Design of Biodegradable Quadruple-shaped DRA for WLAN / WiMax applications

A novel Quadruple-shaped Dielectric Resonator Antenna (DRA) excited by a coaxial feed is investigated. The dielectric used for investigation is biodegradable polymer based dielectric material having a dielectric constant (r) of 3.45 and dielectric loss tangent (δ) =0.02. The resulting antenna offers broad impedance bandwidth of 63.2% for |S11| < -10 dB from 2.8 to 5.2 GHz frequency band. This antenna is suitable for practical use in WLAN (5.15–5.35 GHz) and Wi-Max (3.4–3.69 GHz) applications. The results show the peak gain of the antenna is 4.5dBi at resonant frequency 3.8 GHz. It is also observed that the new proposed structure of Quadrupleshaped DRA offers broadside radiation patterns and high efficiency for the entire operating band. The simulated and experimental results are well in agreement.


I. INTRODUCTION
To enrich with new services to more users higher bit rate is indispensable that's why wireless systems is shifting towards the microwave frequency bands with low interference and more available bandwidth.However, at these frequencies, conductor loss is significant, which affects the gain and efficiency of fabricated metallic antennas.For better utilization of microwave frequencies regarding wireless applications, antennas with simple fabrication, higher efficiency, and larger impedance bandwidth is statutory.Dielectric Resonator Antennas (DRAs) have abundant captivating features such as wider impedance bandwidth with higher radiation efficiency due to the deprivation of surface wave and conductor

Design of Biodegradable Quadruple-shaped DRA for WLAN/Wi-Max applications
Pramod Kumar, Santanu Dwari, losses [1].Challenging part is a fabrication of DRAs as they are made of high permittivity ceramics, which are naturally hard and extremely difficult to cut.The fabrication of these three dimensional (3D) structures are onerous at microwave frequencies where the size of the antenna is reduced to the millimeter or sub-millimeter range.
In recent years the most standard dielectric resonators (DRs) configurations such as rectangular, hemispherical, cylindrical etc., have been modified in the pursuit of enhanced operational bandwidths [2][3][4][5][6][7].As compared to cylindrical or hemispherical DRA, Rectangular Dielectric Resonator Antenna (RDRA) provides more degrees of freedom which can be used to control the impedance bandwidth of the antenna [3].
Significant efforts with different techniques are adopted on DRAs such as Stacked DRs [8], Multisegment DRs [9], as well as more tortuous structures as see in [10][11] led to enhanced band widths of about 35%-40%.In [12] stacked and embedded DRs also claimed the possibility to achieve prodigious operational bandwidths up to more than 60%, but make the manufacturing process of antenna complicated and expensive.The use of multiple resonating modes was adopted in [13].More recently a new class of DRA known as Super-shaped DRA [14] is introduced.These antennas render freedom for the design of the DRs.Quadruple-shaped DRAs overt the good flexibility for matching bandwidth and radiation patterns while preserving broadside radiation and ease of manufacturing.
In this paper a novel rectangular DRA based on Quadruple-shaped profile excited by a coaxial feed is developed and investigated for WLAN/Wi-Max applications.The dimensions of DRA along x-, and y-, directions are equal (length = width = 18mm) and along zdirection is height H (H = 29 mm).The antenna is fed from the bottom side via the coaxial feed at the The theoretical formula for resonant frequency (  ) of a RDRA is given as [1]: where "  " is the dielectric constant of the RDRA, "c" is the speed of light in free space and symbol "  ", "  " and "  " represents the wave numbers in the x, y and z directions respectively.The The permittivity of the proposed antenna is altered with the perforated slots cut in the DRA and it will become effective permittivity (ε eff ) of the DR with a smaller value.Thus, as the ε eff decreases, the Q factor decreases and hence the bandwidth is improved significantly.Hence, ε eff of proposed DRA can be calculated by modifying the dielectric waveguide model (DWM) equation as [17]: where V and V slots are the volume of proposed DRA and air gap slots of DRA respectively.The height and feed of the alumina based DRA is optimized to matche the resonance frequency.Here, heigt = 25.3mmtaken for alumina based DRA while 29mm used for proposed DRA.Fig. 11 shows the comparative plots of reflection coefficient versus frequency and the variation of peak gain versus frequency is shown in Fig. 12.It is observed that polymer based DRA achieves wider bandwidth compare to alumina based DRA.It is also noticed that the peak gain over the operating band is remain same.Except these entire advantage polymer based DRA have also many more benefits such as low cost, easy fabrication, light weight and higher thermal conductivity as tabulated in Table III.

IV. EXPERIMENTAL RESULTS
The structure of proposed DRA is fabricated by additive manufacturing i.e. 3D printing (3DP).It is a layer-by-layer approach of fabricating 3D objects.Commercially available SMA connector is used to provide coaxial feed.Fig. 13 exhibits the fabricated DRA.
The reflection coefficient is measured using ROHDE & SCHWARZ, FSH8 handheld network analyzer, while the radiation characteristics are measured using a basic antenna measurement setup.
The comparison between simulated and measured variations of reflection coefficient versus frequency for proposed DRA is shown in Fig. 14.The simulated results shows that the proposed DRA resonates    It is also noticed that the gain is almost stable for the complete bandwidth of operation.Thus, the proposed DRA offers lower losses and are comparable to the other regular geometries in the DRA family.Table IV shows the judgment of the proposed DRA with the superlative cases of existing inspects.It can be observed that the proposed antenna not only achieves wider bandwidth, but also compact volume.The peak gain of the DRA is 4.6 dB at the resonant frequency with high radiation efficiency.To the best of our knowledge, such geometry has not been explored so far for a DRA.Thus, the varieties of eco-designed DRA are possible by the adoption of polymer dielectric resonators with very effective cost.

Fig. 5
Fig. 5 Reflection coefficient versus frequency for reference antenna (RDRA), modified antenna with side cuts and proposed antenna (Quadruple-shaped)

Fig. 6
Fig. 6 Variation of reflection coefficient versus frequency curve for different value of cutting depth

Fig. 7
Fig. 7 Variation of reflection coefficient versus frequency curve for different value of cutting bottom edges

Fig. 8
Fig.8shows the 3D gain pattern of proposed antenna at its resonant frequency 3.8GHz.The peak gain 4.5 dB is dignified at resonant frequency with broad side pattern.Fig.9shows the electric field distribution in proposed DRA at its resonant frequency 3.8GHz.The field distribution clearly verifies the presence of Quasi-TM 111 like mode in reference to the corresponding cylindrical resonator mode[18].

874Fig. 10
Fig. 10 Comparison of antenna efficiency as a function of frequency for Quadruple-shaped DRA

875Fig. 11
Fig. 11 Comparison of reflection coefficient versus frequency for the polymer based DRA and alumina based DRA

at 3 . 8
GHz, offering an impedance bandwidth of 63.2%, while the measured result shows that DRA resonates at ~3.7 GHz, offering 64% impedance bandwidth for |S 11 | < -10 dB.Thus, simulated and measured results are showing very good agreement across the operating band.The small difference in the results is mainly due to the fabrication inconsistencies like air gaps between the DRA and the ground plane, imperfections while drilling the hole for probe insertion, etc.

Fig. 17
Fig. 17 Measured gain as a function of frequency for proposed DRA of rectangular DRA along with Quadruple-shaped based geometry for biodegradable polymer based material has been successfully implemented.This antenna has a simple-interesting structure and relatively compact volume of 6.08cm 3 .The DRA resonates at 3.8 GHz, offering an impedance bandwidth of 63.2% for |S 11 | < -10 dB from 2.8 to 5.2 GHz.Proposed DRA has many more advantages over the conventional DRA such as low cost, easy fabrication, light weight and a wider bandwidth.This makes the DRA suitable for practical use in the wireless communication systems.Practical applications are like WLAN (5.15-5.35GHz) and Wi-Max (3.4-3.69GHz) system.

TABLE II :
VALUE AND EXPANSION OF ABBREVIATIONS