Polarization-diversity techniques are increasingly applied at base stations of most wireless communication systems due to their potentiality to significantly enhance the radio channel capacity and reliability by mitigating the detrimental effects of multipath fading, polarization mismatch and interference. Good quality multi-polarized operation can be accomplished by aperture-coupled microstrip patch antennas which, besides their inherent polarization purity, also offer many additional benefits including their compactness, conformability, easiness and low cost of fabrication. In particular, planar microstrip arrays with a central patch provide pencil beam radiation with low side lobes (H. Legay and L. Shafai, IEE Proc.-Microw. Antennas Propagat., vol. 144, pp. 67-72, 1997), which perfectly fits pattern requirements of wireless local area network (WLAN) base stations and access points, and makes them well-suited as feeders of reflector antennas in point-to-point links.

Recently, a two-dimensional microstrip patch subarray producing dual-linear and circular polarizations has been presented (A. Vallecchi, ETRI J., vol. 27, pp. 250-256, 2005). However, this design features a too narrow impedance bandwidth to be appropriate for use in wireless communication applications, where bandwidth requirement is typically around 10%. Such a poor performance could be easily enhanced by using a thick foam substrate and near-resonant aperture, but this way the size of the antenna would increase, reducing the allowable scan range when it is operated into an array, and the front-to-back-ratio would significantly deteriorate.

In this contribution, a planar printed antenna exploiting stacked-patch technique is developed to cover either the PCN and PCS services, or the IEEE 802.11-b WLAN applications, with the advantage of polarization agility operation as well as low back radiation. Specifically, the proposed radiating configuration consists of four edge-fed stacked patches suitably connected to a central aperture-coupled stacked patch to form a dice-five-like array configuration. The excitation is dispensed to the whole array through the element at the centre, which is directly coupled to a pair of microstrip lines by a cross-shaped aperture. As a result, polarization agility operation can be simply achieved by using a single phase-shift circuit to deliver appropriate input signals at the antenna ports. Two slightly different antenna configurations have been conceived. In one of these, simplicity is favoured over a more elaborate design, and a single-layered feed network is employed, which implies breaking the symmetry of the antenna. To counteract the resulting loss of isolation between the antenna ports, capacitive coupling between the terminal stubs of the feed lines is applied, which helps to substantially preserve cross-polarization performance. Indeed, fairly low cross-polarization is also obtained by virtue of the symmetric arrangement devised for the radiating part of the antenna, which implements a sort of antiphase feeding technology. Furthermore, to comply with applications where very low cross-polarized radiation is an issue, a fully balanced feeding scheme for the crossed slot has also been implemented. This alternative configuration, while involving the adoption of a more complicated multi-layered structure for the feed, yields high polarization purity of the radiated fields.

In this work a compact dual-band microstrip patch antenna for WLAN applications is presented. The two operative frequency ranges are 2.40 -2.50 GHz and 5.15 - 5.35 GHz. The proposed antenna is made of two stacked patches printed on different layers, thus resulting in a planar, light-weight structure, suitable for low-cost mass production. The lower patch, operating at the lower frequency range, is printed on Taconic substrate with εr=2.50. The upper patch is printed on Arlon foam substrate with εr=1.20. The reduced antenna size allows for the integration in conformal or planar 2-D arrays, the maximum dimension of the resulting elementary cell being smaller than 75% of a wavelength at the higher frequency (5.25 GHz). Both radiating elements are fed by a SMA connector, the upper patch being directly connected, and the lower one fed by proximity coupling (a hole allows the connection with the upper patch). A key element of the proposed structure is represented by the post placed at the centre of the lower patch, which has the important function of suppressing the second resonance mode, that otherwise would strongly affect the radiation pattern at the higher frequency band. The antenna thus produces in both frequency bands a broad smooth beam, the maximum placed nearly broadside. This feature is a crucial requirement for the integration in 2-D arrays. A prototypal antenna has been fabricated and the return loss has been measured. Results are in good agreement with simulations and confirm the feasibility of the proposed solution.

MIMO (multiple-input-multiple-output) systems make use of multiple antennas at both ends of a communication link to exploit the spatial dimension for increasing the capacity. Correlation of the antenna signals has an affect on the capacity of a MIMO implementation. High correlation levels occur mainly for compact antenna realizations, where the separation of the antenna elements is small and the mutual coupling is strong. The antenna mutual coupling distorts the antenna radiation patterns and the input impedances, and therefore affects the correlation level and the efficiency. The concept of total Shannon capacity for a MIMO antenna system is introduced. Typically only the Shannon capacity at a single frequency is discussed, assuming that the system is narrow band and the properties of the antenna system is invariant over the frequency band. For an OFDM (orthogonal frequency division multiplexing) system with many narrow frequency bands, those assumptions could be valid for each band. However for densely packed antenna elements, the properties could vary rapidly between the different used frequency bands.A differential Shannon capacity could be introduced for each used frequency band and the total Shannon capacity as the sum of those differential Shannon capacities. The coupling has mainly two effects; a decrease of antenna efficiency and thereby reduced (SNR) signal-to-noise-ratio and Shannon capacity, and a reduction of correlation between received signals increasing the Shannon capacity. Usually, only the later effect is considered, leading to the conclusion that coupling is beneficial for a MIMO system. However, in reality the decrease in SNR is more severe than the correlation effect, at least in a noise limited environment. The coupling between the antenna elements could be counteracted by using passive coupling compensation networks; however complete coupling cancellation and matching can only be achieved for a single frequency. In this paper, typical total Shannon capacities are shown as a function of separation between two half wavelength long dipoles. The theory could be extended to arbitrary number of antenna elements. As an example consider two dipoles separated by one tenth of a wavelength, the correlation between the isolated patterns is 0.92, leading to an estimation of the Shannon capatity as 6.25 bits/s/Hz if the SNR is 15 dB. Inclusion of the coupling effect reduces the correlation to 0.35, if the antennas are matched to free space. Assuming the SNR being the same, the Shannon capacity is raised to 7.97 bits/s/Hz. Howerever by assuming that the incident power is the same, the SNR is reduced to 13.23 dB, due to decreased effiency and the Shannon capacity is reduced to 6.90 bits/s/Hz. Introducing a coupling compensation network, eliminating both the correlation and losses, will increase the Shannon capacity to 8.14 bits/s/Hz at the center frequency of operation. Assuming that the relative operational bandwidth of the system is 10%, the average capacity over frequency is reduced to 7.70 bits/s/Hz, due to the fact that the matching and coupling compensation cannot be maintained over the whole frequency band.

Monopole antennas are very popular among antennas designer for UWB systems applications because of its broad bandwidth performance and easy to construct. However in many applications antenna must perform in the vicinity of large ground planes and solutions are needed for wide band antennas with a built in ground plane. In this paper we will describe the performance of an antenna designed to fit on a PCMCIA card in a laptop. The antenna on ground plane is designed by using 2 copper plates bent into the inverted U-shape with unequal arms and connected by a thin microstrip line which is fed by a SMA connector over the ground plane the size of PCMCIA card. With the introduction of notches and slots into the copper plates a significant frequency bandwidth can be achieved. Fig.1 shows the geometry of the antenna which is just 7mm high. Two coupled novel antennas are situated over the ground which is credit card in size.

The antenna properties were simulated using CST microwave studio 5 and the result for matching bandwidth or return loss of this antenna is shown in Fig.2. Currently we have achieved a bandwidth of 5.8 – 10.3 GHz with a return loss of better than –8 dB. Over most of this band the return loss is much better than -10 dB and we are currently working on the peak at 8.5GHz to further improve the matching.

The paper will present full details of antenna performance across the frequency band together with installed performance in a laptop computer.

For mobile communications, circularly polarized antennas are very attractive [1]. Moreover low cost and fabrication simplicities are of great advantages. At least, single fed structures are often useful because of their simple integration and of the amount of the space available beside or behind the antenna.

A novel single probe fed circularly polarized patch antenna is proposed. The working frequency band is chosen to be in the IEEE 802.11a band [5.15;5.35] GHz.

The substrate is inexpensive FR4 of relative permittivity 4.4 and of thickness h=1.52mm. The structure is constituted of a ground plane and of a square printed patch of dimension L=6.3mm (figure 1). Feeding is done with a probe located at distance S=3.0mm from the patch centre. Generally, circular polarization (CP) for single feed structures is achieved thanks to asymmetries on the radiating patch [2] (corner truncated patch, nearly square diagonally fed patch, diagonal slot, ...). In our approach, CP is achieved thanks to a hole through the whole structure, placed on the diagonal of the patch. When properly dimensioned, the characteristics of this hole (radius R and distance from patch centre T) make it possible to obtain a minimal axial ratio (AR).

This new principle had firstly to be validated. The simulations were performed with the software HFSS. In a first time, the radius R was varied for a fixed distance T=5mm. For a radius of R=1.85mm, a dip point was observed in the smith chart, that denoted the correct generation of CP. A minimum AR was then achieved (AR=0.26 dB at the frequency f=5.252 GHz) and the 3dB AR bandwidth was 91 MHz ( ). The generation of CP thanks to the hole had been so validated. Similar parametric studies were done in a second time for various distance T, showing that the 3dB AR bandwidth depends (among others) on this parameter. A comparison with an optimized corner-truncated patch antenna of same dimension was done, showing that similar performances could be obtained. At least, to validate the theoretical results, an antenna was fabricated and a quite good agreement had been found between theory and experiment.

The simulated and measured results will be presented in the final paper, which will show that the great advantages of this structure are first that the CP is easy to tune by simply enlarging the hole with a drill and second that the opening can be used for an additional function, as for a camera aperture for example.

References:

[1] Kin-Lu Wong, Planar Antennas for wireless Communications, Wiley edition, 2003
[2] P. C. Sharma and K. C. Gupta, Analysis and optimized design of single feed circularly polarized microstrip antennas, IEEE Trans. Antennas Propagat vol 31 n°6 nov. 1983, p. 949-955.
Figure 1: the proposed antenna.

A simple triangular monopole antenna fed by a tapered-CPW line is presented. The simulated and measured return loss results are presented. The proposed antenna offers a wide bandwidth from 4.14-9.84 GHz (81%) covering WLAN frequency bands for IEEE 802.11a and HiperLAN2 as is seen from the measured results.

Introduction:

With rapid advances in communication technology, a lot of attention is being given to antennas that cover more than one application bands. This aim is accomplished by using a multiband or a wideband antenna. Dualband antennas that cover WLAN applications for IEEE 802.11a/b have been widely reported in the literature [1]-[2]. It is estimated that in the future, the 5 GHz band would be more popular for WLAN applications. Recently a tapered-CPW fed annular monopole antenna with coplanar trapezoidal ground plane was proposed for UWB applications [3]. In this paper we present a simple triangular antenna with tapered-CPW feeding arrangement which offers a wide bandwidth covering IEEE 802.11a and HiperLAN2 bands.

Antenna Design and Experimental Results:

The geometry of the antenna is as shown in Fig.1a. A tapered feed line is used to increase the bandwidth of the CPW-fed Triangular antenna. The antenna is fabricated on a GML substrate having dielectric constant as 3.2 and thickness 0.762 mm. The width of the feed line near feed point is 1.834 mm and near the triangular patch is 0.5 mm. The Gap between ground plane and feed line is 0.1 mm near the feed point and increases as width of the feed line is decreasing towards the antenna. The simulated and measured performance of the tapered-CPW fed triangular antenna are compared in fig. 1b. The proposed antenna exhibits two bands: 4.14 GHz-9.84 GHz (Bandwidth 5.7 GHz) and 11.1 GHz-16.2 GHz (Bandwidth 5.1 GHz). The antenna can be used in the lower band which covers WLAN frequency bands for IEEE 802.11a (5150-5350, 5725-5825 MHz) and HiperLAN2 (5150-5350, 5470-5725 MHz).

Conclusion:

A triangular antenna fed by a tapered-CPW line is proposed. The simulated and measured return loss results compare well. The proposed antenna offers a wide bandwidth from 4.14-9.84 GHz (81%) covering WLAN frequency bands for IEEE 802.11a and HiperLAN2 as is seen from the measured results.

References:

1. C.-Y. Huang and P.-Y. Chiu, "Dual –band monopole antenna with shorted parasitic element," Electronics Letters, Vol 41, No21, 2005.
2. Horng-Dean Chen, Jin-Sen Chen and Yuan-Tung Cheng "Modified inverted-L monopole antenna for 2.4/5 GHz dual-band operations" Electronics Letters, Vol. 39, No. 22, 2003, pp. 1567-1568.
3. X.-L. Liang, S.-S. Zhong, W. Wang and F.-W.Yao, "Printed annular monopole antenna for ultra-wideband applications", Electronics Letters, Vol. 42, No. 2, 2006, pp. 71-72.

Growth in mobile communications has prompted interest in compact and integrated front-ends. Much of the transceiver circuitry can be integrated in a single MMIC. However, low microwave frequency MMIC antennas are uneconomically large or, if made electrically small, suffer performance degradation. One alternative is to have a small off-chip antenna electromagnetically fed from the MMIC. The PIFA is a suitable candidate as it retains the low profile, light weight and low fabrication costs of a microstrip antenna, with reduced size.

In our novel integrated PIFA, a pair of differential signals is combined in the middle of a feed line to couple energy via an aperture to the PIFA. The differential signals create a virtual short circuit in the middle, similar to the virtual short created by the stub in conventional, single-feed aperture coupling. The antenna and the feed circuit are constructed on separate substrates, divided by a common ground plane, allowing independent selection of the two substrates.

In the proposed structure, Fig. 1, impedance matching is achieved by controlling the dimensions of the aperture and the relative position of the PIFA. Fig. 2 shows measured and simulated odd mode return losses. Both are better than -20dB with a frequency shift of only 1%. Normalised radiation patterns, shown in Fig. 3, are similar to those of a conventional PIFA. The simulated and measured gains are 3.1 and 2.7 dBi respectively.

This approach, combining the size advantages of an unbalanced antenna with the circuit advantages of a balanced feed, is suitable for mobile applications. It facilitates tight integration of antenna and transceiver systems, with the PIFA as the only off-chip entity and no output bond connection.

In this paper the effect of an EBG multilayered surface as substrate of a patch antenna is investigated (Fig.1). To this end two layers of dielectric interchange with two copper layers. The first layer is a copper-clad standard PTFE laminate of 62 mil thickness having dielectric constant 2.21.The second layer is of the same material but it has 31 mil thickness and dielectric constant 2.17. The lowermost dielectric layer carries a biperiodic array of circular, copper patches, which are arranged symmetrically with respect to the main axes of the structure into four sub-arrays. The overlying dielectric layer accommodates the square patch antenna and a second, biperiodic array of copper patches, also arranged into four sub-arrays, and located exactly above the patches of the underlying layer. Four central patches are omitted to make space for the antenna. Six more patches in a line are omitted, so that the antenna is symmetrically accessed by two, perpendicular, microstrip feed lines. Furthermore, two more patches are omitted in the vicinity of the entry point of either port, in order to improve the matching between the antenna and the feed lines. As regards the feed lines themselves, they are exactly the same for both ports. Every patch is grounded by use of a via through its centre.

Our simulations manifest that the return loss at both ports is below -9.5 dB from 2403 MHz to 2421 MHz.The average value of S₁₁across the aforesaid, 18 MHz, band is -15dB. Due to the asymmetry of the structure, the value of S₂₂ is slightly different in frequency, ranging from 2409 MHz to 2428 MHz. Port 2 is better matched than port 1, with S₂₂ averaging -18dB in the aforementioned band. The coupling between ports, i.e S₂₁ and S₁₂ range from -48dB to -37 across the band 2404 MHz to 2422 MHz.

The antenna , thus framed by the EBG structure, is approximately 24% smaller in area than a square patch placed over a standard laminate with the same overall dielectric thickness and a continuous ground plane. Had our square patch antenna been considered over such a surface and matched with a quarter wavelength transformer, as is the case with the proposed antenna, it would resonate at 4.41 GHz. Thus 45.2% reduction in the operating frequency is achieved because of the proposed EBG frame.

References:

[1] D. Sievenpiper, L. Zhang, R.F.J. Broas, N. Alexopoulos, and E. Yablonovitch, "High-impedance electromagnetic surface with a forbidden frequency band," IEEE Trans. Microw. Theory Tech., vol.47, no.11, pp.2059–2074, Nov. 1999.

[2] P. de Maagt, R. Gonzalo, Y.C. Vardaxoglou, and J.M. Baracco, "Electromagnetic bandgap antennas and components for microwave and (sub) millimeter wave applications," IEEE Trans. Antennas Propag., vol.51, no.10, pp.2667–2677, Oct. 2003

[3] C. Balanis, "Antenna Theory, Analysis and Design", John Wiley and Sons, New York(1997).

This paper deals with an optimized architecture of an antenna and its RF Front End compliant with the IEEE802.11a/b/g standards. The proposed solution is schematically represented on Figure 1. It is based on the use of two printed end-fire antennas [1], where each antenna has separated accesses at 2.4GHz and 5GHz [2]. These two accesses are sufficiently decorrelated and well matched on their respective bands. The use of separated access at the 2 bands connected to a single broadband antenna allows the suppression of components such as diplexers and relaxes the constraints on others components such as DPDT (Double Port Double Through). Thus a lower cost and better performances RF Front End is achieved. This antenna-RF Front End could be used in a 2*2 MIMO system, compatible with the Cardbus format.

Figure 1 : Antenna and Front End RF Architecture for IEEE 802.11a/b/g standards

Nowadays, Wifi ADSL modems for home use allow establishing an Internet connection towards a wireless card on a PC. Due to both limited number of channels in the 2.4 GHz band and the limited throughputs with the IEEE 802.11b standards, the 5 GHz band is more and more used for such applications and most of future products have to work on both 2.4GHz and 5GHz bands and should be IEEE802.11a/b/g compatible. In addition, it has been proven that the use of antenna diversity techniques improves the robustness and the coverage range of wireless links [3]. Based on these facts, this paper proposes an innovative implementation of a dual band antenna with its associated RF Front End optimized on both cost and performance point of view. Main advantages of this solution will be explained and compared with a traditional architecture. Also, the design of the associated antenna will be presented where simulations agree well with measurement results.

The emergence of intelligent textile systems for personalized health and wearable computing requires the development of low-cost and lightweight antennas that are fully integratable and ensure wireless communication between those garments and the environment. Respecting the main characteristics of garments such as flexibility and comfort, these antennas should preferably be made out of textile materials.

Applying novel electrotextiles, which are electroconductive, we have developed several prototypes of textile antennas operating in the 2.45 GHz ISM band. In this paper we will describe a dedicated methodology for designing wearable antennas, since the approach differs from the classical design of antennas on printed circuit boards.

First, a careful selection of textiles is required: highly conductive electrotextiles for the antenna and the ground plane and non-conductive textile materials for the substrate on which the antenna is mounted. Second, a suitable antenna design was made, taking into account the higher tolerances of textile material when cutting it, compared to conventionally used techniques such as etching and milling. For the antenna and the ground plane, we propose three different conductive fabrics that are readily available from Less EMF Inc.: Flectron®, ShieldIt™ Super and Zelt. As for the antenna substrate a fleece fabric was depicted because its thickness of 2.56 mm and its permittivity of 1.25 resulted in an antenna with sufficiently wide bandwidth and high radiation efficiency. A rectangular ring geometry, as shown on the figure below, provided an antenna with a bandwidth exceeding the required 83.5 MHz. Additionally, a nearly circular polarization of the textile antenna is found in the complete 2.45 GHz ISM band. Moreover, the textile antenna prototypes had an efficiency of at least 70% and robustness against bending, hence proving their feasibility.

With these promising results, this research paves the way for a new generation of communicating garments.

Despite the great interest in the improvement of cellular networks and systems there are still some components, like the base station antennas, that remain non-optimized. This is regardless of the fact that a lot of work has been done in improving the different issues and configuration schemes within a cellular network. Yet the main part of this previous work deals only with specific problems on a system level, like power allocation, or when related to the antennas, it considers configuration problems such as downtilt or ideal beamwidth. To our knowledge the computation of new radiation patterns in elevation has, with certain exceptions, not been considered or it has been done only dynamically in the form of a smart antenna. As a result an essential component of all cellular systems remains non-optimized, namely the base station antenna.

In order to optimize base station antennas the backward problem between system behavior and the antenna gain pattern has to be known. However, knowing how to change the antenna pattern in order to obtain a better system performance is in most cases not possible with conventional dynamic and static simulators because of the complexity of all the variables involved in a cellular system. Therefore a simplified model, which allows the formulation of a backward problem for antenna design accurately representing cellular networks problems like coverage, interference and handover, has to be defined. With this model an ideal antenna can be computed, and based on it a real antenna of N elements can be synthesized. For this purpose an optimum synthesis algorithm must be chosen.

In previous efforts to synthesize antennas that compensate for free space propagation, several methods have been tested. One of the most used ones is known as the Orchard-Elliott algorithm, with which an adequate current distribution for generating a pattern consisting of a shaped region and a sidelobe region can be obtained. In this paper a novel modified version of the original Orchard-Elliott algorithm, based on the considerations made for an ideal base station antenna in UHS cellular systems, is proposed and tested with a 16 element antenna array, whose performance is additionally compared to that of a commercial Kathrein antenna with the help of the theoretical model presented here and a Ray Tracing Simulation in an urban region (see Fig. 1).

The work will be structured as follows. In section II the simplified analytical model used for solving the antenna backward synthesis problem will be presented. Out of it a set of evaluation criteria will be obtained with which an ideal elevation pattern for CDMA Networks under assumption of Ultra High Sites will be defined. Subsequently, in Section III, the chosen synthesis method will be presented. At first, the original Orchard-Elliott Method will be introduced and then the sidelobe variation approach used for the final antenna synthesis will be shown. In section IV theoretical and simulated results based on the simplified analytical model and a ray-tracer scenario will be presented.

Switched-beam arrays (SBAs) are considered as an attractive alternative to adaptive arrays, in order to increase system capacity and spectral efficiency. SBAs are mostly preferred due to easy installation and inexpensive maintenance. In this paper, a new technique is proposed in order to design dual-band switched-beam dipole arrays. The scope of the proposed technique is to design arrays with balanced radiation characteristics at both frequency bands.

This is accomplished by joint optimization of the array¢s technical characteristics and excitation coefficients for operation at both frequency bands, while properly exploiting mutual coupling among array elements. Coupling is modeled using the induced ElectroMotive Force (induced EMF) method, while joint optimization is performed using a custom Genetic Algorithm (GA).

Numerical results are presented in the case of a uniform, circular, eight-element dipole array, with radiation characteristics specified for the 2.4-2.5GHz and 3.4-3.6GHz frequency bands and excellent agreement to design specifications is demonstrated. The normalized voltage radiation patterns of the array at 2.45GHz and 3.5GHz are illustrated in Figure 1. The array¢s radiation pattern bandwidth is 100MHz and 200MHz at 2.45GHz and 3.5GHz respectively. Further research on the field includes compensation of failed (broken) array elements in single- and dual-band array designs.

Figure 1:Dipole array¢s normalized voltage radiation patterns at 2.45GHz and 3.5GHz.

Radio planning tools are widely used today by the operators to simulate the roll-out of cellular or broadcast networks, to evaluate the coverage and the quality of service, as well as to optimise the network parameters. These simulations are based on the prediction of the radio coverage around transmitters obtained by two main components: the loss along the predominant propagation paths and the antenna gain in the emitting path directions. The paper focuses on the latter component, stressing on the importance to predict accurate antenna gain even outside the main lobe.

In general, the antenna manufacturers do not provide 3D measurements of the antenna patterns, though ideal to minimize prediction errors. Only the horizontal and vertical 2D patterns, resp. G_H(az) and G_V(el), are available. Thus the antenna gain in the actual emitting direction (not included in the horizontal or vertical plane) must be calculated by an extrapolation method.
The method predicting the antenna gain from the sum G_H(az)+G_V(el) (gains in dB) gives acceptable results as long as the emitting direction is in the main lobe, i.e. within the half-power beam width. But the simulation of radio networks (especially for interference-limited systems) requires today that the propagation estimate is consistent with reality even outside the main lobe:

Shok Theng Low and Zhipeng Wu

The University of Manchester, School of Electrical and Electronic Engineering, PO Box 88, Manchester M60 1QD
shoktheng.low@ieee.org, zhipeng.wu@manchester.ac.uk
tel: +44 161 3064684 fax: +44 161 3068712
Contact person: Prof. Zhipeng Wu

The simple microstrip-feed scheme enables the integration of dielectric resonator antenna (DRA) with other microwave circuits in a planar arrangement. In the past critical coupling or impedance matching of a microstrip-coupled DRA is found by adjusting the distance between DRA and microstrip line (or offset distance). No systematic study has been made on the effect of offset distances to the impedance matching properties of a DRA. Hence the investigation reported in this paper is to provide an insight of how a microstrip line can effectively couple e.m. energy to the rectangular dielectric resonator antenna (RDRA).

The antenna under investigation is a made of a block of ceramic with relative dielectric constant of 37 and dimensions of 18x18x9.5 mm³. The effect of the offset distances of the RDRA to the microstrip line is studied in which the RDRA is offset both from the centre axis of the microstrip line (dy), and from the end of the line (dx) as shown in Figure 1. Four PCB types on which 50Ω microstrip lines are printed have been investigated including RT/duroid 5880 (ε_r = 2.2) and FR4 (ε_r = 4.4) substrates with thicknesses of 0.79 mm and 1.6 mm. It has been found through simulation and experiments that antenna's resonance frequency, return loss, bandwidth, and radiation patterns are affected by the choice of PCB parameters such as substrate thickness and substrate material and the offset distances.

In this paper, the measured results of resonance frequency, return loss and impedance bandwidth for an RDRA in different combinations of offset distances will be presented. For the same offset distances the resonance frequency of the RDRA on thick RT/duroid 5880 substrate is higher than that on thin RT/duroid 5880 substrate. Furthermore, the use of FR4 substrates result in lower resonance frequency compared to RT/duroid 5880 substrates. The impedance bandwidth from measurement at VSWR< 2.0 may vary up to 5%. It has been found that broad bandwidth can be achieved by using a thicker substrate layer. In the thick substrate case FR4 is a better choice for broader bandwidth and in thin substrate case, RT/duroid 5880 is a better choice. Results of radiation patterns and cross-polarisation levels will also be presented in this paper. Sample results of resonant frequency are shown in Figure 2.

The mobile communication systems are involving rapidly, demanding higher performance and increased capacity of the mobile terminals. The focus for the future wireless communication lies on the high speed data transference which sets some limitations on the present antenna solutions for reception and transmission of the radio signals. To improve the performance by decreasing the effects of the multi-path propagation and to enable higher data rates a multi-channel antenna technique and the underlying mechanisms of antenna diversity can be introduced on the mobile terminal.

The performance of the multi-antenna solution in a multi-path environment is usually presented in the terms of the effectiveness of the antenna diversity, Diversity Gain and Diversity System Gain. A number of parameters are used to describe the performance. The mean effective gain of the antenna is one of them, comprising the antenna performance for a specific reference propagation environment. The correlation coefficient is another parameter, describing the correlation between the incoming signals at the antenna ports. These parameters can be calculated from three-dimensional far field complex radiation patterns or from the scattering parameters that are obtained by numerical methods or from measurements.

In this paper we are focusing on the practical, engineering way of evaluating and characterizing the multi-channel antenna solutions on the mobile terminal. The two methods of calculating the parameters are described and the practical use of the methods in simulations and measurements are presented and evaluated. The time saving aspect of using the scattering parameters is lifted forward but also its limitations. For comparison, a statistical method for calculating Diversity Gain is also presented.

This evaluation is performed for two multi-channel antenna solutions with different characteristics for the purpose of characterizing different diversity performances.

A dual and wide-band printed dipole antenna is presented for IMT-2000 and 5.2/5.4/5.8 GHz WLAN applications. The proposed balun-fed antenna consists of a double-sided printed dipole antenna with a microstrip-fed disk-dipole printed on the top side of the substrate, and a modified annular-ring dipole and the groundplane printed on the bottom side. For the proposed antenna, two resonant bands are observed. The first band is about 14.2% for the center frequency of 2.1 GHz and the second band extends from 4 to 8 GHz with a fractional bandwidth of 80 %. The measured peak gains over these bands are 1.8 and 3 dBi, respectively.

I.Antenna Design
A photo of the dual-band printed dipole antenna, with SMA connector, in both sides is presented in Figure 1. The feeding line connected to a disk-dipole (R1) is on the top layer of the substrate whereas the modified annular-ring radiator ( R2, R3) and the groundplane are printed on the bottom layer of it. The proposed antenna was etched on CuClad substrate with h = 0.8 mm and er = 2.17. The dimension of the antenna is 75 x 50 mm.

II.Results
The measured return-loss is illustrated in Figure 2. Experimental and simulation results have shown, that the first resonance is mainly determined by the inner radius R1 of the central disk, and not influenced by both the inner and outer radii R2 and R3 of the modified annular-ring radiator.

The resonance in the upper band 4.1-8.1 GHz is influenced by radii R2 and R3, whereas the bandwidth is related to the width DR’ = R3-R2 of the modified annual-ringThe resonant mode at 2.1 GHz band is achieved by the first resonance of the feeding element (microstrip + disk) whose length is about 36 mm (approximate l/4= 35.1 mm at 2.1 GHz). For the upper band, we explain from simulated surfacic currents, that the lowest operating frequency is due to the longest current path along the inner boundary of the modified circular-ring dipole.Figure 3 shows the measured far-field radiation patterns of co-polarization for the E-plane (x-z) and H-plane (y-z), at the resonance frequencies 2.1, 4.5, 5, 5.2, 5.4 and 5.8 GHz, respectively.

III.Conclusion
The proposed structure exhibits dual resonance bands suitable for IMT-2000 and WLAN band applications. The wide impedance bandwidth at the two resonant bands, stable radiation characteristics, and the simple design and feeding of the proposed antenna make it a good choice for multiband/wideband wireless systems. This structure could be useful, for example, as radiating element in a sector zone base station antennas.

Rerferences
1. G.Y. Chen and J.S. Sun: 'A printed dipole antenna with microstrip tapered balun', Microw. Opt. Technol. Lett., 2004, 40, (4), pp. 344-346
2. Vasiliadis, T., Vaitsopoulos, E. and Sergiadis, G.: 'A wideband printed dipole antenna with optimized tapered feeding balun for ISM, and FW bands', Microw. Opt. Technol. Lett., 2004, 43, (5), pp. 437-441

Base station antennas with omnidirectional pattern in the azimuth and directive pattern in the elevation are an important requirement for many wireless communication applications [1]. these patterns could be obtained by a center fed collinear dipole array[2] . However, other way to produce an omnidirectional pattern with a high gain has been recently studied [3]. This method consists to use cylindrical dielectric EBG structure to enhance the directivity of a monopolar wire patch antenna. The matched band of this antenna does not exceed 1% due to the high quality factor of the EBG material.
In this paper we present the study of coaxial metallic EBG antenna which can be used for HiperLAN2 applications [5.15-5.35] GHz and the method used to match the antenna throughout this band (3.8%).
In order to achieve this task, a 1D cylindrical metallic EBG structure which is composed of 12 metallic stems is used to increase the directivity of the feeding source. To bypass the matching problem, the antenna has been fed by a dipole with different arms length which resonate at two close frequencies. The dipole is printed on a metallic core which is inserted in the center of the structure to act as a ground plane. In order to reduce the ripple in the omnidirectional pattern of the azimuthal plane, a second dipole is printed on the other side of the center core.
The antenna is vertically polarized. The maximum simulated directivity is around 9.2 dBi with a minimum of 8 dBi between 5.15 and 5.35 GHz. The half power angular width in the elevation is 10° and the ripple in the azimuthal pattern is less than 1dB.
The matched bandwidth is 200 MHz around the central frequency, so if we compare our designed antenna to the one presented in [3], we can see that the bandwidth icreased from 1% to 3.8%.
A prototype of the antenna has been fabricated to validate our simulation results, and the measure process is in progress.

REFERENCES:
[1]: G.Palikaras, A.P.Feresidis, J.C.Vardaxoglou, "cylindrical electromagnetic band gap structures for directive base station antennas", IEEE antennas and wireless propagation letters, Vol. 3, 2004.
[2]: http://www.radialllarsen.com/products_terminalinfrastructure.html
[4]: L.Freytag, E.Pointereau, B.Jecko, "Novel dielectric EBG antenna with omnidirectional pattern in azimuth", URSI EMTS, 2004.

Compact folded dipole antenna, which covers 5 multi bands (GSM850, GSM900 GSM1800, GSM1900 and UMTS), is investigated. Frequency matching is done by changing parameters. The antenna height is less than conventional 4 band antenna (GSM900, GSM1800, GSM1900 and UMTS) that is already on the market(stick phone with PIFA type 8-10mm). In this paper, we will compare simulation result and measured result.

Microstrip (patch) antennas for DMW band work in rather small relative frequency band however absolute frequency band is sufficient to allow their usage in high-speed telecommunication systems.
Radiating structure of a conventional patch antenna is formed by a single conducting patch (section) located above a screen, with a dielectric layer normally placed between the patch and the screen. The section can be excited from an inner conductor of a coaxial cable or in some other way. In all cases section dimensions are not to be smaller than a certain limited value.

Dimensions of the patch antenna can be reduced by using a radiating structure consisting of two sections having the same or almost the same dimensions, for example, by using half-round sections separated by small gap (fig. 1a).

Fig. 1 — Construction, equivalent schematic diagram and input impedance frequency dependence of the antenna

A coaxial cable 1 is attached to the first section 2, with its outer conductor being connected to this section and a screen, and its inner conductor connected to the second section 3 through the matching reactance 4. To ensure that the structure is symmetrical, a conductor 5 having diameter equal to the diameter of the coaxial cable outer conductor is attached to the second section and connected to the screen.

Equivalent circuit of each section is similar to a parallel oscillating circuit (fig. 1b). Loss resistance represents radiation. Active impedance Roe of the circuit at a resonance frequency f₀ is more than ñ/2, where ñ is a wave impedance of the cable. There are two points near the frequency f₀ where circuit active impedance equals to ñ/2 (fig. 1c). Matching of the antenna can be ensured at these frequencies by compensating the residual reactivity of the two circuits with a corresponding reactance of the matching element. Our experiments showed that antenna with air dielectric having height of 0,05ë and diameter of about 0,16ë, where ë is a wavelength can be satisfactory matched in relative frequency band of about 1%.

The discussed principle can be used as a basis for creating a great variety of types of sectioned patch antennas to bring along some other important advantages in addition to the reduced sizes. For instance, three-sectional antenna having small number of reactive elements can perform rotating polarization of radiating field. Four-sectional antenna has a slightly more complicated construction, but is able to provide more uniform pattern in horizontal plane. Also, under certain conditions, antenna with two inputs and two opposite feeding cables can radiate field having two independent opposite rotating polarizations.
The report is dedicated to theoretical models, ways of experimental tuning and some experimental results relevant to the suggested antennas.

The application of the Fractal theory in the domain of the antennas has become a very popular subject during the last years. Fractal antennas like Koch or Sierpinsky monopole and dipole are generally used in order to reduce the size of the radiating element or to get a multi resonance effect.
In the literature there are numerous articles describing Fractal antennas (Patch, Monopole, Dipol...) having several resonant frequencies. It means that those antennas exhibit multi frequency or multi band behavior but not a wide band behavior.

We present in Figure 1, a very compact Fractal Sierpinsky dipole working in a wide band of frequency.
The idea is to implement a large Bow Tie shape with Fractal geometry, to minimize the size of the radiating element. The two arms of our element are constructed with two opposite Sierpinsky triangles. The arms of the Dipole are linked together with conductive shunts at the the dipole edges.

The tuning is finalized with the addition of a "Balun" (balanced unbalanced) which enable to get a good matching (Return Loss S11 under -10dB or VSWR 1:2) on a frequency range larger than 1 octave. This dipole is fed by a SMA connector soldered to a parallel line. The performances of this kind of compact Fractal dipole are very similar to a classical large Bow Tie dipole in terms of gain and pattern.

This element is protected by a patent and is the proprietary of ELTA Systems Ltd (Group & Subsidiary of Israel Aircraft Industries Ltd). Figure 1: Sierpinsky Fractal dipole with 2 Shunts and Balun

In Japan, current mobile communication systems use the W-CDMA system, and antennas are arranged vertically in a row in base station (BS) antennas to obtain the necessary gain for coverage. Because of the recent increase in the number of W-CDMA system subscribers and the communications capacity in conjunction with advances achieving high speed, prompt measures should be taken to deal with the demand to increase the W-CDMA system capacity. A shaped beam pattern for the BS array antenna, for example cosecant beam pattern, is preferred because the shaped beam pattern in a service area provides enough received power and influence of interference to other areas is small. There are many studies that focus on the shaped beam pattern. Examples of the studies on the shaped beam pattern for BS array antennas in the W-CDMA system describe methods in which the excitation amplitudes and phases are controlled [1] and [2]. The phases of the top and bottom elements in the vertically stacked antenna array are controlled in order to shape a beam pattern. This paper proposes the formation of a beam pattern for the BS array antenna to maximize the downlink system capacity of W-CDMA systems using a simplified method in which only phase of the bottom elements are controlled.

Figures 1(a) and 1(b) show the change in the system capacity when the phase shift and the number of elements at the bottom of the array are changed. The total number of elements is 16 and the spacing between each element is 0.67$B!&(B in these figures. These figures indicate that as the number of elements is decreased from 2 to 6 of the total elements a phase shift of -60 to -150 degrees occurs for system capacity. This paper investigated the beam shaping of radiation patterns for the downlink capacity of the W-CDMA system. A change in the number of elements from 12.5% to 37.5% produces a phase shift of $B!#(BV60 to $B!#(BV150 degrees yielding an increase in the system capacity of 180% to 190%. The result of phase optimization shows that the simplified method has an enough effect to increase downlink system capacity of the W-CDMA system.

Reference
[1] Y. Yamada and M. Kijima, "Low sidelobe and tilted beam base-station antennas for smaller-cell systems,"IEEE AP-S, pp. 138-141, 1989.
[2] M. Kijima and Y. Yamada, "Determining excitation coefficients of dual-frequency shaped beam linear array antennas for mobile base station,"IEEE AP-S, pp. 932-935, 1991.

In this paper, an experimental study on different microstrip antenna shapes, such as e.g. triangles, quadrads, pentagon, hexagon shapes, is described. With each shape, we measured radiation pattern, antenna gain and SWR. By this, we found an optimum shape for a microstrip antenna, which comprises a symmetrical, ripple free radiation pattern in E- and H-plane respectively. The relative impedance bandwidth is comparably large to traditional microstrip antenna structures and the antenna gain is twice as much as patch-antennas. Furthermore, the manufacturing of this antenna seems suitable for mass-production at low cost.

Patch array technology has been applied to base station antennas in mobile communications systems for many years, because of their well-known characteristics of low profile, potential low cost, reliability and flexibility in achieving contoured beams. The pattern requirements for a base station antenna are normally a sector pattern in azimuth and shaped in elevation, which are obtained with a passive linear array excited by a proper feeding network. Radiating elements must provide sufficient bandwidth, low levels of cross-polarisation, low back radiation, high efficiency and power handling. With the introduction of the new UMTS services, developing antennas covering the GSM1800 and UMTS bands has become very attractive. This band covers from 1710 MHz to 2170 MHz, thus implying 24% bandwidth. It is well known that the one of most significant disadvantages of planar antennas is their narrow bandwidth. Consequently, some techniques have to be applied to improve it in order to reach the desired bandwidth. However, as a result of the bandwidth improvement other requirements that have to be considered in the array design and that are critical specification for an array design are degraded. Therefore, a careful design balancing all of the requirements has to be made. Special attention has to be taken in back radiation and coupling between element.
In mobile communication is common using two orthogonal linear polarization (±45°) to improve the system performances by polarization diversity. Dual polarization patches are also known. However, the typical coupling between ports is about -25 dB in the proposed patches. In the mobile communications a isolation between ports greater than 30 dB is usually required, thus implies than the isolation in the radiation element has to be better than this value.
In this paper the design of a dual polarization broad band is presented. The design considers the bandwidth specification for return losses. And it also addresses the other critical specifications that would allow using it as radiating element of a base station antenna: coupling between elements, coupling between polarizations and back radiation. The designed radiating element is a two stacked patch antenna, which is fed to the input microstrip line by means of a slot. The simulation of the reached design are presented and it can be seen that the main goals are matched.
A manufactured prototype and its measurement are also presented. The obtained results match the design specifications confirming that the designed radiating element could be suitable to be used in a base station array. This prototype of a broadband patch antenna with high isolation between ports is shown in figure 1.

The paper presents circularly polarized scanning phased array antenna terminal, developed for ground DBS reception in the frequency band 12.2 - 12.7GHz.

Design of the antenna utilizes microstrip printed circuit technique to achieve thin flat profile of the product, easy manufacturing and compatibility with MMIC phase control devices. The array is electronically steerable in elevation, as the phase control is realized with 5-bit phase shifters. Scanning in azimuth angles is mechanical. Amplitude control is not applied. The grid shape and spacing are considered to cover large tilt angles (up to 65deg). Radiating elements are circular probe-fed patches. The antenna is purposed to the ground mobile users and provides hand over from beam to beam, depending on which particular beam provides the best link margin. Practically antenna can support also linear (horizontal , vertical or tilted) polarizations. Main components and blocks of the antenna terminal are discussed in the proposed paper, including radiating element, feed network, vertical structure, microstrip layers, LNA design, beam coverage. Measured antenna patterns and parameters are presented. Live tests using antenna installed on a car roof were provided in USA and Europe.