BACK RADIATION SUPPRESSION IN MODIFIED APERTURE COUPLED MICROSTRIP ANTENNA BY USING PATCHES UNDER THE SUBSTRATE

415 | P a g e M a y 2 5 , 2 0 1 3 BACK RADIATION SUPPRESSION IN MODIFIED APERTURE COUPLED MICROSTRIP ANTENNA BY USING PATCHES UNDER THE SUBSTRATE Richa Sharma, Ms. Amanpreet Kaur, Dr. Rajesh Khanna M.E., Student, ECED, Thapar University, India richa102030@gmail.com Assistant Professor ECED, Thapar University, India Amanpreet.kaur@thapar.edu Professor, Thapar University,India rkhanna@thapar.edu ABSTRACT


INTRODUCTION
Microwave slot antennas have broad interest in military and commercial applications mainly due to the low profile, low cost as well as being simple to manufacture. Moreover, the slot antennas are quite easily incorporated with planar and non planar surfaces and have more degrees of freedom than a conventional design [1]- [3]. In spite of these advantages, it has the main disadvantage of back radiation, which limits its use in mobile communication. This back lobe is undesired because it shows power loss. It increases SAR (Specific Absorption Rate) for the mobile users [4].
Several methods have been proposed to suppress the back lobe, such as by using array topology [5], a tapered-loaded antenna [6] and a 2 wavelength Microwave Leaky Wave Antenna with coaxial probe coupled patch antenna arrays [7]. In addition, these methods also supported parallel plate modes, thus producing undesired radiation.
To solve such problems, a slot antenna with new aperture coupling design is proposed in this paper. The proposed design includes the slot etched on the substrate and the patches and the feed line are employed on the opposite side of the substrate. The purpose of employing the patches beneath the substrate layer is to reduce the radiation into the half space that they occupy and to increase the radiation in the other half space. Also, this design is compact, since it includes a single substrate layer as compared to conventional aperture coupled antennas.
The design is simulated using CST (Computer Simulation Technology) and the return loss, radiation pattern, gain and directivity are measured. In this paper, we will first study a simple slot antenna without patches under the substrate. This simple slot antenna shows the bidirectional nature of the antenna in the absence of the patch. Next, we will study the design, operation mechanism and simulation results of our proposed antenna.

Antenna Structure Without Patch
The figure below shows the Fig. 1 (top view) and Fig. 2 (bottom view) view of the cell structure of a simple aperture coupled microstrip slot antenna. The design is a simple slot antenna with 125Χ130 mm 2 substrate layer with of Rogers RO4232 dielectric material with dielectric constant 3.2 and a slot is cut into the ground of this design which rests on the substrate layer. The microstrip feed line is on the opposite side of the slot and its width is set for 50Ω characteristic impedance. The slot in the above design is incorporated to radiate bidirectionally. The S11 parameter and the E field pattern are shown in Fig. 3  The S11 parameter is -25.469dB and the antenna resonates at 5.57GHz.The E field pattern shows the bidirectional radiation distribution which is due to the alternating standing wave distribution along the slot axis which reverses over a half wavelength [8]. Since the field is distributed towards both sides, the power density of the antenna hence becomes low.

Antenna Structure With Patch
This is the proposed slot antenna in which there are two slots above the substrate and the patches are under the substrate. The idea of placing patches under the substrate is that the slot electric field perpendicular to the slot length appears to have a standing wave distribution with positive and negative nodes along its axis. The direction of this electric field is reversed after propagating over a half wavelength. Hence, by sliding the patch toward or away from the center of the slot, we can adjust the phase of the patch 180º by coupling it to the negative voltage node or the positive voltage node on the slot line [9]. The proposed antenna is simulated for different displacements of the patch position and their corresponding field patterns are also studied for better front to back ratio. The geometry of the proposed structure is as follows. M a y 2 5 , 2 0 1 3

Design And Simulation Results
The configuration of the antenna is shown in the figure below. In this paper the antenna is expected to operate in 4.0 Ghz to 5.0 Ghz band. The slots are cut on the ground plane of 125Χ130 mm 2 . The effective dielectric constant of the substrate is calculated by using equation (1). After calculating effective dielectric constant, the guided wavelength (λg) is calculated by using equation (2) and the length and width of slot is given by L1 and W1 in the Table. The thickness of substrate layer is h mm. The distance dy = 29mm. The length of the slot is calculated by using equation (3). The remaining dimensions of the antenna are calculated using [10] and are given in the Table 1.
Where c = 3Χ10 8 m/sec Length of slot (L1) 54mm Width of slot (W1) 2mm Length of patch (L) 17.5mm Width of patch (W) 17.5mm According to the operating mechanism described above, the spacing "dx" between the two patches is very significant in affecting the antenna performance. Therefore, the antenna structure is simulated for different values of dx and at those values of dx the return loss, gain, E field pattern and directivity are compared.

Effect of Spacing (dx)
Since the effective length of the slot with the partly covered patches will be different from that without patches, the distance of the patches from the center of the slot is chosen to be 1/8 to 1/2 wavelength in the substrate [11]. This range allows the patches to cover the positive or the negative standing wave nodes when they move toward or away from the center of the slot. The antenna is simulated for four values of the dx spacing: 9, 13, 17 and 21. The S11 parameter for the different spacing between the patch has been simulated. The resonant frequency for dx =9mm is 4.93GHz, for dx = 13mm is 4.53GHz , for dx = 17mm is 4.39GHz and for dx = 21mm is 4.87GHz The resonant frequency is shifted to the left as the distance between the patches increases except for dx=19mm where the frequency starts shifting to the right. Thus, the resonant frequency decreases with increase in the distance between the patches up to a particular displacement of the patch. Beyond that displacement, it again increases. The E field patterns for different patch spacings are as follows. The electric field pattern for the different displacements of the patch shows a better front radiation at dx = 13mm as shown in (b) in fig. 9. The back radiation is suppressed due to the reason that the effective length of the patch is about half wavelength, it causes voltage null movement in the slot as it moves along the axis of the slot as compared to (a), (c) and (d) in fig. 9 where there are significant side lobes and the back lobe.  Table 2 shows the comparison of various result parameters for the different patch displacements.The gain is 6.774 dB and the directivity is 6.255 dBi for dx = 12mm whereas it is less for the other three cases. Also, the coupling is also better at dx = 13mm as compared to dx = 17 and 21mm. Thus, dx =12 mm is the point of voltage null in the slot where the back lobe suppresses to a significant point, hence giving good simulation results.

CONCLUSION
The back radiation is successfully suppressed in a microstrip slot antenna with modified aperture coupling by varying the patch distance within 1/8 to 1/2 of the wavelength from the centre of the slot. The design of this work has given good results in term of return loss, gain and directivity. This design is very useful for the mobile communication where the back radiation from the cell phone is not desired. This design is also compact in size. However, the proposed design has the simple structure and it can be constructed with a lower cost.