^{1}

^{2}

^{2}

^{1}

^{2}

This work presents the analysis of monopole microstrip antennas with truncated ground plane and patch geometry inspired on the Mandelbrot fractal curve for applications in UWB systems. The proposed antenna geometry is analyzed using the Wave Concept Iterative Procedure (WCIP), a full-wave method. Results for the proposed antenna operating frequency, bandwidth, VSWR, gain, and radiation pattern are obtained and discussed. The WCIP results are compared with simulation results provided by HFSS software, for validation purpose. In addition, a prototype antenna is built and measured. A good match between WCIP theoretical and simulation, HFSS simulation, and measurement results is observed for the antenna frequency response.

The continuous advance in wireless communication systems has motivated many researchers worldwide to investigate and develop antennas for ultrawideband (UWB) system applications. The great interest in this particular antenna topic is due to several factors such as immunity to the spreading of multipath, difficulty in accessing signal invader users, less interference when compared to narrowband communications, and low cost. Particularly, microstrip antenna structures are an option of great attractiveness to design UWB antennas with excellent performances.

An UWB signal is defined as every signal that occupies a minimum bandwidth of 500 MHz in a particular 7.5 GHz band (from 3.1 GHz to 10.6 GHz), based on the spectral overlap, since much of this spectrum band is used by existing services and technologies [

This paper describes the analysis of a monopole microstrip antenna printed on textile substrate for applications in UWB systems. The use of textile substrate is justified by the increasing need to develop lightweight and flexible devices that could be attached to garments and applied in several areas. For example, several authors have investigated the use of textile substrates, such as microstrip antenna substrates, for applications in biomedical fields [

The patch geometry of the proposed antenna is based on the Mandelbrot escape fractal curve [

Over the years, several authors have studied the application of fractal geometry in the design of planar antennas and printed antennas for UWB applications [

For comparison purpose, simulations are performed using the finite element method, implemented in the commercial software Ansoft HFSS. In addition, a prototype has been fabricated and measured. Good agreement has been observed between the results obtained by WCIP formulation, HFSS simulations, and experiments.

The WCIP method is a full-wave iterative method whose formulation is based on the electromagnetic wave concept [

The WCIP formulation is based on the relationship between incident and reflected waves at the interface of the circuit under investigation [

The relationship taking place between the incident and reflected waves in front of the circuit interface under analysis is described in (

The relationship between the incident and reflected waves in the propagation medium apart from the circuit interface under analysis is described in (

The iterativity of the WCIP method is obtained using the parameters’ relationships in (_{x}_{y}_{mn}_{mn}_{mn}

The Fourier modal transform (FMT) allows you to define the amplitudes of the waves as expansions of TE and TM modes, in the modal domain. This transformation allows the transition of the waves from the spatial domain to the modal domain. The antitransform or inverse Fourier modal transform (FMT^{−1}) allows the return from the modal domain to the spatial domain [

The antenna structure is shown in Figure

Mandelbrot monopole patch antenna on textile substrate: (a) antenna patch illustration, (b) antenna structure illustration, and (c) photographs of the fabricated prototype (front and back views).

In this work, the Mandelbrot set has been used as a basis to develop the microstrip patch antenna structure. The Mandelbrot sequence has been applied in a circle of radius

The brim santista textile material has been used as microstrip substrate, which has the following structural parameters: thickness _{r} = 1.92, and loss tangent tan

Figure

WCIP-simulated return loss results for monopole microstrip antennas on different substrates.

In addition, according to the results presented in Figure

To validate the WCIP results presented in Figure

Frequency responses for fractal monopole microstrip patch antenna on brim santista substrate: (a) return loss and (b) VSWR.

Considering a maximum value reference of 2 for VSWR and −10 dB for return loss, the obtained simulated and measured results are lower than the reference values over the entire frequency band. In addition, a good agreement between simulation and experimental results is verified for the antenna bandwidth.

Figure

Surface current distribution on the fractal patch geometry of the monopole microstrip on brim santista substrate.

Figure

Radiation patterns (gain results) for the monopole microstrip antenna on brim santista substrate: (a) 2D and (b) 3D results.

Table _{11}, for the resonant frequency _{r}, and the maximum VSWR value in the UWB band, and the maximum gain for −100° <

Simulation and Measurements Results.

Parameter | WCIP | HFSS | Measurements |
---|---|---|---|

_{r} (GHz) |
6.51 | 6.63 | 6.67 |

VSWR | 1.16 | 1.49 | 1.60 |

Gain | 0.640 | 0.632 | — |

Based on the results shown in Table

A microstrip antenna for UWB applications printed on a textile substrate has been analyzed using the WCIP method. The monopole antenna is based on a resonant patch element with Mandelbrot fractal geometry. WCIP formulation has been used in the determination of the antenna resonant frequencies, return loss, and radiation pattern (gain results). The WCIP theoretical, HFSS simulation, and measurement results indicate that the proposed antenna can operate in the whole UWB band. In addition, the monopole antenna has presented omnidirectional radiation characteristics. Finally, an excellent agreement between measured and simulated results has been observed confirming the good performance of the proposed antenna and efficiency of the used numerical analysis technique (WCIP method).

The authors declare no conflicts of interest regarding the publication of this paper.

This work was partially supported by CNPq, under Covenant 573939/2008-0 (INCT-CSF), CAPES, Federal University of Rio Grande do Norte (UFRN), and Federal Rural University of the Semi-Arid Region (UFERSA).