Design and performance analysis of small-sized multiband microstrip patch antenna on custom-made biopolymer substrate

1 Introduction

The recent multiprocessor, powerful computer-oriented technological advancements spurred the rapid and wide growth of today’s wireless pervasive environment. For providing integrated services through collaborative wireless communications, there is a great demand for environment-friendly, inexpensive, compact, portable devices. The antenna as core component in wireless communication devices are requested to design for multifunctional operability. This will enable the device to interconnect with more than one network and ensure cohabitation, interoperability, and unified transfer of signals. The antenna having multiband functionality can substitute a number of single-band antennas and thereby are advantageous by ensuring compactness, reduced cost, and improved aesthetics. Besides the radio/TV receivers, these day vehicles are well equipped with various wireless communications devices such as vehicle tracking, WiFi, GPS system, RFID, and satellite link. However, the automotive market is in search of finding a suitable multifunctional antenna to cover maximum numbers of wireless applications. Therefore, the design of the antenna with multiband functionality should have stable radiation, acceptable gain profile, and efficient resource utilization [1]. Nonetheless, the planar microstrip antennas are a good choice due to their merits in terms of low profile, light weight, simple structure, ease of fabrication, and integration to other system components. For the past couple of years, the potential researchers have devoted their valuable time in extensively investigating the development of multiband/multifunctional antenna as a communication component. The multiband planar antennas are studied by many researchers, and these antennas differ in types and are application specific. The recent literatures reported planar antennas of more than one band functionality as, for example, double U-shaped [2], E-shaped [3], open stub-loaded antenna [4], patch with parasitic [5], slotted monopole [6], and multilayered patch [7]. The extensive review of the recent literature has revealed various techniques that are employed by researchers to obtain acceptable bandwidth and multiple resonant frequencies to cater more than one frequency band. Typically, the researches are well known with different feeding techniques [8, 9], reduced/defected ground [10], integrating stubs/parasitics [11], creating electromagnetic band-gap structure [12], inclusion of metamaterial [13], cutting slots of different shapes/sizes [14], and using different fractal shapes [15] for attaining the desired frequency bands. Moreover, the slotted patch antennas have gain a considerable research interest among researchers. The slot-loaded patch antennas are easy to design and fabricate, giving flexibility in the research to regulate the shape/size/orientation of the slots for achieving wide-band and multiband functionality.

The microwave and electronic circuitry has been always demanded for long-lasting, ecofriendly, nontoxic materials. The durable form of biopolymer substrate can be an alternative to the conventional petrochemical-based plastic substrate [16]. The research community has investigated several types of polymer material substrates for achieving compact dimension, desired antenna performance, and ecofriendly characteristics. Reviewing through the published literatures, a number of custom-made substrate material-based antennas can be found, such as a dual-band antenna on liquid crystal polymer-based substrate [17], compact-sized patch antenna on artificial magnetodielectric substrate [18], a sandwich structure of glass-silicon-glass substrate-based patch antenna with partial substrate removal [19], and a multilayered sandwich substrate of different dielectric material [20]. However, there is a scarcity of organic- or biopolymer-based substrate materials for developing compact patch antenna with desired gain and bandwidth. A widespread research work has to be carried out to find an environment-friendly, bio-based polymer substrate that can substitute the conventional fossil petroleum-based substrate material.

A rectangular patch antenna consisting of four arc slots at the corner and a square wide slot at the middle is proposed to cater services in multiband operations. Utilizing the most reliable EM field solver, the planar antenna has been designed for and fabricated on a 1.0-mm-thick environment-friendly, custom-made, ceramic-filled biopolymer substrate material with a dielectric constant value of 10.0 and loss tangent of 0.002. Throughout the design process of the antenna, it has always been attempted to keep the geometrical structure as simple as possible by avoiding every kind of hard-to-control skills such as complex shape of slot or parasitic/stub, tiresome fractal design, and stack or layer of substrates. The analysis of the measured data collected from the antenna prototype shows that there is a good correlation between the numerical simulated results and the experimental results. The proposed planar antenna has achieved the fractional bandwidths of 90.35% (0.355–1.067 GHz), 38.78% (2.92–4.3 GHz), and 30.71% (6.55–9.01 GHz) for the functioning frequency bands centered at 0.788, 3.34, and 8.01 GHz, respectively. The measured results also affirm that the average antenna gain has achieved values of 1.14, 2.54, and 3.14 dBi for the lower, middle, and upper operating frequency bands, respectively. The analysis of the test results of the antenna as radiation profile can be visualized as almost stable and symmetric radiation patterns in both E- and H-planes. The adequate bandwidth, desired resonance mode with good reflection coefficient, symmetric radiation, and appreciable gain profile keep the antenna ahead for serving RFID, 3.5 GHz WiMAX, and C/X-band applications.

2 Design of the antenna module

For the past couple of years, the development of planar antenna with reduced dimension has gained a lot of interest to cope with contemporary wireless communication services. One of the effective ways is to use high dielectric ceramic composite material as a substrate, which can substantially minimize the overall size of the radiating area [21]. This study has been carried out by proposing a multiband planar antenna fabricated on ceramic-filled biopolymer substrate material. Figure 1 shows the multilayered, ceramic-filled sandwich structure substrate that is manufactured using the biopolymer material and ceramic powder. Nowadays, ecofriendly, bio-based polymer and composite material substrates have become more prevalent than the conventional one, as they provide durability besides biodegradability wherever required. The permanence of the bio-based substrate materials can be further upgraded by applying biopolymer composites in their durable plastic form. According to the intended application, the assorted ceramic powder with a polymer binder is sintered by employing the polymer sponge method. For this process, 0.25-mm-thick biopolymer sheets are crafted from several organic biomass sources such as cornstarch, vegetable oil, and palm oil. The depth of ceramic powder layer between the biopolymer layers is determined by estimating the required material properties. After covering the top and bottom surfaces of the three-layered biopolymer-ceramic-biopolymer stacked materials with 35 μm copper foil, a multipress machine is used for preparing final substrate.

Figure 1:

Composition of the ceramic-filled biopolymer substrate material.

The detailed geometrical configuration of the proposed multiband planar patch antenna is shown in Figure 2 with corresponding optimal parametric values. The design of the antenna has been numerically analyzed using the commercial full-wave High-Frequency Structural Simulator (HFSS) (Ansys, Inc., Canonsburg, PA, USA) based on finite element method (FEM) [22] and suitable values of different parameters are determined through optimization. The design process first starts with the estimation of the area of radiating patch and the overall dimension of substrate material depending on the anticipated operating frequency band. For a typical rectangular, metallic patch antenna, the dimensions are easy to calculate using the standard formulation processes stated in Ref. [23]. However, the equations may not suitable for other types of shape and cutting various types of slots on radiating element assist to lower down the resonant frequency. Hence, it is certainly required to explore the estimated dimensions wisely by considering the further modification of the radiator. The overall dimension of the biopolymer substrate and hence the antenna is 17.0×24.0×1.0 mm³ (W×L×H), whereas the radiating element only cover an area of 17.0×17.0 mm² with slots inside. The final optimal parametric values are used for fabricating the antenna on a 1.0-mm-thick custom-made biopolymer substrate material with a dielectric constant ε_r=10.0 and loss tangent tan δ=0.002. The rectangular, slotted radiating element of the antenna is excited through a 50 Ω microstrip line feeder of 1.2 mm wide and 8.0 mm long, which is soldered to the middle conductor of the SMA connector. The surrounding conductor of the SMA connector is linked to the reduced ground plane of 17.0 mm wide and 7.0 mm long. The dimension of the ground plane is a key factor for regulating the operating bandwidth associated with the resonant mode of frequency and the commonly used method by researchers to attain the projected bandwidth [24, 25].

Figure 2:

Geometry and dimensions of the proposed multiband planar antenna.

Different slots are positioned on the radiating element for obtaining the required frequency bands. The numerical results in terms of change of reflection coefficients (S11) with the inclusion of different slots and inductive elements are shown in Figure 3. As shown in the figure, the length of the radiating area is primarily accountable for generating the lower resonant frequency, which is approximately 2.0 GHz. Integrating the wide circle slot perturbed the surface current path and distributions, thus helping to shift the resonant frequency toward low frequency at 1.02 GHz and produce the high-frequency resonance at near 8.1 GHz. The inclusion of a rhombus inductive element inside the wide slot circle increases the overall radiating area, which helps to radiate more EM energy and thus further improve the bandwidths and reflection coefficients with a slight shift of frequency bands. The loading effect of a wide square slot at the center of the patch can be clearly observed, as it assists in the further shifting of resonance and increasing the operating bandwidth. The surface current distributions on the radiating patch are also carefully analyzed and discussed in the next section to validate the effect of slots and inductive element inclusion.

Figure 3:

Variation of reflection coefficients vs. frequency for different antenna modules.

3 Results and discussion

The optimal parameters from the numerically synthesized model have been used to fabricate the physical antenna prototype and the performance criteria are measured in a standard anechoic chamber to validate the numerically estimated results. The fabricated antenna prototype is presented in Figure 4 and the standard rectangular, anechoic chamber of 5.5×4.5×3.0 m (W×L×H) is situated at the Faculty of Engineering and Built Environment, University Kebangsaan Malaysia. The floor, wall, and ceiling of the anechoic chamber are covered by a pyramidal radiation-absorbent material. For the whole measuring process, an Agilent E8362C vector network analyzer (Agilent Technologies, Santa Clara, CA, USA) that ranges up to 20 GHz is used. To conveniently visualize the comparison results, the numerically simulated and measured reflection coefficients and measured gain are plotted against frequency in Figure 5. The measured impedance bandwidths for S11 less than -10 dB are 712 MHz (0.355–1.067 GHz) resonates at 0.788 GHz band, 1.38 GHz (2.92–4.3 GHz) resonates at 3.34 GHz band, and 2.46 GHz (6.55–9.01 GHz) resonates at 8.01 GHz band. As illustrated in the figure, the measurement results well agreed with the simulated results. The little deviation between the measured and simulated reflection coefficients may be contributed by imperfection in soldering, fabrication, and measurement setup. The gain of the multiband planar antenna is measured by using the three-antenna method where two identical horn antennas are used as reference antennas [26] and the result is plotted in Figure 5. The maximum gains over the three operating bands are observed as 1.37, 2.8, and 3.56 dBi for the lower, middle, and upper frequency bands, respectively. Meanwhile, it can be noticed that the gain is significantly reduced when there is impedance mismatch, as most of the EM energy could not radiate to the free space. Considering the impedance bandwidth, resonant frequency, and acceptable gain profile, it can be concluded that the proposed antenna is suitable to cover the typical bandwidth requirement for RFID, 3.5 GHz WiMAX, and C/X-band frequencies.

Figure 4:

Photo of the fabricated antenna prototype: (A) top view and (B) bottom view.

Figure 5:

Measured and simulated reflection coefficients and measured gain against frequency.

The simulated surface current distributions of the proposed antenna at 0.8, 3.35, and 8.0 GHz are shown in Figure 6. It can be clearly perceived that the distribution of currents at three resonant frequencies is different. For the lowest resonant mode at 0.8 GHz, the intensity of current distribution is much weaker and seen concentrated near the feed and bottom areas of the radiator. A much stronger current distribution is observed on two sides of the inductive-loaded rhombus for the frequency of 3.35 GHz. At the resonant frequency of 8.0 GHz, the surface currents are seen much disturbed and the concentrated area of coverage increased. The phenomenon of current distributions also verifies the effect of the inclusion of arc slots and inductive element in generating the resonant frequencies.

Figure 6:

Simulated surface current distributions at (A) 0.8 GHz, (B) 3.35 GHz, and (C) 8.0 GHz.

For the designed antenna, the input impedance and voltage wave stranding ratio (VSWR) are examined through the Smith chart as shown in Figure 7. The VSWR 2:1 circles are marked in the Smith chart and the resonant frequencies with corresponding VSWR and impedance values are also depicted. Figure 8 shows the measured radiation patterns of the proposed antenna in E- and H-planes normalized for three different frequencies at (a) 0.8 GHz, (b) 3.35 GHz, and (c) 8.0 GHz. Both the copolarization and crosspolarization radiations of the antenna are shown in the figure. The E-plane copolar radiation for all three frequencies is perceived as omnidirectional and symmetric, whereas the H-plane copolarizations are little bit disturbed. The symmetric radiations are due to the symmetric geometry of the radiating element and nearly uniform distribution of currents. The asymmetric radiation profile at higher frequency may be due to the increased x-directed radiations and concentration variations of the surface current. The crosspolarization radiation values are less than -30 dB and the variations are less than -10 dB throughout the bands as expected. The prototype antenna is approximately symmetric and steady radiation patterns of the proposed antenna reasonably make it suitable to furnish the services associated with the intended frequency bands.

Figure 7:

Simulated Smith chart presentation of the proposed multiband planar antenna.

Figure 8:

Normalized E- and H-plane measured radiation patterns of the proposed antenna at (A) 0.8 GHz, (B) 3.35 GHz, and (C) 8.0 GHz.

4 Conclusion

A new type of rectangular, slotted patch antenna printed on a 1.0-mm-thick custom-made, ceramic-filled biopolymer substrate material has been presented. The antenna has a very simple structure that is formed with the combinations of circle, rhombus, and square excited through a 50 Ω characteristic, impedance-compliant SMA connector. The shortcomings of the proposed high-permittivity, biopolymer substrate-based antennas are narrow bandwidth, which has been mitigated using a reduced ground plane. However, the reduction of the ground plane leads to an increase of backward radiation. Therefore, further research can be conducted to improve the operating bandwidth without an effect on radiation characteristics and low-permittivity biopolymer materials can also be adopted. The combinations of capacitive slots and inductive elements generate the desired resonant modes and noticeably offer a sufficient bandwidth of 712 MHz (0.355–1.067 GHz) for RFID, 1.38 GHz (2.92–4.3 GHz) for 3.5 GHz WiMAX, and 2.46 GHz (6.55–9.01 GHz) for C/X-band frequencies. The experimental radiation patterns and maximum gains confirm sensible performance characteristics and a good understanding between the simulated and measured results. The good radiation characteristics of the antenna prototype and more than 0 dBi gain over the operating bands confirm the antenna as a potential candidate for use in RFID, 3.5 GHz WiMAX, and C/X-band wireless communications.