum, the optimization is carried out by an adaptive version of the Simulated Annealing algorithm. We need an efficient and versatile tool to get results in a reasonable time in every optimization step. To fulfill this requirements the analysis is performed by an hybrid method based on the Segmentation technique, the Finite-Element Method and a Matrix Lanczos-Padè algorithm (SFELP) [1], adapted to problems with axial symmetry. The field in the radiation port is expanded across a sum of spherical modes that allow the definition of equivalent current and voltage.

As a result from the optimization process, an optimized profile monopole antenna is presented here. This design present an ultra wide-band response over the 3 to 20 GHz frequency band. In this frequency band return losses of less than 10dB have been achieved and in the frequency range defined between 4 to 20GHz, return losses have been reduced to less than 17.8dB. To make this analysis, the antenna has been simulated using an infinite ground plane. Results from this simulation will be compared with those of a similar simulation using the HFSS commercial analysis tool. Simulations using different radius of the finite ground plane will also be presented. In all cases radiation patterns from the analysis will be also presented. All this data will be of great interest to design and build a prototype in any desired frequency band to check the optimization method.

[1] J.Rubio, J.Arroyo and J.Zapata, "SFELP-an efficient methodology for microwave circuit analysis", IEEE Trans. Microwave Theory Tech. vol.49, no.3, pp. 509-516, 2001

A small printed antenna for Ultra-Wideband (UWB) applications is presented in this paper. The antenna is made on silicon substrate with thickness of 0.6 mm. The antenna operating frequency is in the lower band of UWB frequency range of 3.1- 5.1 GHz. The presented antenna achieved a liner gain with omni-directional patterns and very low look-angle dispersion. The small size of antenna makes it suitable for different applications such as body area network.

Antenna Design:

The UWB system transmits tremendously short pulses without any carrier and occupies bandwidth of more than a few GHz. As a result, antenna plays an important role in the UWB system than in any other system. The structure and dimension of the proposed loop antenna is illustrated in Fig. 1. The antenna made on a very tiny Silicon substrate with thickness of 0.6 mm. To minimize the error on the antenna frequency band due to transmission line, the transmission line length and width are considered as a part of the radiating element.

Result:

The 2 GHz (3.1-5.1 GHz) bandwidth has been achieved for VSWR<2, as illustrated in Fig. 2. The antenna gain is illustrated in Fig. 3, it can be seen that a liner gain has been achieved in the total frequency band. The azimuth field distribution is quite uniform (omni-directional) for three different frequencies, 3.1, 4.1, and 5.1 GHz. The elevation pattern is nearly omni-directional. The radiation pattern is remained approximately same in the each frequency (3.1, 4.1, and 5.1 GHz), which shows there is no or very low look-angle dispersion. The antenna radiation pattern at 4.1 GHz is shown in Fig. 4.

Conclusion:

A well suited printed antenna for commercial application of the UWB frequency operation of 3.1-5.1 GHz is presented in this paper. The antenna has attractive features same as, liner gain, VSWR<2 in the total band, low dispersion, nearly omni-directional patterns, and very tiny structure, which makes it very strong candidate for very small UWB wireless devices and wireless body area network.

In UWB systems, antenna design is a of primary importance. The evolution of their radiation performance with frequency alters the antenna impulse response and consequently the shapes of transmit pulses. In this context, characterization of antenna behaviour allows to forecast their ability to efficiently radiate pulses depending on their spectrum and bandwidth. In this article, a Time Domain measurement procedure used to calculate the antenna transfer function in different directions is presented. This method is applied to validate the design optimization of a coplanar printed UWB antenna for lower pulse distortion. The results obtained experimentally are also compared with Transmission Line Matrix (TLM) simulations to prove the validity of this numerical approach.

The measurements were performed on an UWB Time Domain facility constituted by a pulse generator, and a fast sampling scope. The filtered signal of [2-12 GHz] bandwidth is applied at a transmitting pre-calibrated tapered slot antenna while the antenna under test is placed on reception. Both antennas are set in an anechoic chamber. At the reception, all components are deconvoluted to isolate the antenna and channel effects from the measured pulse [1]. The propagation channel is characterized using the Friis formula and the transmit and receive antenna transfer functions are differentiated in regard of the derivation effect [2]. All the data processing are performed in frequency domain using Fourier Transform. The transmitting transfer functions of the antenna under test are plotted versus frequency and direction in xz and xy planes. These characterizations allow us to calculate the distortion of a reference pulse [3,1-10,6 GHz] radiated by the antenna in these planes.

A novel triangular CPW-fed printed antenna with specific ground planes is measured (fig.1)[3]. The obtained transfer function show that its distortion (in term of group delay) is depending on radiation direction. A work is carried out on ground planes shape to obtain more constant and omnidirectional diagrams with frequency. Time domain characterisation are conducted to verify experimentally the efficiency of antenna improvement on pulse distortion.

A Time Domain numerical simulation was also performed to model the transmit/receive antenna and propagation channel link with the same excitation pulse than the experimental one. The home-made TLM-based software (FP-EMMA-TLM [4]) used is implemented on an IBM-SP4 parallel computer which allows calculating a large amount of data. The simulated results offer good agreement with measurements (fig.2). This demonstrates that FP-EMMA-TLM could be an efficient CAD tool to improve the key parameters on antenna design for UWB telecommunications and particularly the antenna's environment influence on link performance.

References

[1] B. Denis, J. Keignart, « Post-Processing Framework for Enhanced UWB Channel Modeling from Band-Limited Measurements », in Proc. IEEE UWBST 2003, Reston, Nov. 2003, pp. 260-264.

[2] J. Kunish, J. Pamp, « Consideration regarding the correlation between UWB antenna transmit and receive responses », URSI EMST, 2004.

[3] N. Fortino, G. Kossiavas, J.-Y. Dauvignac, and R. Staraj, « Novel antennas for ultra-wideband communications », Microwave and Optical Technology Letters, V.41, N.3, pp.166-169, May 2004.

[4] FP-EMMA-TLM software, http://www.antennasvce.org/Public/Software

In this paper, a directive sinuous slot antenna for UWB applications is presented. This antenna is feeding by a microstrip taper balun, which can be used as a coaxial line to Lecher line transition. To obtain directive antenna a reflector has been added and optimised. Finally we realized, a compact directive UWB slot sinuous antenna which is included in a 5cm3 volume. The balun and the antenna was modelled with CST Microwave Studio. The conception of the antenna has been validated by measurements.

The sinuous slot antenna

This antenna was first defined in 1996 [1] and its geometry looks like the sinuous antenna of R. H. Duhamel [2]. But this antenna is not a self complementary antenna, so its impedance is not close to 290 Ohms. In fact, this antenna modelling as a strip antenna, which is really easy to calculate, has an average input impedance of 350 Ohms. After, we use the Babinet's principle and our strip antenna became a slot antenna (the complementary antenna, Fig. 1). So this slot antenna has an average input impedance close to 100 Ohms.

The balun

In this work, a microstrip taper balun for ultra wideband applications has been optimized with a total length of 5 cm. The geometry is presented on Fig. 2. The width of the substrate is .762 mm , and the permittivity is 2.17. This balun has been realized and measured with a load of 82 Ohms connected to the lecher line port. The -10 dB measured bandwidth is between 10 MHz and 13GHz and the –15dB measured bandwidth is between 1.5 GHz and 6.7 GHz (Fig.3).

Realization and measurements

To obtain a directive antenna, we have added a ground plane to this ultra wideband antenna. The size of the ground plane is 5cm by 5cm. The influence of the distance between this limited ground plane and the radiating part has been studied. This antenna has been realized and mounted on a dielectric support (Eccostock-LoK) with low permittivity (1.7). The distance between the limited ground plane and the radiating part has been chosen equal to 4 cm. The measured return loss of the UWB sinuous antenna is below -10dB between 2.4 and 6.9 GHz (Fig.3). Radiation pattern and gain has been analysed and the directive behaviour of this antenna has been verified.

References

[1] X. Begaud, P. Poey, J. P. Daniel, G. Dubost, "Antennes à fentes à double polarisation et à très large bande passante et procédé pour sa réalisation". French pattent 96 15027, December 1996.

[2] R. H. DuHamel, "Dual Polarized Sinuous Antennas," U.S. Patent 4 658 262, Apr. 14, 1987.

[3] X. Begaud, P. Poey, J. P. Daniel, and G. Dubost. Design of wideband dual polarized slot antenna. presented at Proc. Millennium Conf. Antennas and Propagation, Apr. 12, 2000.

UWB systems promise to offer great data rates thanks to the use of a very wide frequency range. Theses device constrains are mainly due to the frequency range greater than 100 %. In this transmission system among all the devices, the antenna is one of the most difficult device to design. Indeed, its caracteristics can not be considered constant [1] like in narrow band systems, in particular concerning the gain which depends on the frequency and the relative spatial orientation of the antennas. So it can be interesting to have a model of the antenna that permits to take into account all the characteristics in a complete transmission system composed of the emitter, the channel and the receiver.

We propose to model the antenna in a circuit form, to improve its matching to an emitter or a receiver system or to take into account the antenna effects in a system simulation. In the first case, only the input impedance of the antenna has to be modelled [2]. In the second case, the antenna has to be represented in reflection and transmission. The proposed model consists in representing the antenna by a band pass filter which parameters depend on the spatial variables. The circuit model is mainly composed by many resonant cells in series.

To determine the values of the various components, we have to identify the S parameters of the antenna. To obtain these parameters, we use the representation (fig 1) of the link composed of an antenna, the channel and an antenna. The antennas are supposed to be identical and are set in the same position symetrically. We can obtain the parameters of the antenna thanks to the simulation or a measurement of the link parameters.

The circuit model of the antenna is based on a band pass filtering structure which depicts the general performance of an antenna. The chosen structure is a symmetric T-structure (fig 2) because of the symmetry of the S parameters. The T horizontal branch impedances are noted Ra, the vertical one Rb. Rb is composed by several RLC parallel resonant cells which represent the various resonances of the antenna even if UWB antennas can not be described as resonant structures. Ra impedance is also composed by resonant cells and by other types of structures which debrief the antenna feeding for example. To determine these two impedances, we consider the Z-parameters of the antenna inferred from S-parameters.

Thanks to an iterative algorithm, it should be possible to obtain the values of the various cells parameters.

Thus, the paper proposes a transmission antenna model which allows to integrate the antenna characteristics in a global transmission system to define transfer function. It can also be used to optimize the matching between the antenna and components connected to it, or to analyze the functioning of the antenna by giving a physical meaning to the impedance cells.

[1] S. Zwierzchowski, "Antennas for UWB communications: a novel filtering perspective", AP-S, June 2004

[2]I. PELE, "simultaneous modelling of impedance and radiation pattern antenna for UWB pulse modulation", AP-S, June 2004

This paper further investigates the design of a UWB antenna based on [1] by means of both measurement and analysis, to determine if further improvement could be made by further careful optimisation of the design and to then evaluate the antenna's performance in terms of the excitation of particular current modes on its surface using an FDTD model.

The antenna proposed in [1] is a trident-fed, square, metal monopole antenna. The trident feed is intended to ensure that the excited current distribution on the antenna is uniform across the antenna, corresponding to the lowest excited mode of the antenna.

A candidate UWB antenna (Figure 1) has been investigated, based on the design approach in [1] but optimised for UWB-operation. The input characteristics of the optimised antenna (Figure 2) show considerable promise for use across the UWB spectrum. Its radiation patterns are relatively frequency-independent across the frequency range and these will be presented at the conference.

The good agreement between the experimental and simulated results in Figure 2 suggests that FDTD simulation will be an accurate tool for further analysis of the antenna that is difficult or impossible to carry out by experimentation, e.g. assessment of current distributions.

This antenna is under current development with a view towards further analysis and then optimisation by means of experimentation and simulation, the results presented herein being the first step in this process.

References

[1] Kin-Lu Wong, Chih-Hsien Wu, and Saou-Wen (Stephen) Su, "Ultrawide-Band Square Planar Metal-Plate Monopole Antenna With a Trident-Shaped Feeding Strip", IEEE Transactions on Antennas and Propagation, vol. 53, no. 4, April 2005.

The fast growing field of ultrawideband technologies requires creating adequate techniques for measuring characteristics of UWB antennas. These can be measured either in the Frequency Domain (FD) (using Vector Network Analyzer) or in the Time Domain (TD). Currently the FD measurements result in much better S/N ratio than direct TD measurements. With TD measurements one is limited with sampling rate of the measurer that restricts time resolution, the counterpart in FD is the frequency band. At this we are limited not only at high frequencies (contemporary VNAs usually have 6 GHz or 20 GHz high frequency limit) but also at low frequencies due to limitations of the measurement environment – the anechoic chamber. Having measured the FD S₁₂ parameters of the antennas we need to apply Fourier transformation to convert data into TD and get impulse characteristic. At this stage band limitness should be overcome in some way. One possible solution is zero padding that introduces numerical artifacts into the impulse; other approach is windowing that results in widening the pulse width. The best approach is spectrum extrapolation that allows both removing numerical artifacts and improving time resolution due to reconstruction of high frequency part of the spectrum.

Here we propose a technique for such extrapolation that exploits Pencil of Matrix for restoring high frequency part of the spectrum and Gershberg-Papoulis algorithm (modified by Vovk) to restore the low frequency part with explicit assumption of zero DC component.

The proposed technique is illustrated at example of Resistively Loaded Vivaldi Antenna. The antenna consists of coplanar strips exponentially tapering with both exponential impedance profile. The strips smoothly bend back and via corrugation connect to resistive narrowing strips that are connected at back (there may be additional lumped RC circuit here). Such a configuration suppresses only unwanted currents thus not affecting the impulse radiating performance. The use of the loading allows significantly reduce ringing in the antenna, while using exponential impedance profile results in shorter impulse width.

The antenna has been measured at DTU-ESA Spherical Near Field Antenna Test Facility. Two identical antennas were measured in coaxial configuration in 0,22 – 6 GHz band with 20 MHz step. Then the spectrum of impulse response of a single antenna was extracted and the extrapolation algorithm was applied to reconstruct low and high frequencies. The resulting NIR is shown in the figure. The half amplitude duration of the reconstructed NIR is 66 ps, though the high frequency of the measuring (6 GHz) would allow time resolution of only ~160 ps. The detailed discussion of this measuring and description of the algorithm will be given in the full paper.

An algorithm is proposed to process band-limited measuring of impulse responses that allows one to remove Fourier transform artifacts and significantly improve time resolution.

Microwave baluns are realized in either distributed or lumped circuits. The formers have shown their capability in many hybrid-based circuit designs. However, because of their non-planar structure and large size, few of them are adopted printed circuit processes. Conversely, the lumped circuit baluns, for their compact size, can play important roles in printed circuits and planar antennas.

Among the lumped circuit baluns, the simplest one is the lattice balun pass filter structure. In this structure, the 3 dB cross frequency is the centre of the balun bandwidth in which a phase difference of 180° between outputs is naturally obtained. In order to increase the balun bandwidth, a basic idea can be applied. This would consist of increasing the order of the two filters. Recently, in [1], a second order balun has been proposed where each filter has been upgraded to a T structure and a complementary filter has been added in a parallel way at the output of both filters in order to equalize the phase difference of 180° over the common bandwidth of the two filters. Such structure is similar to the lumped element rat race or Ï 3dB hybrid [2], which can also act as a balun. Furthermore, the structure proposed in [1] achieves very large bandwidths (in theory, up to 4:1) and can also provide impedance transformation between input and outputs.

The design presented in this paper tries to achieve similar bandwidth with impedance transformation to feed UWB printed balance antennas. To do this, high order filters designed under the basis of impedance geometrical progression are analysed; it means that, in the general case, the parallel output filters introduced in [1] are avoided in order to minimise the number of components and their losses. The analysis has shown that the phase-opposition is only maintained over a large bandwidth for second order filters. (It must be pointed out that the robustness of the circuit has been computed, showing that a 1:3 variation in impedance leads to acceptable mismatching at the output.

Furthermore, both theoretical and experimental work, show the extremely important sensitivity to parasitic effects (tolerances, self resonances, size and bondings). For instance, the small asymmetry in the circuit leads to a 20° error in the phase difference that must be compensated with a short length transmission line. The prototype was intended to work in the 250-800 band with impedance transformation from 50 to 75 ohm. Figure 1 shows good agreement between simulated and measured results. Figure 2 shows the circuit built. A second realization to feed UWB dipoles over 800 to 2500 MHz, 50 to 114 ohm will be shown at the conference.

[1] D. Kuylenstierna, P. Linner, "Broadband lumped element baluns with inherent impedance transformation," IEEE Trans. Microwave Theory Tech., vol. 52, pp. 2739–2745, Dec. 2004.

[2] S. J. Parisi, "180-degree lumped-element hybrid," in 1989 IEEE MTT-S Dig., pp. 1243–1246.

Introduction

Research activities on antenna miniaturisation are particularly important to meet systems' requirements in terms of compact, integrated antennas including broadband antennas for high data rate communications. IETR has been involved with such a topic for several years. As far as compact and broadband antennas are concerned, one has to deal with specific characterisation issues. Among different factors, the environment effects and the excitation devices are key parameters to consider. This paper focuses on some of these parameters and their influence on gain, impedance or efficiency measurements, and describes some possible solutions to control the reproducibility of these measurements. In this paper we will present different hard point we have identified during our measurements. The advantages of some specific test environments such as reverberation chamber and near field test set up will also be discussed.

The compact UWB antennas are difficult to measure properly as the electrically small antennas because of the limited size of the ground plane. Therefore, when such an antenna is connected to a measuring device, a current will flow in the outer conductor of the cable connecting the antenna, creating spurious radiation. This can be so important that it will mask the characteristics of the antenna under test : perturbation and modification of the radiation pattern, gain measured will be overvalued and the impedance measurements are be affected. Some techniques are used usually for ESA to minimise the effects of the spurious mode such as metallic cap placed on the cable, ferrite cores and shorted sleeve baluns. The major limitations of these techniques are that they are efficient in a small bandwidth and for low frequency (ferrite core, up to about 1 GHz) and are not sufficient to get accurate measurement for UWB antenna.

Then validation of the design of the radiating part needs specific procedures. Moreover, another important issue is the validation of the antenna behaviour at system’s level. Experimental protocols must therefore includes specific environments that are not always compatible with conventional test set ups.

The near field technique enables to quantify the projection of electromagnetic field on a series of spherical harmonics ( in spherical coordinates). Given the geometry of antenna, it is possible to make a distinction between the contributions of the antenna and spurious radiation from the feeding cable. It is also a very convenient tool for optimising the feeding device position with respect to the antenna. However, it requires some expertise to make sure that the field sampling is adequate and to select appropriate mode filtering.

Efficiency seems to be a very important indicator for UWB antennas, probably more significant than gain and radiation pattern. Reverberation chambers enable to integrate the total radiated energy for various combinations of environment factors, with much less problem of test set-up installations.

The final version of this communication will illustrate through measurement examples the ability of these test environments to carry out some useful experiments to quantify the quality of small UWB antennas without and within their final environment.