Compact and Hexaband Rectangular Microstrip Patch Antenna for Wireless Applications

A Compact Corner Split Ring with Split C Slot Rectangular Microstrip Patch Antenna fed by a 50 Ω microstripline is discussed. A Corner split ring with C shaped slot has been etched in rectangular microstrip antenna. The slot increases the length of the surface current for the dominant mode TM10 leading to the decrease in resonance frequency. Size reduction and hexaband is obtained for the proposed antenna. The proposed antenna provides both size reduction and hexaband and is best suited for wireless communication. The proposed work is simulated using 3DEM of Mentorgraphics and validated. The results show that Hexaband with compactness is achieved.


Introduction
Since the introduction of wireless networking and portable devices, the need for light weight antennas is growing. Because of the low profile and planar configuration, the Rectangular Microstrip Patch Antenna (RMSA) is used widely. Nevertheless, the size of the low frequency RMSA becomes large and the bandwidth is small. Many techniques for obtaining both the compact and broadband antennas were published in the literature, such as high relative permitivitty substrates (ε r ), shorting pins at appropriate position inside the radiating patch and etched slits etc., [1][2][3][4][5]. Advancements in the wireless communication field involve innovative RMSAs which provides dual or triple band to avoid using two or more antennas and that can also allow size reduction. Mobile telephony systems for example, allow portable devices 1 3 compliant with GSM900/DCS1800/ UMTS2000 technologies and the same devices will also link users to WLAN networks based on 802.11 standards (2.4/3.6/4.9/5/5.9GHz) specifications. Hence, it is of great interest to develop patch antenna with multiple resonances [6,7]. Many researchers have made huge efforts to design slot/slit based antennas on the radiating patch and ground plane, which can be used for multiband operations [8][9][10][11][12][13][14][15][16][17][18].Reconfigurable antennas are still attracting many researchers for designing multiband antennas [19].Metamaterials are the promising new candidates for designing multiband antennas [20][21][22][23][24][25][26]. Fractals have been investigated for multiband antenna operations [27]. Stacking of antennas for multiband operation has also been investigated [28,29].
Effort has been made in proposed work to obtain compactness and Hexaband. The Compact Corner Split Ring and Split C Slot Rectangular Microstrip patch Antenna (CCSR-SCS) is simulated using Mentor graphics 3DEM simulation software and validated using Vector Network Analyzer (VNA).

Antenna Design
The design of RMSA is carried out based on transmission line model proposed by Munson. First and foremost substrate material is selected. For the selected substrate, the main electrical properties taken into considerations are relative permittivity (ε r ) and loss tangent (tan δ). Substrate with higher relative permittivity results in smaller patch, reduces impedance bandwidth and it results in tighter tolerances in fabrication. A high loss tangent decreases antenna performance in terms of antenna efficiency and increases feed loss. The substrate thickness (h) is so chosen to be as high as possible in order to maximize impedance bandwidth and efficiency but not as high as the chance of excitation of surface waves. Usage of substrate material having higher relative permittivity decreases radiation losses due to electromagnetic field between the radiating patch and the ground. The value of h is selected as per the equation, where c is the speed of light in cm and f 0 is maximum operating frequency in GHz. Glass epoxy is one of the low cost and easily available substrate, it is commonly used in printed circuit board (PCB). The antennas are commonly fabricated using glass epoxy substrate, for testing the novel designs. For known values of ε r , h, resonant frequency f r and λ 0 , the design of RMSA is as follows.

Design of Elemental Width (W)
The elemental width of RMSA is

Design of Extension Length (∆l)
The extension length ∆l is usually deducted from the calculated length L of RMSA in order to retain actual length of the RMSA. The extension length virtually appears due to fringing fields as where ε e is the effective dielectric constant. It is calculated by using

Design of Elemental Length (L)
Once the extension length (∆l) and effective dielectric constant (ε e ) are determined using Eqs. (3 and 4) then the elemental length of RMSA is found by using the equation.

Design of Microstripline Feed
The design of 50 Ω microstripline feed is carried out by calculating W/h ratio for the known values of characteristic impedance Z 0 and ε r . The design equations are By using Equations. (6) to (9) the width of microstripline feed W f may be determined by multiplying the value of h to the value obtained from Eq. (6) or (7). The feed length L f for single element RMSA in order to keep minimum loss in microstripline feed is given by where and The 50 Ω microstrip line feed of length L f and width W f (designed as per the above procedure) will be connected at the center point (C p ) along the width W of the RMSA as shown in Fig. 1. But the impedance offered by the patch at C p may not be equal to 50 Ω. Hence, microstripline feed need not be connected at this point as impedance mismatch occurs. In such case a matching transformer must be used between C p and 50 Ω microstripline for better impedance matching.

Design of Quarter Wave Transformer (λ g /4)
A very important circuit element in most microstrip designs is the quarter wave transformer. As its length is one fourth of the guide wavelength (λ g ) so the name quarter wave transformer. The lg/4 transformer matches the impedance of two dissimilar sections. The impedance of λ g /4 transformer Not only quarterwave transformer is used to match between two seperate impedances, but also should be used wherever there is a possible impedance mismatch. Like for the single patch antenna, Z t should be used between 50 Ω microstripline feed and the input feed location C p of the patch. For instant, the impedance at C p along its width is 256.99 Ω (obtained in the present study). This impedance is not same as that of the microstripline impedance of 50 Ω. Therefore a quarter wave transformer may be used between the microstripline and center point C p along the width of rectangular patch element for matching their impedances.
The following equations are used to determine the impedance R in at C p along the width of conventional RMA where Impedance of λ g /4 transformer is given by Z t = √ R in × Z 0 . The width of λ g /4 transformer Wt is obtained by using Eqs. (6)(7)(8)(9). The length of λ g /4 transformer L t is obtained by using Eqs. (10-16) by replacing Z 0 by Z t . The designed geometry of center fed single element RMSA is as shown in Fig. 1.

Design of Ground Plane
The size of the ground plane is selected as per the following equations. Figure 1 shows the geometry of RMSA having length L = 11.33 mm and the width W = 15.22 mm for 6 GHz operating frequency. The antenna uses a low cost glass epoxy substrate having relative permittivity of ε r = 4.4 and tan δ = 0.0245 and thickness h = 1.6 mm.The antenna is fed using microstripline feed through λ g /4 transformer feed line of length L t = 6.3 5 mm and width W t = 0.47 mm. Microstripline is having feed length L s = 6.29 mm and width W s = 3.16 mm. Figure 2 shows geometry of the proposed CCSR-SCS microstrip antenna, which is obtained by etching rectangular slot of dimension L1 = 8 mm × W1=9 mm and then placing a small patch L2 = 7 mm × W2 = 8 mm at the centre of the RMA and the ring so obtained is split by placing 0.5 mm copper material at the corner, further a split C slot is embedded in the existing design by leaving 0.25 mm from perimeter,along the length amd 0.5 mm along the width of the patch and etching 0.5 mm copper from the patch.

Results and Discussions
Mentor graphics 3DEM software is used to simulate the proposed CCSR-SCS antenna and later the simulated results are validated. Table 1 shows the comparision of simulated and validated results of RMSA antenna and proposed CCSR-SCS antenna.
It is evident from the Table 1 that the resonant frequency of CCSR-SCS has decreased significantly thereby giving a size reduction of 60% and Hexaband resonance.It is further seen from the table that simulated and measured results are in good aggrement. The same can be confirmed from Fig. 3. Figure 4a shows the simulated returnloss characteristics of ANT1 (RMSA) resonating at 6 GHz. Also Fig. 4a shows that Ant2 resonates at 8.  Figs. 12, 13, 14, 15, 16 and 17 respectively. Current is much stronger around the edges of the CCSR-SCS antenna along with uniform distribution of current over entire patch indicating that patch size plays important role at lower frequencies and current distribution for upper bands is mainly from the split ring as strong current distribution moves towards the center of the patch. Figure 18 shows the variation of gain, efficiency and directivity of CCSR-SCS antenna with respect to frequency. It is noted from Fig. 18

Conclusions
A CCSR-SCS antenna is studied. Simulation and Validation results are in good agreement. It has been observed that CCSR-SCS gives a size reduction of 60% along with Hexabands, the compactness is due to larger path length of surface current. The radiation patterns of all proposed antennas are of broadside in nature.     Funding No funding source.

Data and Material Availability Not applicable.
Code Availability Not applicable.

Conflict of interest
The authors declare no conflict of interest.