A Single Plane Low profile Patch Antenna π-Shaped Slots and Low Side Lobe Level for Directive Wireless Communications

The patch antenna was used recently for ISM band (5.725 – 5.875) GHz, the patch antenna suffers of being an antenna with low gain as well as narrow operating bandwidth with high side lobe level (SSL) radiation pattern. A single plane with low profile and easy fabrication microstrip patch antenna has been proposed in this work was presented for directive communications and operates in Industrial, Scientific, and Medical (ISM) band 5.8 GHz with low SLL radiation pattern and gain of 5.20 dB for pint to point (directive). The gain of antenna has been increased while SLL decreased due to shifting the radiating patch and feeder of antenna with specific distance toward –ve side of the X-axis without any need for using techniques which would increase overall dimensions of antenna. The antenna has been printed on Arlon AD 250 substrate. The simulated results of antenna’s gain, operating frequency, bandwidth, return loss, and radiation pattern are presented. CST Microwave studio software was used in this design simulation.


Introduction
The microstrip patch antenna has certain advantages such as being low cost antenna with easy fabrication requirements as well as being low weight antenna which has ability of operating with multiple operating frequencies [1]- [3].On the other hand this type of antenna shows many disadvantages such us being low gain antenna which suffering from high side lobe level radiation pattern with narrow operating bandwidth as well as suffering from surface currents which reduce antenna's efficiency [1][2][3].Many research activities have been conducted during last years to improve the operating bandwidth of microstrip patch antenna as well as achieving wideband operation [2][3][4][5][6][7][8][9][10][11] .On the other hand many researches had shown focus on enhancing gain of microstrip patch antenna and many techniques had been developed for this issue.Using grooved ground caused gain enhancement more than 10 dB for a patch antenna operating at 10.45 GHz in compare with a patch antenna with conventional ground plane [12].Using array of patches is an ordinary technique to improve the gain of antenna, while using an array of 256 patches in arbitrary shapes caused to improve the gain of antenna as well as reducing SLL of radiation pattern [13].In [14] using defected patch antenna led to achieve gain of 4.12 dB at 2.69 GHz ,while covering 4×1 patch array with three layers of high refractive index metamaterial with split ring resonator caused gain enhancement of 3.4 dB [15].Moreover a triple operating frequencies using metamaterial loaded to patch antenna and caused gain of 3.1 dB at 5.8 GHz [16].Using reflector layer with slot etched from ground of patch antenna caused to achieve gain of 7.35 dB [17].In [18] using defected ground structure caused improvement around 1 dB in antenna's gain while adding a reflector layer under antenna's ground with a specific distance caused an enhancement of 2 dB in gain.Using multilayer antenna structure is very helpful technique, where the gain of conventional circular patch antenna was increased up to 10 dB at 5.8 GHz due to adding an FR4 superstrate with frequency selective surfaces above the patch [19].Locating 155×155 mm 2 with 5 × 5 partially reflected surfaces on its bottom led to an enhancement in gain around 5.5 dB at 5.8 GHz [20].Also gain of an elliptical patch antenna had been increased from 8.5 dB to 14.1 dB at 5.8 GHz after adding 100×100 mm 2 superstrate with 7×7 partially reflected surfaces located on upper side of superstrate [21].Using three superstrate layers of Roger RO3006 without any metallic objects also can increase gain of patch antenna and reduce SLL after studying height of each superstrate layer from antenna's ground which caused an increment in gain from 8.23 dB to 11.7 dB at 5.8 GHz [8], while multilayer antenna in [22] increased gain up to 12.7 dB due to adding three layers of Rogers RO6006 superstrate as well as adding a reflector under antenna's ground with optimizing height of each of four layers.Another effective technique to enhance the gain of patch antenna is etching slots from surface of radiating patch [10].Moreover, the effects of thickness of copper of the patch on antenna's gain have been investigated to achieve better performance in general with highest gain at a certain frequency [11].In this article a single plane low profile patch antenna with πshaped slots operates at ISM Band at 5.8 GHz has been presented.The gain of patch antenna has been increased and SLL of radiation pattern have been eliminated based on investigating position of both microstrip line feeder and patch without need for using ordinary techniques; which described earlier in this section, to increase antenna's gain and reduce SLL.The proposed antenna achieved gain of 5.20 dB which is higher than 4.30 dB and 4.12 dB achieved by antenna array proposed in [7], and defected patch structure proposed in [11], respectively.The proposed antenna has a radiation pattern with eliminated SLL and reduced size compared to antennas in [7][8], [16], and [19].

Antenna Design
The design of the proposed antenna is a single plane low profile patch antenna which is composed of semi-rectangular patch printed on Arlon AD 250 substrate with dimensions of (L×W×h) for length, width, and height, respectively and dielectric constant ( ) 2.5.The dimensions of the rectangular patch are (P L ×P w ).Two circles with different radius for each were located on the upper right and left edges of the rectangular patch with radius of (r R ) and (r L ) for the right and left hand circles, respectively.Both circles have an etched shape of three quarters of a circle with radius of 0.4 mm.Three rectangular slots were etched from rectangular patch to form the πshaped .These slots are composed of two vertical slots with length and width of (L VS ) and (W VS ), respectively.The third slot is a horizontal slot with length and width of (L HS ) and (W HS ), respectively.The patch antenna has been fed using microstrip line feeder with length of (F L ) and width of (F W ). the thickness of annealed copper of the patch is (t).The feeding microstrip line as well as the radiating patch have been shifted with distance (a) from the center of the substrate toward (-ve) X-axis to achieve minimum SLL in radiation patter as well as increasing gain of antenna.Top view for the single plane low profile antenna with π-shaped slots is shown in Fig. 1 and list of antenna parameters' values are presented in Table 1.

2-
To improve performance and achieve an operating frequency within the ISM band 5.8 GHz; two circles have been added on the upper right and left hand edges of rectangular patch with radius of r R and r L , respectively, both circles have an etched shape of three quarters of a circle with radius of 0.4 mm.Adding these two circles caused shifting in operating frequency to 5.876 GHz.Moreover the reflection coefficient improved to be -30.55dB while operating bandwidth decreased to 120.81 MHz with gain of 3.17

4-
Etching second rectangular vertical slot; which is in parallel to the first slot with distance of 2.3 mm separating between them, improved the performance and caused resonance at 5.796 GHz with S 11 of -26.50 dB and gain had been improved to 3.70 dB while the operating bandwidth decreased to 123.40 MHz.5-Etching horizontal slot to form π-shaped slot; which is isolated from two vertical slots, led to enhance performance of antenna in terms of operating frequency which found to be 5.8 GHz, reflection coefficient which found to be -25.44 dB, operating bandwidth with increment up to 141.46 MHz, and gain enhancement to 4.09 dB.

6-
As shown in previous steps an optimized performance had been achieved but SLL in antenna's radiation pattern still high> In this step SLL will be eliminated due to shifting microstrip line feeder and radiating patch with a distance (a) toward -ve X-axis of the substrate as shown below: 6 -A: Shifting microstrip feeder and radiating patch with distance of (a = 3mm) will cause to have a resonance at 5.796 GHz, S 11 = -24.08dB, operating bandwidth of 127.92 MHz, and gain increased up to 4.49 dB due increment in SLL of antenna's radiation pattern.

-B:
Shifting microstrip feeder and radiating patch with distance of (a = 6mm) will cause to have a resonance at 5.796 GHz, S 11 = -22.92dB, operating bandwidth of 123.43 MHz, and gain increased up to 4.64 dB with SLL reduction.6 -C: Shifting microstrip feeder and radiating patch with distance of (a = 9mm) will cause to have a resonance at 5.788 GHz, S 11 = -25.50dB, operating bandwidth of 150.73 MHz, and gain increased up to 5.20 dB with SLL reduction.Tabulated data for steps of design of the proposed antenna presented in Table 2. Fig. 2 shows reflection coefficient for designing a single plane patch antenna with low SLL.The radiation patterns of the proposed antenna; before and after shifting microstrip feeder and radiating patch with distance of 9 mm, are presented in Fig. 3, where the red curves refer to the far field region and blue lines refer to the direction of the main lobe of radiation pattern while green circles refers to the lowest back and SLL.

DISCUSSION
The proposed single plane low profile patch antenna with π -shaped slots provided improvement in antenna's performance due to shifting microstrip line feeder as well as radiating patch toward the (ve) X-axis.The proposed antenna achieved comparable gain compared to conventional rectangular and circular patch antenna's proposed by [14], [19].The proposed antenna had achieved gain higher than 2×1 patch array proposed in [7]  Shifting the microstrip feeder and patch with a specific distance toward negative X-axis from center of substrate caused to eliminate the SLL of antenna's radiation pattern without need for using any technique which will lead to increase the overall dimensions of antenna.This led to achieve size reduction compared to antennas presented in previous works with reduction in SLL as shown in Table 3. 5.80 9.85 99% [22] 5.80 12.60 99%

CONCLUSION
The performance of microstrip patch antenna can be improved due to studying effects of position of microstrip feeder and radiating patch from the center of substrate on x-axis.The achieved enhancement in performance was in terms of increasing gain of antenna as well as covering the ISM band at 5.8 GHz with eliminating SLL in antenna's radiation pattern without using techniques which would increase the antenna's dimensions and led to present a single plane with low profile antenna compared to low SLL antennas in previous works.

dB. 3 -
To achieve an operating frequency closed to ISM band (5.725-5.875)GHz than achieved in Step (2), a vertical rectangular slot have been etched from surface of patch which caused improvement in operating bandwidth up to 168.55 MHz and resonance frequency of 5.86 GHz with S 11 of -22.42 dB and gain of 3.22 dB.

Figure 2 .Figure 3 .
Figure 2. Reflection Coefficient for Steps (1 -6-C) . The proposed antenna shows 0.6 dB of gain variation over ISM band (5.725 -5.875) GHz as shown in Fig. 4. The proposed antenna radiation pattern has a Half Power Beam Width (HPBW) of 71 degree and front-to-back (F/B) ratio of 22.58 dB and proposed for directive point-to-point communications applications.

Table 1 .
List of Feeding Patch Parameters' Values.

Table 2 .
Results of Steps of Designing Proposed Antenna.

Table 3 .
Size Reduction Achieved by Proposed Antenna.