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A functional antenna tuner for slot patch antenna.

1. INTRODUCTION

Microstrip antenna has advantages of low profile, low cost and easy integration to the microstrip circuits [1, 2] but disadvantages of low bandwidth [3]. Since microstrip antenna with wide bandwidth is very important for Wireless-LNA, Bluetooth and Video-interface [4], techniques of modifying bandwidth have been developed a lot. Structurally, it includes integrating several microstrip resonance structures into one antenna [5, 6], adding more structural layers to antenna [7], changing dielectric constant of substrate [8,9], modifying probe feed structure [10-14] and printing functional slot to the radiator as well as the ground [15-18], etc. Lately, by using of stepped [19,20] and slotted ground [21-23] for size reduction, parasitic elements for shorting the patch [24-26] to modify the bandwidth of microstrip antenna has been reported. In this study, the functional tuner which is used to improve the power transfer by matching the varied impedances, was designed and analyzed to regulate the bandwidth and gain of the antenna for the applications to WiMAX Ap system in the band of 2.5 GHz (2.5-2.69 GHz) [27,28].

2. DESIGN CONFIGURATION

The schematic of the slot patch antenna studied is shown in Figure 1(a). The constructed prototype was implemented on a FR4 substrate ([[epsilon].sub.r] = 4.4) with the overall size of 69.5 x 51 x 1.6 [mm.sup.3]. A slot line (33.5 x 45 [mm.sup.2]) and a microstrip (26 x 37 [mm.sup.2]) being coupled through the gap (3.5 mm) were printed on the ground. A T-shape probe-fed circuit includes two microstrip components with the sizes of 28.5 x 3 [mm.sup.2] and 3 x 24[mm.sup.2]. The operation frequency was designed at 2.6 GHz with the impedance bandwidth (-10 dB return loss) for the studied antenna The tuner was pin-coupled to the rectangular microstrip patch (28.5 x 42 [mm.sup.2]) by using of a M17/113-RG316 cable line [29]. The length 1.5 cm of the cable line was used. The constructed prototype of the antenna with tuner is show in Figure 1(b).

Rectangular microstrip 24 x 3 [mm.sup.2] is connected to the feed line (signal transmission line) of microstrip 28.5 x 3 [mm.sup.2] and gap (0.5 mm) coupled to the rectangular plane 28.5 x 42 [mm.sup.2], the probe-fed input matching impedance is 50 [ohm] to the slot patch antenna. The optimized return loss was obtained by adjusting of the length of transmission line and the width of rectangular microstrip. Figures 2(a) and 2(b) shows the measured return loss. As in figures, when the length of the rectangular microstrip was increased from 24 mm to 42 mm, the return loss at 2.6 GHz was changed from -7dB to -16dB. In contrast, when the width of the rectangular microstrip was reduced from 3 mm to 1 mm, the return loss was changed correspondingly as well. Therefore, the size of the rectangular microstrip for the slot patch antenna is optimized to be 24 x 3 [mm.sup.2].

In Figures 3(a) and 3(b), simulation results (by Ansoft HFSS10) show the surface currents on the patch (rectangular plan). The pro-feed T-shape microstrip transmitted current distribution could be adjusted by the angle [theta] at the connection point which is pin coupled to the tuner for matching impedance.

Figure 4 shows modification of the return loss with the values of angle [theta]. As in Figure 4, the angle [theta] is determined to be 13[degrees] based upon the best value of the measured return loss.

In Figure 5, schematic drawing of the connection point design for a square pin (location of pin-coupling) with cross section of 1 x 1 [mm.sup.2] is demonstrated. For the coordinate origin O was defined to be (0, 0) shown in the Figure 5, the output peak voltage at different locations with different coordinates of y = 1 mm, 3 mm and 5 mm to the origin O responded to the input power of 30 dBm (calculated by Ansoft HFSS10) was 4.56 V, 3.16 V and 2.34 V respectively. On the other hand, if the coordinates of x = 0 mm, 2 mm and 4 mm, the output peak voltage to the tuner is 3.16 V, 3.17 V and 2.86 V respectively. In this study, the coordinate of the coupling point is located at x = 0 mm and y = 3 mm to the coordinate origin point O, the output peak voltage to the tuner was thus 3.16 Volt which is shown in Figures 6(a) and 6(b).

In Figure 7(a), four different matching impedance designs of the tuner are presented. In the circuit of matching impedance at 25 [ohm] and 30 [ohm], [C.sub.1] was designed to 1.5 pF that includes two 1 pF series connected capacitors and a parallel connected 1 pF capacitor. In the design of matching impedance at 75 [ohm], [C.sub.1] is consists of six pieces of 1 pF capacitors series and parallel connected to catch 0.75 pF. However, in the circuit of matching impedance at 75 [ohm], [L.sub.1] includes two pieces of inductance of 1.6 nH being parallel connected to catch 0.8 nH. At matching impedance of 25 [ohm], the [L.sub.2] circuit includes 2 pieces conductance of 1.6 nH parallel connected to catch 0.8 nH. As shown in Figure 9(b), the functional tuner was constructed by using of the circuits printed on FR4 plate with the thickness of 0.4 mm being welded with inductance of SMD 0603 as well as capacitors and resistors. The constructed prototype of the tuner is demonstrated in Figure 7(c).

Basically, the tuner consists of four passive components [L.sub.1], [C.sub.1], [L.sub.2] and [R.sub.1] operated at 2.6 GHz with matching impedance 25 [ohm], 30 [ohm], 50 [ohm] and 75 [ohm] to regulate the power transfer from the pro-fed circuit. Components of the tuner circuits used for impedance matching are listed in Table 1.

The Smith charts of matching impedance for the tuner are shown in Figure 8. It demonstrates three steps to match the impedance. First, determine the characteristic impedance [Z.sub.0] when the source current goes through cable to [L.sub.1]. Then, [Z.sub.0] is modified to [Z.sub.1] along with the path marked as '1' with colour red in Figure 8. Secondly, when the current goes through L1 to the microstrip [C.sub.1], [Z.sub.1] is modified to [Z.sub.2] along with the path marked as '2' with colour blue. Finally, when the current goes through [C.sub.1] to the microstrip [L.sub.2], [Z.sub.2] is modified to [Z.sub.3] along with the path marked as '3' with colour black. Through the pin-coupling, the microstrip tuner can access the power from the probe-fed circuit to the slot patch antenna and then be activated to operate at 2.6 GHz with matching impedance 25 [ohm], 30 [ohm], 50 [ohm] and 75 [ohm] correspondingly.

Details of the impedance characteristics of each component on the tuner circuit are calculated by using of software ADS2008 and are given in Table 2.

3. MEASUREMENTS AND SIMULATIONS

The measured return loss and voltage standing wave ratio (VSWR) of the antenna studied are shown in Figures 9(a) and 9(b). The bandwidth BW of the slot patch antenna was demonstrated 3% without coupling of the tuner. The BW of the antenna was improved to 11% when the tuner was pin-coupled to provide 75 [ohm] impedance matching to the slot patch microstrip antenna. Comparatively, when the tuner was pin-coupled to the slot patch with matching impedance 50 [ohm], 30 [ohm] and 25 [ohm], the BW of the antenna would be increased to 14%, 18% and 21% correspondingly. The measured BW of constructed prototype is listed in Table 3.

The equivalent circuit of slot patch microstrip antenna was calculated by using of Smith Chart Tool in the software ADS. In this study, two parts are comprised to analyze the equivalent circuit. Part 1 is the equivalent circuit of slot patch microstrip antenna without the tuner. Part 2 is the equivalent circuit load [Z.sub.L] for the antenna which is the impedance of the tuner. In Figure 10, the equivalent circuits of the slot patch microstrip antenna with the tuner are demonstrated in part 3. If the matching impedance Z = 75 [ohm], the equivalent circuit of the slot patch microstrip antenna with the tuner is proposed with the parallel connecting of two inductors. In comparison, when Z = 30 [ohm] and Z = 50 [ohm], the equivalent circuit consists of a series connected inductor and a parallel connected capacitor.

The radiation pattern defines the variation of the power radiated by an antenna. It is described as a function of the direction from the antenna. In Figure 11, the measured field patterns of the proposed antenna are demonstrated. The tuner circuit with different values of matching impedance to the slot patch antenna does not change the radiation patterns specifically. The structure of the antenna is the major factor to dominate the radiation patterns. The studied slot patch antenna was originally designed with the maximum radiation patterns along the +Z axis [30-32], however, the rectangular slot line reversely creates the maximum radiation pattern occurring along the -Z axis [33,34]. Generally, it is defined the radiation E plane is the YZ plan. H plane is the XZ plan. Therefore, co-polarization planar direction is the same with the E direction and the cross-polarization one is perpendicular to the E direction. In Figure 11, it is observed that the [E.sub.[PHI]] pattern (cross-polarization) being reduced with the matching impedance. Additionally, the small value of the difference between [E.sub.[theta]] cos [PHI] and [E.sub.[PHI]] sin [PHI] agrees with the observations of having small radiated pattern cross-polarization ([E.sub.[PHI]]) [35].

The circuit load of the tuner influences the bandwidth and the antenna gain significantly. In Figure 12, the antenna gains can be observed to be affected by the functional tuner at different matching impedance loads. In this study, it is realized that if the impedance is lower, the BW of the antenna will be broader; however, the antenna gain will be reduced.

4. CONCLUSION

The bandwidth and antenna gain of a slot patch antenna can be modified by adding a circuit load. If the tuner functions at the best matching impedance, it can use the power provided from the antenna through cable line to resonate at the frequency being operated. The measurements confirmed that the studied antenna with the tuner has good performance.

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Chia-Ching Chu (1), Lih-Shan Chen (1), Hsien-Chiao Teng (2), and Shen Cherng (3), *

(1) Department of Electronic Engineering, I-Shou University, No. 1, Sec. 1, Xuecheng Rd., Dashu Dist., Kaohsiung City 840, Taiwan

(2) Department of Electrical Engineering, ROC Military Academy, No. 1, Weiwu Rd., Fengshan Dist., Kaohsiung City 830, Taiwan

(3) Computer Science and Information Engineering, Chengshiu University, No. 840, Chengqing Rd., Niaosong Dist., Kaohsiung City 833, Taiwan

* Corresponding author: Shen Cherng (cherng@msu.edu).

Received 3 June 2013, Accepted 6 July 2013, Scheduled 9 July 2013

Table 1. Reactance of the components of the
tuner at matching impedance in Figure 8.

Impedances   Re.     Im.    Re.     Im.

[Z.sub.0]     51      0      51      0
[Z.sub.1]    51.0   13.1    51.0   26.1
[Z.sub.2]    25.2   -27.1   30.2   -32.1
[Z.sub.3]    25.2    2.3    30.2    3.8

Impedances   Re.     Im.    Re.     Im.

[Z.sub.0]     51      0      51      0
[Z.sub.1]    51.0   29.4    51.0   37.6
[Z.sub.2]    52.9   -28.2   74.8   -17.0
[Z.sub.3]    52.9   -2.1    74.8   -3.9

Table 2. Components of the circuit for the tuner used for matching
impedance.

Impedance         [L.sub.1]   [C.sub.1]   [L.sub.2]   [R.sub.1]
matched [[ohm]]     [nH]        [pF]        [nH]       [[ohm]]

25                   1.8         1.5         0.8         51
30                   2.2         1.5         1.6         51
50                   1.6          1          1.8         51
75                   0.8        0.75         2.3         51

Table 3. The measured BW of the
slot patch microstrip antenna
with the tuner.

Matching     Return     BW [%]
Impedance   Loss [dB]
[[OMEGA]]

0              16         3
25             17         21
30             19         18
50            30.36       14
75            30.42       11
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Article Details
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Author:Chu, Chia-Ching; Chen, Lih-Shan; Teng, Hsien-Chiao; Cherng, Shen
Publication:Progress In Electromagnetics Research C
Article Type:Abstract
Geographic Code:9CHIN
Date:Aug 1, 2013
Words:3175
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