Analysis and implementation of shorting pins in microstrip patch antenna for SAR reduction

Specific absorption rate (SAR), is the safety measure to quantify human exposure to electromagnetic radiation from mobile phones and other wireless devices, which must be kept within the permissible levels to avoid various health concerns. This paper provides an experimental study on the effectiveness of SAR reduction, using shorting pins, and its corresponding effects on antenna performance. A compact, single-element, microstrip patch antenna design, using shorting pins, with reduced SAR value, is also presented. The surface current distribution on the patch can be modified by the strategic positioning of shorting pins to produce a mushroom shaped radiation pattern having nearly omnidirectional nature in the +Z direction with a near-field null in the vicinity of the user’s direction. This completely minimizes the back lobes in the E-plane, with minor back lobes in the H-Plane creating a deep null, which reduces the SAR value without compromising the signal coverage. The antenna is fabricated and cross-verified by experimental evaluation in an anechoic chamber for return loss, gain, and radiation pattern.


Introduction
Recent years have witnessed a rapid expansion in the field of mobile communication technology.With the development of compact devices and the increase in the number of services and applications, mobile devices have now become an indispensable part of human life.The handset antennas require not only compactness but a wide-coverage radiation pattern to receive signals from all directions, as these devices are used in arbitrary orientations with signals coming from arbitrary directions.Since mobile handheld devices are often used in the vicinity of the human body, it is also important to consider the effect of electromagnetic radiation exposure on the human body.The problem with omnidirectional antennas is that they have significant backward radiation directed towards the user's body, which is undesirable.Therefore, the handset antenna radiation pattern must have a near field null towards the user's body, for low SAR, while maintaining a broad beam width for good signal coverage in the opposite direction, as shown in figure 1.This creates demand for a modified antenna having a broad beam width and low specific absorption rate (SAR), which is compatible with the decreasing electronic component sizes.Rectangular microstrip patch antennas (RMPA) are used in a variety of applications due to their low profile, thin and conformal properties, which can be readily integrated with the printed circuit board of the communication system.Shorted-patch antennas (SPAs) are a class of microstrip antennas that uses shorting pins or plates for achieving various added functionalities like compactness, frequency agility and polarization diversity [1][2][3][4][5][6][7][8].They can also be used to enhance the antenna bandwidth [9][10][11].The design of antenna for the unlicensed ISM (industrial, scientific and medical) bands (2.4 GHz to 2.48 GHz) has gained a lot of attention among the researchers because of its wide range of applications such as Wi-Fi, Bluetooth, ZigBee and RFID.More recently, mobile phones has included the capability of the ISM 2450 band for Wireless LAN or Bluetooth applications.Several antenna designs are available in literature for operations at ISM2450 bands.

Specific absorption rate-related work
The primary measure used to evaluate electromagnetic (EM) absorption by human tissues is the specific absorption rate (SAR).The unit of measurement for SAR is watts per kilogram (W/kg), which is defined as the amount of power dissipated per unit mass of tissue.The safety limits of SAR value have been established by countries and international organizations.SAR testing must be done on all wireless devices that have radiating parts closer than 20 cm to the human body or head.As per ICNIRP guidelines [12], the localized SAR limit for the head and body is 2 W kg −1 , averaged over 10 g of tissue in a cube form, whereas the FCC [13] SAR limit is 1.6 W kg −1 averaged over 1 g of tissue for all mobile terminals.All saleable handsets must meet the aforementioned guidelines within these stringent conditions; hence, low SAR antenna design techniques are at the cutting edge of mobile and wearable antenna designs.
This paper presents a novel design of a microstrip patch antenna, using shorting pins, for SAR reduction that has not been extensively reviewed in previous studies.A simple, compact, single-layer, rectangular microstrip antenna, with two pairs of shorting pins, has been proposed and experimentally investigated for low SAR value and improved beam width.To analyse the exposure to far-field radiation on the human head, we have also computed the maximum average SAR values in 10 g of mass of tissue.SAR values are computed for different feed and short combinations at the 2.45 GHz ISM band using the SAM phantom model of the human head, provided by CST software, and tabulated to enumerate the difference.The present structure is simple and easy to manufacture as compared to complex electromagnetic band gap (EBG) and artificial magnetic conductor (AMC) structures.It provides a beam width of around 106 degrees, in the broadside direction, with a null on the rear side for low radiation exposure to the human body.The present study employs a visualization-based exploration to understand the concurrent improvement in SAR values without compromising signal coverage.
Various techniques have been presented in the literature to reduce the SAR in handheld devices [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31].It can be inferred from the previous works that one of the straightforward and uncomplicated ways to lower the SAR value is by increasing the distance between the antenna and the human body.However, the increment in the distance is limited due to the shrinking size of mobile devices.The use of auxiliary antenna elements, like absorbers and reflectors, can also reduce SAR.The drawback with these methods is that a separate antenna element is required, which leads to increased cost and size.Careful optimization is also required for proper working.Another method is to use a phased array of two active elements.But this requires a phase shift feeding in each element.Size, cost and careful design are required in this case to avoid high SAR under special user conditions.Special high-impedance surfaces, known as artificial magnetic conductors (AMC) or electromagnetic band gap (EBG), and metamaterials have also been shown to reduce the SAR values.[31][32][33][34][35][36][37][38][39][40][41][42][43][44][45] However, all these structures are intricate and require more development.Therefore, a simple and costeffective method to reduce SAR, without increasing the antenna size and volume or employing additional circuitry, is the need of the hour.

Low-SAR antenna-theory and design
In this section, some standard electromagnetic field theories are discussed and their relationship to SAR is explored.The goal is to establish a relationship between these theories and the procedure selected to design a compact low SAR broad beam-width antenna.The important parameter, which determines the directivity as well as the resonant length of a microstrip patch antenna, is the relative dielectric constant ε r of the substrate.The resonant frequency of the patch antenna is inversely proportional to √ε r .Therefore, improving the dielectric constant of the substrate will reduce the resonant length of the patch as well as its directivity.However, the use of a high dielectric constant substrate has direct drawbacks like bandwidth reduction, lower efficiency and higher material cost.A better strategy is to vary the effective permittivity by loading the antenna with shorting pins/plates.Shorting pins increase the substrate's effective permittivity by capacitively coupling it to the resonant circuit of the patch.
A simple way to model the rectangular patch antenna is to assume the radiating edges, located at and opposite the feed edge, as slots of width t radiating into a half space.The two antenna edges are approximately half of a free space wavelength apart when the substrate used is air (ε r ≈ 1.0).According to the slot model of the rectangular patch antenna, this spacing yields an array spacing that yields maximum directivity.As the dielectric constant of the substrate is improved, the slots will come closer in terms of free space wavelengths, and they no longer form an array to generate directivity as high as in the case of free space.As the dielectric constant of the substrate is increased, the directivity of the rectangular microstrip patch antenna decreases.This causes the antenna to resonate at a quarter wavelength, which is substantially smaller than a normal microstrip antenna's half-wavelength characteristic dimension [2,3].

Relation between SAR and surface current
Energy is absorbed by human tissues as electromagnetic waves transit through them.SAR is therefore defined as the amount of power (heat) deposited in tissue by RF irradiation per unit mass, and it is measured in terms of watts per kilogram.The SAR value calculated locally is, therefore, related to the electric field induced in the tissues, which can be calculated using the formula where E denotes the local electric field induced in the human tissue due to electromagnetic radiation, σ depicts the electrical conductivity of human tissues and the local mass density of human tissues is denoted by the letter ρ m .According to Maxwell's equations, By substituting (2) in (1), we get the SAR value equation in terms of the magnetic field as Therefore, the radiated magnetic field of the antenna is a prime factor that controls the SAR value of the antenna.Thus, the antenna's SAR value can be reduced if its emitted magnetic field is attenuated.According to the boundary conditions of the ideal conductor surface, n H J 4 s ( ) ´= Equation (4) implies that the intensity of the radiant magnetic field, generated by an ideal conductor, is determined by the surface current density.Hence the antenna's distribution of surface current decides the magnitude of the radiated magnetic field.The magnetic field radiated by the antenna is proportional to the surface current on the antenna.The greater the radiated magnetic field of the antenna the larger the induced magnetic field in the human tissue, and the higher the antenna's SAR value.As a result, decreasing the surface current density on the patch will lower the SAR value [32][33][34].The use of shorting pins can change the surface current distribution of the antenna such that an increase in the current density can be observed at the position of the shorting pins, which reduces towards the open end.Therefore, the surface current distribution can be controlled by using shorting pins inside the patch and hence the radiated magnetic field can also be controlled.Placing shorting pins inside the patch will also alter the permittivity of the substrate.Therefore, the permittivity and surface current distribution can be controlled by strategic positioning of shorting pins inside the patch, which results in compact broad-beam width operation.

Designs and analysis of structures
The evolution of patch antenna for low SAR values is relatively straightforward.By incorporating shorting pins at specific locations, the surface current distribution of the reference antenna can be changed to reduce the magnetic field radiated by the antenna terminal.The basic width and length are designed based on standard equations (5) (6).The dimensions are then optimized using a full-wave simulator after parametric analysis.An iterative process is employed to attain the optimum design for a broad beam width antenna with a near-field null along the Z-axis.
In cellular communications, the contact of mobile antennas with the physical body is a major consideration.In most cases, phantom measurements or computer simulations are used to assess the SAR.For SAR computations, the finite-difference time-domain (FDTD) approach is currently widely used.CST Microwave Studio is used for quantifying the effect of radiation exposure on the head tissues and SAR.A specific anthropomorphic mannequin phantom head, available in the library of CST Microwave Studio Suite, is used as the human head model [15].The proposed work therefore mainly focuses on obtaining a radiation pattern that minimizes the backward radiation, while maintaining signal coverage in all other direction of the antenna as shown in figure 1.A compact antenna with mushroom shaped radiation pattern is therefore presented.An antenna prototype is developed to understand how the implementation of shorting pins can be used to control the antenna backward radiation.As an initial step we have developed a single band antenna in the ISM2450 band to analyse the results.The work also focuses on the effect of exposure to electromagnetic radiation on the human head as the radiation exposure is mainly directed towards the head.It is presumed that for a particular operating frequency, the dielectric properties of the head remain the same unless there are any specific changes [34][35][36][37][38][39][40].Using the FDTD method, the solver divides the head cells into smaller units of cells.Specific meshing properties are assigned to each cell unit before simulation.The mesh type used is hexahedral.The lines per wavelength in the mesh density control are set to 8 and the mesh line ratio limit is set to 10, which results in a total of 1797120 mesh cells.Throughout the study for computing SAR values, the patch antennas are mounted as it is placed on a handset in line with the upper edge (See figure 11 for location details).The distance between the antenna and head is fixed at 5 mm.The following section deals with the parametric study and evolution of the proposed structure indicating the variation of SAR value with the number and position of shorting pins.

Reference antenna
Consider a simple rectangular coaxial-fed patch antenna.The antenna element here is printed onto a substrate of a thickness of 1.6 mm with a patch size of 28 mm × 26 mm and a ground plane dimension of 48 mm × 38 mm.Larger metallic ground planes are selected to reduce the back lobes of the patch antenna [37].However, this structure has significant backward radiation as shown in table 1.The present work proposes a way to minimize this backward radiation to obtain lower values of SAR for antennas used in handheld telecommunication devices.The structure of the antenna is shown in figure A, table 1.

Parametric analysis of different feed locations
A parametric analysis is carried out to find the position of the feeding pin inside the patch with minimum backward radiation.Full wave simulations are carried out by placing the feed at different locations inside the patch.The most relevant results are summarized in table 1.In this case, the rectangular patch is operated as a half-wavelength antenna.The patch radiates along the broad side direction where the E plane pattern is slightly wider than the H plane pattern.It can be noted that the antenna has significant backward radiation irrespective of the position of the feed.The gain in the -Z direction, at phi =180 degrees (towards the user side), is listed in table 1.It can be seen that the antenna has a directional radiation pattern with significant radiation towards the user's side.
However, when the feed is placed along the x-axis at x = l2 and y = 0 (i.e., along the length of the patch), the radiation pattern gets slightly tapered towards the backside and takes a conical shape.Precisely, the backward radiation in the E plane is reduced.However, for this arrangement, H-plane has backward radiation as shown in table 1.For all other feed locations inside the patch, the radiation pattern exhibits an oval shape and has significant backward radiation in both E and H planes. Since the symmetrically opposite location of the feed shows the same results, it is not shown in the table.Other parameters, along with SAR values of the antenna at 2.45 GHz, for different locations of feed, are also computed and tabulated in table 1.From the results in table 1, it is evident that maximum reduction in backward radiation and SAR is observed when the feed is placed along the x-axis at x = l2, y = 0 or x = −l2, y = 0. Hence, the feed position for further analysis is taken as x = l2 and y = 0.

Design I
After fixing the feed position to further reduce the backward radiation, a single shorting pin is added to the structure.Figure .(a) in table 2 shows the configuration of a rectangular microstrip patch antenna shorted using a single pin.

Parametric analysis of different shorting pin locations
A detailed parametric study is then carried out to find a suitable shorting pin position with minimum backward radiation.The feed is placed at x = l2 and y = 0, and the shorting pin position is varied, as shown in table 2. It can be seen that the addition of shorting pin, irrespective of its location, eliminates back lobes in the E-plane.This reduces the gain in the -Z direction, at phi =180 degrees (towards the user side).It also reduces the side lobe levels in the H-plane by tapering the radiation pattern towards the ground portion of the antenna.According to the analysis in [32], a strong shunt-inductive effect is produced when the shorting pins are placed along the centreline of the patch antenna and the effect of shorting pin on the input impedance of the antenna can be quantified using as LC tank circuit.Simulated radiation characteristics of the antenna, when shorting pin is placed at different locations in the patch, are listed in table 2. It is observed that the antenna has a nearly omnidirectional radiation pattern with reduced radiation towards the user side.This implies that the addition of shorting pin reduces SAR values.A maximum decrease in SAR value is obtained when the shorting pin is positioned along the x-axis at x = −l2 and y = 0.

Design II
From the above results, it is evident that the lowest SAR values are obtained when the feed and short are placed along the x-axis at x = l2 or x = −l2.However, in the previous design, there are back lobes in the H-plane, which need to be eliminated for further SAR reduction.Therefore, in the remaining section of the paper, more shorting pins are introduced to modify the surface current of the antenna, as proposed in Design 1, to minimize the back lobes in the H-plane.The shorting pins are additively included in the structure, with the help of rigorous parametric analysis, to control the surface current and weaken the radiated magnetic field towards the user's side and achieve SAR-reduced designs.According to the theory described in section III, the SAR value is proportional to the magnetic field radiated by the antenna, which in turn is dependent on the surface current density.So, SAR values can be reduced by controlling the surface current density.In the proposed antenna, to achieve appropriate surface current distribution to produce the radiation pattern in figure 1, the shorting pins are carefully distributed, which makes the current propagate through multiple paths.The distribution of surface current implies that the shorting pins serve as an inductive loading for the patch antenna, which makes the surface currents concentrate in the region near the pins [32,[39][40][41][42][43][44][45].From the parametric analysis, it can be inferred that the addition of balanced shorting pins along the diagonals makes the surface current concentrate along the periphery, thereby, reducing the surface current distribution at the centre of the patch as shown in figures 2-5.This creates a near-field null along the ground plane in the user's direction, which reduces the backward radiation.However, due to shorting pins, the current flow on the patch and ground is in the same direction, which results in the superposition of radiation in the far field at 2.45 GHz.This results in a mushroom-shaped radiation pattern with low backward radiation towards the user's side.It can be observed from the parametric study that this placement of shorting pin reduces the gain at phi =180 degrees, considerably in the -Z direction, thereby, reducing the SAR value.The antenna, with a beam width of around 266 and 259 degrees, is observed in respective planes in the broadside direction.However, this placement yields a slightly lower value of gain in the +Z direction, which is because of the omnidirectional nature of the radiation pattern (table 3, figure.a).The second design has a slightly unbalanced placement of the shorting pins to further reduce the back lobes in design 1 as well as to increase the gain (table 3,  figure.b).To further increase the gain, a rectangular slot is cut along the circumference of the ground plane in the third design (table 3, figure.c).The increase in the gain is due to the embedded slot in the ground plane, which in turn lowers the form factor and enhances the flow of surface current on the patch [33].The final designs with relevant results are listed in table 3.

Experimental results and analysis
The configuration of the suggested patch antenna, with the shorting pins, is shown in figure 6.The antenna is excited by a 50-Ω SMA connector (coaxial probe) due to its low spurious radiation and easiness of fabrication.The system circuit board is made using an FR4 dielectric substrate of thickness 1.6 mm, which is commonly used in the industry.The antenna contains a rectangular patch with slight asymmetrical loading of shorting pins and a coaxial feed.The rectangular patch has a dimension of 39 mm × 27 mm (L1 × W1) above a system circuit board of dimension 55 mm × 45 mm × 1.61 mm (L × W × H).The coaxial probe is located in the centre line of the patch on the x-axis.The patch is symmetrically shorted to the ground plane on the x-axis, as well as on diagonals, using 5 copper shoring pins with a diameter of 0.5 mm.The diameter of the pin is selected to be the same as that of the feed pin diameter to achieve a balanced configuration.These shorting pins are placed in such a way as to produce a near-field null along the Z-axis.A rectangular ring is cut along the outer edge of the ground plane with a width of 1 mm (S).
A prototype of the antenna was fabricated (figure 7) and experimentally evaluated using the Agilent PNA E8362B Vector Network Analyser (figure 12).The measured and simulated input reflection coefficient (S11) of the antenna is in good agreement as shown in figure 8. Though there is a slight frequency shift due to fabrication tolerances, the obtained bandwidth satisfies design requirements.Anechoic chamber measurements were also carried out to determine the far-field radiation characteristics of the antenna.Figures 9 and 10 show the simulated and measured normalized radiation pattern of the antenna at the resonant frequency.In the E plane, a mushroom-shaped radiation pattern with a null towards the ground plane is obtained.In the H plane, the obtained radiation patterns are mushroom-shaped with a small back lobe.However, this is negligible as these radiation patterns are stable enough to give low SAR values (figure 11) because of the low radiation levels associated with the null at the bottom .The radiation pattern is almost omnidirectional.Most mobile antennas are preferred to be omni-directional.This is because an omnidirectional antenna is able to radiate radio wave uniformly in all directions and offers a 360 coverage in a specific plane (normally in Azimuth plane), which make it very suitable to communicate with multiple users.Omni antennas are typically low gain antennas which are designed for greater coverage and radiates with an average gain of 2.1 dB over an isotropic antenna.The gain comparison method is used to measure the gain of the antenna and a peak gain of 1.50 dB for the resonance band is obtained.
A comparative study of different parameters of the proposed antenna compared to previously reported low SAR antennas is depicted in table 4. Comparing the results in table 4, it can be deduced that the proposed   antenna is compact and has good radiation characteristics, an almost omnidirectional radiation pattern, moderate gain and a low SAR value.The antenna is simple in its design without using complex components, additional circuitry and space.

Conclusion
In this paper, a simple compact single-element SAR reduced patch antenna, implemented using shorting pins with limited exposure to electromagnetic radiation towards the human head, is proposed.The methodology adopted is simple and reduces SAR without increasing antenna size and volume.The proposed antenna occupies an area of 0.33λ0 × 0.23λ0 × 0.0134 λ0 mm2 etched on a substrate with relative permittivity of 4.3 and thickness of 1.6 mm.The antenna is fabricated, and the experimental analysis of the radiation pattern verifies the reduction in back lobes and formation of null, which results in a low SAR value.The simulated and experimental results are in agreement.The proposed antenna is well suited for mobile and wireless applications due to its simple structure and acceptable radiation characteristics while maintaining low electromagnetic radiation exposure towards user direction.The presented antenna finds applications in all wireless devices that resonates in the ISM frequency range, working in the vicinity of human body.The proposed work can be considered for further studies and can even be extended to wearable antennas and biomedical devices using stainless steel and copper conductive threads as shorting pins.

Figure 1 .
Figure 1.Schematic representation of radiation pattern for low SAR antennas.A mushroom shaped pattern with minimum backward radiation.

Figure 2 .
Figure 2. The surface current distributions of design 1 at 2.45 GHz.

Figure 3 .
Figure 3.The magnetic field distributions on the reference plane of design 1 at 2.45 GHz.

Figure 4 .
Figure 4.The surface current distributions of design 2 at 2.45 GHz.

Figure 5 .
Figure 5.The magnetic field distributions on the reference plane of design 2 at 2.45 GHz.

Figure 6 .
Figure 6.Schematic representation of patch and ground of the antenna.

Figure 7 .
Figure 7. Fabricated path antenna in FR-4 substrate with shorting pins and coaxial feed.

Figure 8 .
Figure 8. Simulated and measured reflection coefficients (S11) of the proposed antenna at 2.45 GHz.

Figure 9 .
Figure 9.The simulated and measured normalized radiation pattern of the proposed antenna in the E-plane at 2.45 GHz.

Figure 10 .
Figure 10.The simulated and measured normalized radiation pattern of the proposed antenna in the H-plane at 2.45 GHz.

Figure 11 .
Figure 11.SAR values computed for the test antenna at 2.45 GHz with respect to the phantom model.

Table 1 .
Simulated Radiation Characteristics of the antenna when feed is placed at different locations.

Table 2 .
Simulated Radiation Characteristics of the antenna when a single shorting pin is placed at different locations.

Table 3 .
Simulated Radiation Characteristics of the antenna when a pair of shorting pins are used.
H B Kurup et al

Table 4 .
Low SAR antennas in literature -a comparison with proposed antenna.