A. Van de Capelle

An impedance-matching technique for increasing the bandwidth of microstrip antennas

The nature of the inherent narrow bandwidth of conventional microstrip patch antennas is considered. It is observed that, except for single-feed circularly polarized elements, their bandwidth is limited only by the resonant behavior of the input impedance and not by radiation pattern or gain variations, which usually are negligible over a moderate 10 to 20% bandwidth. Therefore, broadband impedance matching is proposed as a natural to increase the bandwidth. The maximum obtainable bandwidth is calculated using Fanos broadband matching theory. It is found that by using an optimally designed impedance-matching network, the bandwidth can be increased by a factor of at least 3.9, the exact value depending on the degree of matching required. A transmission-line prototype for a proper matching network is developed. The translation of this prototype network into a practical structure (e.g. a microstrip or stripline circuit) is considered. Practical design examples and experimental results which clearly show the validity of the technique are given. >

Mixed-potential integral expression formulation of the electric field in a stratified dielectric medium-application to the case of a probe current source

The well-known procedure for determining the electric field in a structure consisting of an arbitrary number of planar dielectric layers is modified in order to obtain a form specially suited for the analysis of multiprobe multipath configurations. In general, the field is generated by arbitrary currents in the layers and arbitrary sheet currents in the transitions between the layers. The currents may be electric as well as magnetic, and the dielectric layers are isotropic, homogeneous, and lossy. The procedure results in Greens functions especially suited for the analysis of multiprobe multipatch configurations. They can be used in an efficient mixed-potential integral expression formulation. The theoretical procedure is applied in the case of a probe current source situated in one of the dielectric layers of the structure. For this probe current a highly efficient attachment current distribution is derived. Comparison of measured and calculated results for example structures proves the accuracy of both the approach and the attachment mode. >

Study of the capacitively fed microstrip antenna element

The moment method is used to solve the integral equations describing the capacitively fed rectangular microstrip antenna element. This element consists of a ground plane, a radiating patch, and a small patch located between ground plane and radiating patch. The small patch is fed by a coaxial probe. It excites the radiating patch through capacitive coupling. After checking the accuracy by comparing calculated and measured results, the effect of the capacitor patch is analyzed theoretically. A procedure is given to determine capacitor patches which yield elements matched to the coaxial feed. It is shown how a matched configuration can be found for a given capacitor patch height. >

Study of gain enhancement method for microstrip antennas using moment method

The moment method is used to determine the radiation and impedance properties of microstrip patch antennas in multilayered material configurations. The resonance conditions for the layer structure which allow for high gain are studied. The gain, the impedance, the beamwidth, and the bandwidth are discussed. >

Direct Extraction of the Non-Linear Model for Two-Port Devices from Vectorial Non-Linear Network Analyzer Measurements

Non-linear models for microwave and millimetre wave devices are commonly based on DC and S-parameter measurements, due to the absence of vectorial large-signal measurements in the past. At present, accurate prototype measurement systems are being developed, which implies that new non-linear modelling techniques can be explored. We will present a method that allows the direct extraction of the state-functions of a HEMT.

A model for calculating the radiation field of microstrip antennas

Starting from the equivalence principle, an aperture model is developed for calculating the radiation field of microstrip antennas. In this communication the model is applied to the rectangular microstrip resonator antenna. Antenna characteristics, like patterns and radiation resistance, are computed and compared with experimental results. The model and the calculations include the higher order modes as well as the fundamental mode of the resonator antenna.

Broadband active microstrip antenna design with the simplified real frequency technique

This paper deals with the design of broadband active microstrip antennas where the amplifier is integrated with the radiator. Theoretically sound definitions for gain and noise figure of the active antenna are introduced, and their relationships with the definitions for the composing circuit and radiator parts are explained. A sequential design procedure is presented that allows the straightforward and optimal design of transmitting and receiving antennas with multiple active stages, taking into account input and output matching, the gain-versus-frequency curve as well as the noise performance. The theoretical concepts are illustrated with two examples: one of a transmitting active antenna and one of a receiving antenna. The former one is a two-stage design that achieves nearly 25% of bandwidth with regard to gain and matching and 24 dB gain improvement as compared to the matched passive antenna. The second one is a receiving antenna (one stage) with a measured noise figure of 1.2 dB in a bandwidth of over 17% and a gain improvement of 11.9 dB over the corresponding passive antenna. Finally co- and cross-polar radiation patterns in E- and H-plane prove that the antennas also have favorable radiation characteristics in a wide bandwidth (at least 18%). >

Transmission line model for mutual coupling between microstrip antennas

Although more rigorous treatments have been developed for mutual coupling between microstrip antennas, the purpose of this transmission line model is to provide a numerically efficient substitute for them. Therefore, two approximations have been introduced: first, the surface waves have been neglected and second, each rectangular resonator is replaced by two equivalent radiating slots. In most practical cases the approximations are acceptable; this has been proved while comparing the transmission line model with other published results. It is obvious that the efficieney of the transmission model and can be used to include mutual coupling in practical analysis or synthesis routines for arrays of rectangular microstrip antennas.

Wavelet packet based multicarrier modulation

A recent but already popular technique for broadband communications is multicarrier modulation, which divides the channel into several orthogonal, overlapping subchannels. Multicarrier modulation is normally implemented using a Fourier transform to create and detect the different subcarriers. This can be efficiently implemented using an (I)FFT. This transform however has the drawback that it uses a rectangular window, which creates rather high sidelobes. This leads to rather high interference when the channel impairments cannot be fully compensated. Looking at other transforms leads us to the wavelet transform, and more specifically the wavelet packet transform, which has more flexibility and due to the time-overlap can have much lower sidelobes. This article studies the implementation of these wavelet packets, and the effects this implementation has on the requirements imposed in the design of usable wavelets. It is shown that the restrictions imposed by the perfect reconstruction requirement necessitate the use of biorthogonal wavelets. This however influences the performance. Moreover, the frequency behaviour of the wavelet packet transform is not straightforward which limits the practical use of this transform in a multicarrier system.

Calculation of the Bandwidth of Microstrip Resonator Antennas

In this paper one presents an analysis of the bandwidth of microstrip resonator antennas in function of the antenna parameters. Starting from the general expression for the quality factor of a resonator, it will be proved that the bandwidth of a microstrip resonator antenna is directly proportional to the thickness of the substrate, to the square of the resonant frequency and inversely proportional to the square root of the relative permittivity of the substrate material. Comparing the theoretical results with experimental data one finds a good agreement.