Georges Kossiavas

Dual-frequency and broad-band antennas with stacked quarter wavelength elements

Original lightweight, low-cost, and compact air-filled planar antennas with short-circuited elements, fed by a coaxial probe, for dual-frequency (S-antenna) and wide-band applications (E-antenna) are investigated. The two-band frequency antenna is formed of two stacked quarter-wavelength elements, short-circuited along diametrically opposed planes. This structure offers two modes with different radiation characteristics. The ratio between the two frequencies can be closely controlled within a range varying from 1.3-2. A bandwidth of 30% for a VSWR <2 is demonstrated using two stacked quarter-wavelength elements short-circuited along the same plane. Numerical simulation results are compared with experiments and a very good agreement is observed. Radiation patterns and input impedance of both structures are measured and the effects of various physical parameters are presented.

Enhanced diversity antennas for UMTS handsets

In this paper, we present several UMTS multi-antenna systems having radiators with low mutual coupling and high total efficiency. Firstly, two antennas are simply positioned at the top of a small ground plane whose size is representative of the Printed Circuit Board (PCB) of a mobile phone. A solution is then proposed to decrease their mutual coupling and enhance their total efficiency. Then the same neutralization technique is applied to a four-antenna structure. These systems are fabricated and measured. The simulated and measured S-parameters and total efficiencies are presented. The envelope correlation values are also shown and discussed. It is demonstrated that this neutralization technique could be easily implemented at the terminal side of a wireless link to achieve enhanced diversity antennas.

MEMS-based reconfigurable antennas

In this paper, two frequency reconfigurable antennas using the ON/OFF states of a microelectromechanical system (MEMS), are presented. A multifrequency planar inverted-F antenna (PIFA) is designed with a L-shaped open slot upon its main plate. As a proof-of-principle, an ON state RF MEMS switch, modeled by a piece of copper (1x1 mm/sup 2/), is inserted into the slot to achieve frequency switching (while OFF state is modeled without the piece of copper). According to the switch position alone the slot, the GSM operating frequency is not detuned while the other standards become reconfigurable. To validate our concept, a thermal MEMS switch, designed and developed at CEA-LETI is used. The MEMS switch is presented and its scattering parameters are measured and compared with simulated results from an equivalent RLC circuit. The active device is then inserted in the slot to valid the simulated frequency behaviour of the PIFA. Based on the same concept, a planar antenna is designed. It consists in a printed PIFA on a high-resistivity silicon substrate (10 k/spl Omega/.cm). The antenna resonates in the 2.4 GHz WLAN band but the insertion of two small gaps in its main aim which model the OFF state of a MEMS switch allows the antenna to work in the 5.150-5.875 GHz HIPERLAN 2 frequency bands.

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

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

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

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

Diversity characterization of optimized two-antenna systems for UMTS handsets

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

Design Optimization of UWB Printed Antenna for Omnidirectional Pulse Radiation

The use of ultrawideband technology in many applications implies the use of a suitable radiating structure, especially for pulsed systems. The development of a triangular CPW-fed printed antenna with specific ground plane shape is presented. First the triangular element was optimized to minimize return losses and size of the antenna in the [3.1-10.6] frequency band. The evolution of radiation patterns with frequency was also studied, as changes in phase center position and radiation characteristics alter the integrity of transmitted pulses. Thus the antenna ground plane shape was modified in order to maintain almost constant radiation patterns over the entire bandwidth. Time domain measurement methods were used to characterize the antennas and validate these improvements. The procedure is presented and results are plotted in a 3D format and discussed.

Frequency and Time Domain Analysis of Different Approaches to the Backing of an UWB Slot Antenna

The presence nearby of a parallel metallic plane usually impairs the radio-electric performance of planar ultrawideband (UWB) elements such as monopole and slot antennas. Two comparative approaches to this problem are studied, in the frequency and time domains. Conservation of the UWB performance of a slot antenna is investigated in order to obtain a mono-directional quasi-planar element suitable for an UWB array. First, the addition of a planar reflector is studied: up to a certain distance, wide impedance bandwidth is maintained, but the antenna offers a non-uniform gain. Secondly, the addition of a cavity overcomes this disadvantage and makes the cavity-backed slot antenna one of the best candidates for an UWB quasi-planar array.

Exploitation of the UC-PBG structure as ground plane to enhance microstrip antenna bandwidth

The uniplanar compact photonic bandgap (UC-PBG) metamaterial structure has been profitably used for antenna miniaturization and radiation enhancement purposes. This paper presents an additional way to exploit it, when it is employed as ground plane for microstrip antennas. As a matter of fact, if a proper number of UC-PBG unit cells is selected, the antenna bandwidth can be remarkably increased. Such a behavior is here presented in dealing with a classical square patch antenna.

PIFA antenna with tilted circular polarization angle for RFID readers

In this paper, a miniaturized circularly polarized (CP) antenna for RFID readers is presented. This structure, made up of two crossed PIFA has been designed to operate in the European RFID UHF band [0.865-0.868 GHz]. Thanks to two shorts circuits, miniaturization and CP are achieved with only one feed. By optimizing the different parameters of the structure, CP direction can be also tilted in a desired angle. The simulated impedance matching and axial ratio bandwidths are 4% and 16%, respectively. The realized gain is 1.85 dBi at 0.866 GHz. The overall dimensions of the structure are 0.5λ0 × 0.5λ0 × 0.06λ0.