Analysis of Helix Turns Impact on Design of Helix Antenna Operated on 2.46GHz Proposed

for FSO/RF Hybrid System

Michal Márton*, Ľuboš Ovseník*, Ján Turán*, Michal Špes**, Jakub Urbanský**

*Technical University of Košice, Faculty of Electrical Engineering and Informatics, Department of Electronics and Multimedia Communications, Košice, Slovakia

**Technical University of Košice, Faculty of Electrical Engineering and Informatics, Department of Electrical Engineering, Košice, Slovakia

michal.marton@tuke.sk. lubos.ovsenik@tuke.sk, jan.turan@tuke.sk, michal.spes@tuke.sk, jakub.urbansky@tuke.sk

Abstract – Motivation on the research in this area is still growing with regards on expanding demands on communication networks. We live in the ages of communication technologies. Users’ demands are still growing. We need to develop new solutions to resolve the growing demand on multimedia streams in metropolitan networks. Current high speed communication systems are based on an optical fiber transmission. However, in densely populated areas an optical fiber cannot always be embedded. In these areas free space optics (FSO) is a perfect solution. With FSO system optical beams can be transmitted through free space. In the case when the primary optical link has the radio-frequency (RF) backup link, the system is named FSO/RF hybrid system. The RF backup link is used in outage cases. This paper is focused on improving availability of FSO system using RF link with appropriate type of antenna. In this research we consider helix antenna. Design is realized using professional designing tool FEKO. In the optimization process of the RF link we focused on the analysis of antenna properties depending on the number of helix turns. The operating frequency was chosen from non-licensed frequency range free available in Slovak republic. It is 2.46GHz. The main point of this paper is to find appropriate candidate for RF backup link which will be implemented in our experimental FSO/RF system.

Key words – antenna design, FSO/RF hybrid system, helix antenna, turns of helix

I. INTRODUCTION Communication technologies were expanded through

many areas of our lives. These technologies have dominant effect onto private and professional spheres of human lives. Fully digitalized system needs direct communication between all parts of system for sufficient operation. The demand on availability of services are still growing and it needs physical infrastructure which would be able to transmit full data stream. In recent years a lot of communication systems which are able to react on the users demands were invented. The main break point in the invention of communication was the invention of optical fibers. Optical fiber communication systems allow rates of about several Gbps. Nowadays, backbone networks are based on an optical fiber. Their implementation has some drawbacks which is conditioned to the next research. These

drawbacks are: the higher building cost of the physical structure and the implementation time. The optical communication system which is able to transmit optical beams without the need physical fibers Free Space Optics system (FSO). This system minimizes drawbacks of fiber optical communication systems. On the other hand, the FSO system have the drawback too. It is the sensitivity to weather changes. The solution is in implementation of backup radio-frequency transmission link. This backup link makes redundancy in case of soft switching between links. In the case when the communication is realized synchronously using both links, we talk about soft switching. In the case when the communication is realized using primary link and the switching is realized when the primary link has an outage, we talk about hard switching. The experimental model of FSO/RF system is realized in the area of Technical University of Košice. Now we are interested in design of appropriate type of RF antenna operating on frequency from non-licensed frequency range. The main point of this paper is comparison of six helix antennas. Our previous research is published in [1-7].

II. DESIGN OF HELIX ANTENNAS The design was realized in professional tool proposed to

analysis of electromagnetic compatibility from Altair Company. This program suite called FEKO. FEKO is simulation software package for analysis of electromagnetic compatibility. We designed six models of helix antenna with the same dimensions, the difference was the number of turns of helix. The dimensions of antennas are in the TABLE I.

TABLE I. PARAMETERS OF DESIGNED HELIX ANTENNA

Label Parameter Values f Frequency 2.46 GHz N Number of turns 5 d1 Diameter of helix 4.4 cm d2 Diameter of conductor of helix 0.2 cm d3 Diameter of reflector 6.2 cm l Length of helix 12.2 cm

These models operates on 2.46 GHz. The TABLE II. shows the assignment of the number of turns and label of antenna.

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TABLE II. DIFFERENT VALUE OF KEY PARAMETER OF THE MODEL

Variable Number of Turns - N

Antenna No.1 5 Antenna No.2 6 Antenna No.3 7 Antenna No.4 8 Antenna No.5 9 Antenna No.6 10

The structure of helix antenna model is given by Fig. 1. Parameters defined in the TABLE I. are also shown in Fig. 1. These values are given by equations in [8-15].

Fig. 1 The reference model of helix antenna

For design was chosen programme CAD FEKO from the package. Its environment is used to set the elements features. CAD FEKO used Mesh network which distributes element to the grid of same triangles. For these particular elements the Maxwell´s equations will be calculated using method of moments (MoM). The models of designed antennas are in the Fig. 2.

Fig. 2 Models of designed antennas in CAD FEKO Requirements for analysis were chosen in CAD FEKO.

The one of these requirements is FarField. The considered FarField consists from points placed around the designed model of antenna. In this case these points are placed on the surface of virtual sphere in uniform distances between points [16-21].

III. ANALYSIS OF DESIGNED HELIX ANTENNAS The analysis of designed antennas was realized in

program POST FEK. POST FEKO allows analysis of fundamental parameters of simulated antennas. The 3D radiation patterns of simulated antennas is illustrated in the Fig. 3.

Fig. 3 Total gain of designed antennas operating on 2.46 GHz

These radiation patterns were obtained on operating frequency 2.46 GHz. The cut of these radiation patterns (Φ = 90°) could be seen in the Fig. 4.

Fig. 4 Radiation of modelled antennas for Φ = 90°

The radiation patterns in the Fig. 4 are placed in the polar coordinates. The radiation patterns are in the [dBi] units which suggest increased value of gain in comparison

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with the case when the radiation of considered antenna is isotropic.

TABLE III. VALUES OF TOTAL GAIN

Variable Gain Gain [dBi] Antenna No.1 15.5079 11.9055 Antenna No.2 16.022 12.0472 Antenna No.3 15.8382 11.9971 Antenna No.4 15.0451 11.774 Antenna No.5 15.8281 11.4076 Antenna No.6 12.494 10.967

Obtained values of gain are recorded in the TABLE III. In this table, the maximal values of obtained gain denoted are by italic.

Fig. 5 Total gain of designed antenna operating on 2.46 GHz in dBi units

The Fig. 5 shows the dependence of obtained values of total gain of simulated antennas and change of number of turns. The maximum value of total gain was obtained for antenna No.2 with the value 12.0472 dBi.

Fig. 6 Change of angle of radiation for Φ = 90

The most important parameter of antenna is Half Power of Beam Width (HPBW). The HPBW of designed antennas is in the Fig. 6.

TABLE IV. VALUES OF CHANGE OF RADIATION ANGLES

Variable HPBW - Φ = 0° HPBW - Φ = 90° Antenna No.1 46.2746° 45.7531° Antenna No.2 44.6114° 44.0651° Antenna No.3 43.6217° 42.9374° Antenna No.4 43.3116° 42.4555° Antenna No.5 43.7664° 42.7112° Antenna No.6 45.2192° 43.9022°

The obtained values of angle of radiation could be seen in the TABLE IV. In this table, the minimal values of this angle are denoted by italic.

Fig. 7 Angle of radiation at Half Power of Beam Width (HPBW)

The Fig. 7 shows the dependence of obtained values of change of radiation angle of simulated antennas and change of number of turns. The HPBW value for antenna No.4 is the smallest: 43.3116° for Φ = 0° and 42.4555° for Φ = 90°

TABLE V. VALUES OF MAXIMAL RADIATED POWER AND SIDE LOBE LEVEL

Variable Power [mW] Power [dBW] SLL [dB] Antenna No.1 10.0258 -19.9888 15.1271 Antenna No.2 10.4748 -19.7985 15.339 Antenna No.3 7.90759 -21.019 15.4837 Antenna No.4 6.46146 -21.8967 15.0803 Antenna No.5 6.9322 -21.5913 14.3799 Antenna No.6 10.0957 -19.9586 13.6126

The values of reached power are noticed in the TABLE V. In this table, the SLL parameter describes the ratio between maximal lobe and second maximum.

Fig. 8 Maximal radiated power of designed antennas

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The Fig. 8 shows the dependence between obtained maximal power values in [mW] of simulated antennas and change of number of turns.

Fig. 9 The impedance of the designed antennas

The impedance points of designed antennas can be seen in the Fig. 9. In this figure, the points are placed on Smith chart.

IV. CONCLUSION This paper is focused on examination of impact of

number of helix turns on features of designed antennas. We are looking for appropriate RF antenna for FSO/RF hybrid system and on the base of this knowledge we chose the frequency of 2.46 GHz from non-licensed frequency range. The antenna No.2 had the maximal value of gain: 16.022 (12.0472 dBi). The most important parameter for direction oriented application is size of the angle of radiation. We consider the small value represent the direction of radiation into small spreaded beam. The HPBW of antenna No.4 for Φ = 0°is 43.3116° and 42.4555°for Φ = 90°. For non-licensed application the maximal value of radiated power into affected area is very important parameter. The threshold value for the given frequency (2.46 GHz) is 20 mW. The maximal value of radiated power was obtained for antenna No.2 with the value 10.4748 mW (-19.7985 dBW). The last examined parameter was side lobe level (SLL) which shows the ratio between main lobe and second maximal lobe. With regards on this knowledge the maximal value for antenna No.3 was 15.4837 dB. This paper shows that the trade-off is the need. From our point of view is antenna No.2 appropriate candidate for realization.

ACKNOWLEDGMENT This work was supported by following research grants:

KEGA 023TUKE-4/2017, APVV-17-0208 - Resilient mobile networks for content delivery and Technical University of Košice.

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