[ieee 2012 ieee symposium on wireless technology & applications (iswta) - bandung, indonesia...

A Design Of Off Centered Feed Array (OCFA) Antenna for ISM Band

A.A. Azlan (1), M.T.Ali (2), M.K.M Salleh(2)

(1) Center Of Medical Electronic Technology (CMET) Sultan Salahuddin Abdul Aziz Shah Polytechnic;

Persiaran usahawan seksyen u1; Shah Alam Selangor, Malaysia

E-mail:[email protected]

(2) Faculty of Electrical Engineering Universiti Teknologi MARA Malaysia 40450 Shah Alam, Selangor, Malaysia.

Email:[email protected]

Abstract -- In this paper, a patch antenna for 2.45 GHz ISM (Industrial Scientific Medical) band Wireless Body Area Network (WBAN) applications is proposed. The antenna structure consists of a microstrip antenna array consist of 3 by 3 OCFA (off centered feed array) that is coaxial feed through type antenna with Unbalanced characteristic whereby the feed line situated off centered from the OCFA antenna structure. Two matching stub and Circular matching BalUn were introduced to match the impedance of the antenna and contribute high gain antenna about 11.84 dB at 2.45 GHz. Then, the entire proposed antennas were being analyzed in term of return loss, Voltage Standing Wave Ratio (VSWR, radiation pattern and gain. The fabricated proposed antenna only analyzed in term of return loss using Vector Network Analyzer (VNA). Keyword: Industrial Scientific Medical, Wireless Body Area network, BalUn, matching stub, Return loss.

I.INTRODUCTION

With the recent development of wireless communication technology, many researchers pay great attention to the study of wireless body area networks (WBANs). WBANs link between various electronic devices in, on, and the human body. The application of WBANs has been expanding in medical services, national defense, wearable computing, and so forth. Several frequency bands have been assigned for WBAN systems, such as the medical implant communication system (MICS: 400 MHz) band, the industrial scientific medical (ISM: 2.4 GHz and 5.8 GHz) band, and the ultra-wideband (UWB: 3~10 GHz) [1].Not much design focusing on unbalanced type of antenna especially on array method to enhance the antenna gain. Since it is difficult to match the fed of the antenna with conventional balanced type of matching hence it also contributes of multiple side lobes on radiation pattern. So the need of the antenna design for high gain is

antenna is importance to minimize the power and still offer great distance for long range communications. The overall study is base on developing the high gain antenna for ISM band, so it can be used on various electronic devices to exchange the data from the sensor equip with on body antenna for medical sensing purposes to the receive antenna at the electronic equipment. By using the microstrip high gain antenna the amount of supplied power can be reduced and the long range communication between reading equipment and sensor still can be maintain since the system used high gain array antenna. Most design is base on using microstrip antenna technology using low cost FR4 substrate and all the antenna parameters were show in TABLE I.

TABLE I: ANTENNA PARAMETER

Substrate FR-4

Dielectric constant εr 4.9

Tangent loss 0.019 Thickness 0.8 mm Centre frequency 2.45 GHz

II.ANTENNA DESIGN

The design was inspired by the usage of 2 by 2 arrays structure with FR4, 0.8mm substrate. So the aim is to maintain the same substrate characteristic and rectangular quarter wave technique used by that array antenna to perform the design of our proposed OCFA antenna. The main works were focusing not only increasing the antenna gain and its performance but also maintaining the substrate characteristic that used FR4 0.8mm substrate. The design of

2012 IEEE Symposium on Wireless Technology and Applications (ISWTA), September 23-26, 2012, Bandung, Indonesia

978-1-4673-2210-2/12/$31.00 ©2012 IEEE 233

2 by 2 arrays is show in the figure 1 while proposed 3 by 3 OCFA antennas is shown in figure 2.

Figure 1: 2 by 2 conventional array antennas

Figure 2: Proposed 3 by 3 OCFA antenna

The overall length of proposed antenna is 131.6 mm x 130.2 mm and the shape of the antenna is almost like cubic shape, and the overall shape in figure 2 are aspired from figure 1.The structure of 2 by 2 array system is still be maintained and from the figure 2, half of the structure of figure 1 was add to left of the structure at figure 2 and all the dimension are exactly the same as the origin patch. Then, we add another 3 patches to the upper section of figure 2. This upper section is aspired from the 2 patches located at the bottom of the figure 1. The overall structure of the proposed antenna consists of 9 patches of rectangular antenna that be array together with the direct feed using feed through SMA connector from behind of the antenna. Since this proposed antenna contributed to the unbalanced type of antenna since the feeding line located at approximately λ/3 to left of the antenna and 2λ/3 to right of the antenna and the illustration of location of feeding point is

show in figure 3. As reported in [2] any off centered antenna, contribute high impedance up to 200 ohm at the feeding point of the antenna due to unbalanced condition. So to overcome this problem we need to install the BalUn (balance to unbalance) to the proposed antenna to overcome the mismatch problem. As also reported in [2] they install ratio 4 to 1 BalUn so that the impedance is down to 50 ohms that is suitable for most communication standard for transmitting equipment [3]. Besides that, in order to fine tune the antenna especially to resonate to our desired ISM band centered frequency that is 2.45 GHz, there are 2 matching stub installed at the proposed antenna. This both matching stub are Shunt -short type of matching stub. The design of circular Matching BalUn and 2 matching stub are show in figure 3.

A. BalUn and matching stub design

Figure 3: BalUn and Matching Stub

As illustrated in figure 3 the design of shunt short matching stub is expressed in equation (1) through equation (3) and for tuning purposes, if the calculations for the lengths turn up negative we can increase the distance by λ/2

= tan , 0 tan , 0 (1)

Where

(2)

For a short circuit stub is defined as:

= tan (3)

Beside matching stub the BalUn also play the importance role in our proposed antenna matching system. The design of the BalUn is based on the effective area of the patch (length of the rectangular single patch) that is defined as


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and to find the diameter for the BalUn we used the equation /4 to find the diameter of the BalUn. All the calculation and actual dimension of the antenna is shown in TABLE II (all dimension in mm).

TABLE II: BalUn AND MATCHING STUB DIMENSION

B. Mitering Design

Mitering or curving transmission lines is a process of bending the end of the array to minimize the reflection effect of the antenna. Mitering the bend chops off some capacitance, restoring the line back to its original characteristic impedance [4]-[9].The figure 4 below shows the important parameters of a mitered bend.

Figure 4: Array mitering process

From figure 2 our proposed antenna used this mitering process to minimize the reflection effect at the end of the antenna array. The process of mitering process is based on the Equation (4) through Equation (6).

D √W (4)

0.52 0.65 . (5)

(6) Where

D – Chamfer angle, W – microstrip Width and H – Substrate height while on most of our proposed miter design for the proposed antenna is show in TABLE III.

TABLE III: MITERING DIMENSION (IN MM)

C. Rectangular patch and Array design

Figure 5: Single Patch Design

Figure 6: Upper side of proposed antenna

As illustrated in figure 5, this is one of the patches taken apart from of our proposed antenna show in figure 2. The entire dimension can be view as figure 5 while in figure 6 is the half of the upper side of the proposed antenna array design since half of the bottom design identical to upper design. All the equation to obtain the patches and antenna array are showed in Equation 7 through Equation 12

Parameter Calculation Optimization

Parameter Calculation Optimization


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The width of antenna is given by equation 7

(7)

The length of the antenna is given by equation 8

2∆ (8)

The extension for length antenna ΔL is given by equation 9

(9)

Effective dielectric constant (εreff ) is given by equation 10

(10)

The height of the 50 ohm matching patch (Lf) is given as equation 11

(11)

The 50 ohm feed line width and all array design are base on equation 12

(6)

(12)

Antenna structures have been fabricated using FR4 0.8mm substrate using manual DIY fabrication method.

Figure 7: Antenna prototype

III.RESULT AND DISCUSSION

Numerical simulations and measurements were conducted for 3 by 3 OCFA antennas and the analysis are focus on Return loss measurement, radiation pattern, and gain by using simulation CST microwave studio simulator while all measured return loss (S11) value were conducted by using Rohde & Schwarz ZVA 40 vector network analyzer. Besides that, the differences of antenna structure for BalUn and matching stub in term of return loss for 4

cases also was carried out in figure 8a to 8d and all this 4 cases were tabulated in TABLE IV.

TABLE IV: BalUn AND STUB EXPERIMENT CASES

Where Letter R is Remains while E is Eliminated

Figure 8a:S11 data for Case 1

Figure 8b:S11 data for Case 2

The S11 graph as show in figure 8a is a result for CASE1 whereby the BalUn was eliminated while maintaining the matching stub structure. From the simulation CST result for CASE 1, by elimination of the Circular matching BalUn and maintaining the matching stub there is no peak reported at the

Cases R E R E

STUB BalUn

Case 1

Case 2

Case 3

Case 4 (proposed)


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S11 graph at figure 8a. The nearest peak reported was at 2.55GHz at -14.7dB. The results indicate that without the installation of BalUn to our proposed antenna system, it is not only creating mismatch to the feeding point but it’s also result the proposed antenna system did not resonate at our desired 2.45GHz frequency. Since the purpose of matching stub is to do fine tuning, so it is not so much different in shifting the frequency for CASE 2. As revealed in figure 8b the S11 reading for Stub Elimination (CASE 2) is alias at 2.43GHz for value of -10.1 dB and -14dB at 2.56GHz, but it is still beyond our desired frequency. Again there is still no peak reported at our ISM band frequency 2.45GHz which lead to the adding of CASE 3 experiment work showed in figure 8c.

Figure 8c:S11 data for Case 3

As shown in figure 8c the S11 reading show that no peak reported at 2.45GHz. The nearest peak only occurs at frequency 2.28GHz for case 3. The Unbalanced condition create no peak response to almost entire ISM band frequency 2.4 GHz to 2.5GHz if both matching stub and BalUn is remove from the structure of proposed antenna.

Figure 8d: S11 data for Case 4

As shown in figure 8d for case 4 situation whereby both matching stub and BalUn were installed, it show that highest peak obtain at 2.45GHz with reading of return lost (RL) is -33.1dB and this is the best result compare to the other 3 cases stated at table 4.So by using result obtained for case 4, we selected case 4 structure to be fabricated as our proposed antenna and the comparison between measured (case 4) simulation and measured fabricated antenna (case4) is show in figure 8e and all the Resonant Frequency, VSWR, Gain and Return loss value obtained between simulation and fabricated for case 4 is showed in TABLE V.

Figure 8e: Simulation versus measured result

TABLE V: SIMULATION MEASURED RESULT

Figure 8f: Polar plot at 0 degree

Parameter Simulation Measured


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Figure 8g: 3D radiation pattern for OFCA antenna

Figure 8f and 8g is a radiation pattern reading for our proposed antenna by using CST simulation software. Figure 8f is a 2dimension reading while figure 8g is 3 dimension reading of our proposed antenna at zero degree.

IV. CONCLUSION

A new 3 by 3 OCFA (off centered feed array) antenna with operating frequencies 2.45 GHz was designed, fabricated and measured. It has been demonstrated with CST simulation software and experimental results that the antenna is useful for ISM (industrial science medical) centre frequency is at 2.45GHz.The antenna is feed with off centered feeding feed through technique and create the unbalanced to the antenna especially on impedance matching. A comprehensive solution to overcome this problem by installing circular BalUn (Balanced to Unbalanced) and Shunt short matching stub create solve a lot of problems regarding frequency shifting and frequency fine tuning. All the work regarding the installation of the BaLun and Shunt short stub was also demonstrated in this paper and the outcome by all the result indicate that the purpose of the BalUn is acting as the frequency shifter. So by altering the optimization on BalUn it is reported from the result that it can shift the operating frequency and balanced the impedance matching as well. While, Shunt short matching stub is acting as frequency tuner whereby as reported in the result, it is only narrow down the return loss value while maintain the operating frequency. So the 3 by 3 OCFA antennas not only create high gain 11.84dB at 2.45GHz but it also solves some of the unbalanced feed technique especially on array technique. For future development the work focusing on reduced size on array development can be extended with replacement the radiation elements with other geometry shape to see their performance.

REFERENCES

[1] P. S. Hall and Y. Hao, ed., Antennas and Propagation for Body-Centric Wireless Communications, Artech House, 2006.

[2] http://www.w8ji.com?windom_off_center_fed.html

[3] Berge Ayvasian “LTE TDD operator business case &Adoption forecast” white paper march 2011 [4]R.J.P. Douville and D.S. James, Experimental Characterization of Microstrip Bends and Their Frequency Dependent Behavior, 1973 IEEE Conference Digest, October 1973, pp. 24-25. [5]R.J.P. Douville and D.S. James, Experimental Study of Symmetric Microstrip Bends and Their Compensation, IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-26, March 1978, pp. 175-181

[6]. C Wu, k. L. Wu, Z Bi, J. Litva, “Modelling of coaxial-fed microstrip patch antenna by finite difference time domain method”, Electronics Letters 12th September 1991, Vol. 27, issue 19, pp. 1691-1692 [7]. “Different IEEE Wireless Standards” http://compnetworking.about.com/cs/wireless80211/a/aa80211standard.htm [8]. “Introduction to WiMAX standard” http://en.wikipedia.org/wiki/WiMAX\ [9]. Asrokin A., M. K. A. Rahim, M. Z. A. Abd. Aziz,” Dual Band Microstrip Antenna for Wireless LAN Application”, Asia-Pacific Conference on Applied Electromagnetics, Johor, Malaysia, pp: 26-29, 2005.


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