Trapezoidal Microstrip Patch Antenna Array for Low Frequency Medical Applications

This paper presents the design of low cost FR4 substrate trapezoidal patch microstrip feed antenna array for low frequency wireless medical applications. Initially, single element trapezoidal patch antenna having size of 70 × 30 mm2 has been designed and then the 2 × 2 array has been achieved for bandwidth improvement with low loss in same dimension. The array resonates at 1.891 GHz with impedance bandwidth of 80 MHz and low return loss of − 26.19 dB. The VSWR of 1.103 validate the activeness of the proposed antenna array having maximum surface current 133.1 (A/m) and directivity of 4.48 dBi. The antenna array exhibit the H-Field strength of 160.52 (A/m) and E-Field of 36,093.4 (V/m) prove the radiation capability at low frequency on body application. The antenna has been simulated in low frequency L band from 1 to 2 GHz and compared with other works in literature. The radiation property of proposed array validate it uses in low frequency biological imaging application for visual representation of interior body for medical diagnosis and interventions.


Introduction
Microwave antenna is the major parts in now a day's wireless era in all fields for low cost [1] microwave portable diagnostics tools. In microwave head imaging, designing portable and desired radiation properties antenna present great challenge in terms of band of operation [2]. The antenna in the low frequency microwave band within 1 GHz to 4 GHz provides acceptable and suitable qualities in terms of resolution of resulting image and penetration of signal into human head for medical applications. Various configurations of antenna have been proposed in the above frequency band for size reduction [3], antenna 1 3 structure with matching need elimination [4].The wideband, compact size antenna requirements in medical imaging have spurned out several research and innovation in microwave systems [5]. Monopole omnidirectional antennas are widely used in ensuring better images due to its low profile [6] and simple fabrication. Though the directional antenna is complex in nature, they are preferred in medical imaging for less environment interface sensitivity [7]. The directional antennas exhibits smoother gain pattern due to less fluctuation [6] in current distribution. Antenna size, mutual coupling between array elements in limited area [8] and constrain in frequency band are major challenges in antenna research for medical applications. Several types of conventional substrate antenna arrays have been proposed [9] using FR4, Rogers, Duroid etc. In tomography microwave imaging, to map the entire electrical profile [10] of the breast and in confocal imaging [11] for mapping the location of significant scatterers [12], the antenna array made up of wideband elements has been utilized to send and receive the test signals.
Microstrip arrays are now a day's best suited for high performance [13] in present wireless communication. Flat, compact and mechanically convenient radiating structure antennas using microstrip transmission line are now a day preferred for coupling electromagnetic energy into biological systems due to simple to design and fabricate, efficient electromagnetic transmission and reception [14] and low cost and compact profile [15]. For the microstrip antenna to penetrate at best in human body, the antenna tends to be physically too large and produce radiation leakage at narrow band. The design of microstrip antenna for medical applications must be designed to radiate in lossy medium when compared to design for other wireless applications in which the antenna radiates in to free space. The bandwidth enhancement in microstrip antenna can be achieved by different design techniques like optimization of impedance matching, lowering the substrate permittivity, substrate thickness increase [16], inclusion of multiple resonances, choosing the right radiating shape combination, such as comb-shaped slits and radiating section nesting [17], logarithmic architecture design with ultra-wide bandwidth [18], ring slot array antenna for high gain of 10.2 dBi [19]. BY proper selection of material properties [20] to avoid losses, the microstrip arrays are mostly preferred patch antenna for mm (MilliMeter) wave communications.
Microstrip antenna array are mostly used for high gain wireless power transfer medical implants applications. Many array antennas are realized like 1 × 2 antenna array for RF power transfer in pacemaker [21], UWB antenna for human body changes measurement in footwear applications [22], square patch antenna array with complimentary split ring resonators [23], CP implanted patch antenna [24] for impedance matching, rectangular patch antenna with a rectangular slot [25] for ISM band applications, square shape CSRR [26] to reduce the onbody power observation, omnidirectional pattern radiation characteristics analysis in close proximity with human body [27], hexagonal shape patch antenna [28] for ISM application, H shape 2 × 2 metamaterial antenna array [29] for WBAN applications, defected ground Flexible Parasitic Element Patch (FPEP) antenna for ISM biomedical application [30], etc. One of the challenges in antenna array large scale production for medical applications is the use of non conventional flexible substrate like fabric and plastic [31]. The cost efficient, high gain and flexible characteristics of antenna array is much suitable for biotelemetry and health monitoring applications [32].
In medical filed, L band frequency range (1-2 GHz) is mostly use microwave based diagnosis systems [33] to increase the penetration of signal within small space. For maximizing the energy penetration into the tissue for better diagnosis, the new composite material operates from 0.5 to 10 GHz has been reported in [34]. Many antennas are proposed in literature for medical imaging system like 3D wideband antenna [1], asymmetric dipole sleeve transceiver antenna array [35] for reduced SAR (Specific Absorption Ratio) peak and improved SAR efficiency and, ring shaped antenna array with less than 0.5 dB electromagnetic influence [36] and ultra wide band flexible metamaterial antenna array for WBAN (Wireless Body Area Networks) and breast imaging application from 6.5 to 35 GHz [37]. In this paper, a flexible trapezoidal radiating patch microstrip 2 × 2 antenna array has been proposed for all wireless especially for medical imaging applications in 1.89 GHz low frequency. The radiating trapezoidal shape is coated on copper ground plane which radiate on free space with FR4 as substrate. All ideal characteristics support for parametric analysis of designed antenna has been simulated using Finite Integration Techniques (FIT) analyzer.

Antenna Design
Flexible and economical microstrip trapezoidal antenna array for onbody health monitoring applications is realized on 2.4 mm thickness FR4 substrate having 4.4 as relative permittivity and 0.025 of loss tangent. The radiation patterns are drawn on conductive copper foils for the prototype antenna array having 30 × 70 mm 2 size. Generally the conducting patch of any shape and simple half wavelength trapezoidal shape is chosen for simple analysis and performance prediction. Using microstrip basic antenna design procedures and transmission line theory, all the dimensions of the structure are calculated and as per the required specification, they are optimized carefully. The proposed 2 × 2 linear array microstrip fed antenna comprise of single copper conducting layer on one side and FR4 dielectric substrate on other side.
Single layer metallization supports desire radiation pattern and high gain which may degrade when the antenna comes in close proximity with human body. Hence the design procedure used in this proposal involves the design of single radiating element and replication of same trapezoidal element as 2 × 2 array fed by microstrip configuration. The length (L 1 ) and width (W 1 ) of the dielectric substrate has been chosen with a thickness of 2.4 mm as shown in Fig. 1a-b for front and back view of proposal with dimension given in Table 1. Based on transmission line model [33], the width of radiating patch is desired first, length is calculated with fringing field consideration [38] and the trapezoidal shape is arranged to produce circular polarization.

Results and Discussion
The array antenna proposed is constructed, simulated and properties are studied using Finite Integration Technique (FIT) 3D ElectroMagnetic (EM) analysis software called CST and the simulated parameters are explained here. For desired radiation pattern, abrupt change in geometry which create discontinuity for electric and magnetic field distribution has been introduced by changing the feed line width and radiating patch junction.

Return Loss
The return loss characteristic of the simulation is shown in Fig. 2 (a)     the return loss magnitude is below-10 dB and the reflected wave is less compared to incident wave. This represents the good impedance matching between the antenna and its feed line. The fractional bandwidth or percentage bandwidth defines as the ratio of difference between upper and lower frequency by the resonant frequency achieved is 47 MHz. This low and negative reflection coefficient prove that the antenna array is active and effective to deliver the power from source.

VSWR
VSWR is one of another metric of transmission line which signifies the matching of line with load. Next parameters called voltage standing wave ratio (VSWR) which related the efficiency of the antenna is plotted in Fig. 2b. The operating bandwidth (VSWR < 1.5) has been shifted to 60 MHz (1.92 GHz-1.86 GHz). By having the VSWR of 1.103, the proposed array proves that it meet out the practical requirements of (VSWR ≤ 2) at the resonant frequency.

Far Field Patterns
Far field is the region with high radiation effect and the considerable antenna parameters like directivity and the radiation pattern are studied in this region only. In far field region, the power absorbed will not be feedback and hence no reflection. In this region, the radiating field has polar variation with maximum radiation in the radial distance from the origin. Figure 3 represents the 2D and 3D radiation pattern behavior of array carried out at 1.89 GHz. It is depicted that the proposed antenna array array exhibit directional pattern in electric and magnetic planes with gain or major lobe of 4.98 dBi in 4° main lode direction, with 83.4 • 3 dB angular width. It radiates in back also with minor lobe width of − 8.8 dB as shown in Fig. 3b.

Far Filed Radiation Properties
For further analysis of antenna performance, the simulated surface current distribution is plotted as shown in Fig. 4a. It shows that the current distribution has maximum value of 133.102 (A/m) which concentrated at feed line and adjacent to radiating patch beside the feed line. From the 3D radiation pattern, the far filed directivity can be contributed as the ratio of radiation maximum strength to the average density of subjected antenna. The far field parameters of the subjected antenna are depending upon the acquirement of the knowledge of current distribution. The far filed which gives normal electromagnetic radiation is shown in Fig. 3a for the proposed array. From the figure, it is clear that the radiation strength is maximum at the center with magnitude of 4.98 dBi and it tends to decrease as the width of main lobe increases. From Fig. 3b it shows that the proposed antenna array exhibits 4.98 dBi directivity and the variation of directivity with respect to angle in z (conical cut) and x axis (planar cut) directions are shown in Fig. 4c-d.
Generally there are two ways or cuts to represent the directivity of an antenna. First one is the conical cut or theta cut where angle, Theta (θ) is kept constant and the angle, Phi (ɸ), related to x axis is swept from 0 to 360 degrees. The other is the phi cut or planar cut where phi is fixed and theta is swept. This produces the radiation pattern which gives the detail of directivity variation in parallel to the circuit layout plane as shown in Fig. 4c.
The electric and magnetic field variation with respect to x direction angle of 0 and 90 degree are shown in Fig. 4e-f respectively. It shows that the electric and magnetic field have the maximum magnitude of 36,093.4 (A/m) and 160.522 (A/m) respectively. The electrical parameters which prove the efficiency of the designed array antenna is compared with parameters of previously proposed antennas as per the literature as in Table. 2.

Conclusion
In this paper microstrip trapezoidal four element antenna array using FR4 flexible substrate material has been proposed for onbody medical wireless applications. The designed bandwidth of 80 MHz and 4.98 dBi gain with desired radiation characteristics of the proposed antenna array made them highly desirable for low frequency medical applications. The medical application antennas must take diverse shape on human body movements and the interaction with human body characteristics has to be investigated in future with fabricated