диафрагмированные волноводные фильтры / 2483665f-8eef-443d-82b3-2460feb702c8

Abstract—This paper deals with the design of the two-pole filtering antenna based on SIW (substrate integrated waveguide) resonator coupled by rectangular iris to the rectangular patch antenna for body centric communication. Firstly, we present the design of the filtering antenna. Secondly, the change of the antenna parameters influenced by the proximity of the human tissue is studied; especially its gain characteristic and radiation patterns. In order to tune the antenna simplified multilayered model of human tissue is applied. Optimized antenna is then used in the complex model of the human body to verify the antenna properties.

Keywords—SIW; antenna; filter; human tissue; gain characteristic; body centric communication;

I.INTRODUCTION

These days wireless body area network (WBAN) attracts attention thanks to many promising applications in the area of identification, biomedical engineering and many others [1- 3]. Properties such as low profile, low weight and compact size of the antenna are demanded for off-body communication in ISM frequency band 5.8 GHz (ITUR 5.138, 5.150 and 5.280 of the Radio Regulations) due to comfortable and easy placing in proximity of the human body. The ISM radio bands are reserved internationally for the use of radio frequency energy of industrial, scientific and medical purposes other than communications.

Most of RF front-ends use bandpass filter for filtering the unwanted signal. These filters are cascaded directly after the antenna. The number of communication services rapidly increases thus the particular channels are very close to each other. In this case it is necessary to use bandpass filters with high selectivity in order to sufficiently filter the incoming signal. In practice, bandpass filters need to have higher order or multiple transmission zeros in order to effectively filter the unwanted signal even with the lower filter order. Several papers deal with the integration of bandpass filter and radiating antenna into the one single device. Microstrip antennas with filtering function are presented in [4-7]. Another possibility uses electromagnetic horn antenna [8, 9]. The results of such an approach lead to complete filtering antenna (filtenna) without the necessity of cascading bandpass filters after the antenna. Afterwards, the overall miniaturization of

the RF front-end is achieved. Otherwise, it is at least possible to effectively pre-filtrate the signal and then rapidly decrease the demands on the cascaded bandpass filter.

In this paper, the antenna is designed and optimized to operate in vicinity of a human body in ISM frequency band from 5.725 GHz to 5.875 GHz. We used the simplified threelayer chest and voxel duke model for the results verification for simulations. The designed antenna is simulated and optimized in CST Microwave.

II.TWO-POLE FILTERING ANTENNA

The multilayer schematic view of the proposed filtering antenna is shown in Fig. 1. We use two dielectric layers. The lower layer (CuClad217 with ɛr = 2.2 and 0.7874 mm of thickness) consists of excitation structure (bottom), vias representing the SIW resonator and common ground with rectangular slot. We use the same dielectric substrate for upper layer with rectangular patch antenna.

Upper dielectric layer with rectangular patch

Ground with coupling rectangular slot

Lower dielectric layer with vias

Excitation

Fig. 1. Multilayer schematic view of the proposed antenna

The similar concept of SIW resonator and radiating patch is presented in [10]. The whole filtering antenna is based on two resonators coupled by the slot. The first resonator is SIW resonator in TE101 mode. This resonator is coupled by the slot which acts as an admittance inverter. The second resonator is a typical patch antenna in its fundamental mode. From the filter point of view, the input is realized as a microstrip line inserted into the SIW resonator. The output (load) corresponds to the patch antenna. The signal is stored as a resonant mode and the rest is radiated into a free space. The advantage of this approach lies in a possibility of filter synthesis based on circuit model. The circuit model

parameters can be then transformed to the fullwave model [10].

dimensions is shown in Fig. 3. In free space (Fig. 5), antenna provides very good impedance matching (-20 dB in the passband). The realized gain reaches 9.34 dB with almost undeflected main lobe in particular plane (Fig. 7, 8). The realized gain in the dependence of frequency (Fig. 6) shows the sufficient outband gain rejection.

Y

X Z

Fig. 2. E-field in z direction: SIW rezonator (left), patch antenna (right)

Fig. 3. Schematic view of the antenna with defined parametres

III.MODELS OF THE HUMAN TISSUE AND INTERACTION

WITH FILTERING ANTENNA

The primary simulations using antenna placed in proximity of the simplified human body model to obtain general results of interaction between the antenna and the human body model from impedance matching and the radiation properties point of view. This simulation of the simplified model takes less computation time due to lower number of mesh cells than in the complex model.

The human model used for primary simulations was composed of three layers (tissue sample) – skin, fat and muscle. From the propagation of electromagnetic waves point of view, all the described layers are lossy dielectric materials of specific parameters at demanded frequency [see Table I]. The chest model is a block with base 80 mm x 80 mm. The height of the block is given by the thickness of the layers representing the skin, the fat and the muscle [11]. We

chose 5 mm distance between human body model and the antenna with regard to interaction between human body models and the antenna.

TABLE I. THE MATERIAL PARAMETERS OF THE HUMAN BODY MODEL

AT FREQUENCY 5.8 GHZ

The impedance matching of the antenna designed in Chapter II was simulated in the proximity of simplified model of human tissue (Fig. 5). As we expected, the response is slightly untuned. In order to tune the antenna, we used local optimization method called Nelder Mead Simplex. The goal was to have S11 = -20 dB in the band from 5.725 GHz to 5.875 GHz. The optimal response is shown in Fig. 5. The realized gain reaches 9.31 dB at 5.8 GHz. Next, we verify the antenna performance on the complex human model.

The complex, voxel human body model used for advance simulations whose dimension and organ masses correspond to the values defined by the ICRP were developed in [12]. The model was created from computer tomographic or magnetic imaging. The image which sets anatomical body model have been segmented using a fixed spatial resolution (e.g. cubical voxels) over the entire body with resolution 1.0 mm x 1.0 mm x 1.0 mm [12].

Fig. 4. Voxel human body model with antenna placed on the chest

Advanced simulations take more computation time, due to the fact that the voxel model contains more details such as internal organs, veins, hairs etc., however, the results are more realistic.

Fig. 4 illustrates the voxel duke model with antenna placed on the left side of the chest 5 mm from the body. Fig. 5 shows that from the impedance matching point of view the change in response between the antenna with simplified model and complex model is negligible. As we expected, the side lobes

level decreases rapidly due to the presence of the human body. The noticeable side lobe level rejection can be seen in H plane (Fig. 7). The antenna main lobe is almost perpendicular to the body over the whole antenna passband. In addition, the presence of the human body enhances the outband gain reduction as the part of the radiated energy dissipates in the lossy human tissue. The realized gain at 5.8 GHz reaches 9.67 dB. We also studied the influence of the change in the distance between the surface of the human body and the antenna. We came to a conclusion that even though we rapidly change the distance value, the return loss characteristic does not change (Fig. 9).

The final antenna dimensions are summarized in Table II. Please note that the distance between the lower and upper layer is set to 0.5 mm. This gap can be sufficiently realized using the plastic washers and screws in the further antenna manufacturing.

TABLE II. OPTIMAL ANTENNA DIMENSIONS

Antenna dimensions [mm]

Fig. 5. Comparison of the antenna impedance matching for all the models

Fig. 7. Radiation patterns in H plane at 5.8 GHz

Fig. 8. Radiation patterns in E plane at 5.8 GHz

Fig. 6. Simulated realized gain response for E plane and H plane

We presented the design of the compact low-profile twopole filtering antenna working at ISM band (5.8 GHz). The antenna itself provides very good impedance matching and radiation properties, especially outband gain rejection. We used two different models for compensating the human tissue on the antenna parameters; less consuming computation time three layers human tissue model for antenna pre-tuning and the complex human model for the results verification.

The results show that even with very complex human model, antenna has very good impedance matching and from the filtering function point of view reasonable outband gain reduction. The antenna overall dimensions are 60 mm x 60 mm x 2.2 mm.

The manufactured filtering antenna with the all measured necessary data (reflection coefficient, realized gain and radiation patterns) will be presented at the conference.

ACKNOWLEDGMENT

Research described in this paper was financed by the grant of the Czech Science Foundation no. P102/12/1274. The research is a part of the COST IC1102 Action supported by the Czech Ministry of Education under grant no. LD12012. Measurements were performed in the SIX Research Center.

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