Design and Analysis of MIMO Spiral Antenna for GPS , WLAN and UAV Applications

This paper presents a multiband compact microstrip MIMO antenna for Unmanned Air Vehicle, GPS and WLAN applications. It consists of two symmetric triangular spiral shape patch and defected ground structure (DGS). The antenna is designed using low cost FR4 Substrate and analysed by using commercially available electromagnetic simulation software CST. The simulation results shows the designed antenna has resonance frequencies at 1.62 GHz and bandwidth from1.6027 GHz to 1.6391 GHz, at 2.42 GHz from 2.3609 GHz to 2.4667 GHz and 3.79 GHz from 3.6418 GHz to 3.7909 GHz frequency which covers GPS, WLAN and UAV(LTE band) applications respectively. Detailed physical analysis of proposed antenna is presented and the characteristics of antenna are verified by the return loss, insertion loss, gain and directivity. It is found that it has good performance for above applications. The overall size of the antenna is 28 x 20.5 mm2 which is less than the conventional antenna by 84% at lowest resonating frequency of 1.62 GHz.


I. INTRODUCTION
Multiple Input and Multiple Output (MIMO) antenna can greatly increase bit rate without increasing the spectrum.There are various applications of MIMO antenna including Wi-Fi, long term evolution (LTE) and massive MIMO [1].Demonstration and study on MIMO antennas has shown effectiveness of such systems, because they provide special diversity or spatial multiplexing [2].
To increase the performance of MIMO one factor is studying Envelope Correlation Coefficient (ECC).For correlation coefficient there are two things to be studied, first one is effect of two separate channel port correlation and second is mutual coupling between two antenna elements.These two effects are not independent and need to think of lowering signal correlation between the antennas.
Most of the MIMO antennas provide isolation 10-20 dB between the antennas ports and this has been shown to be enough for providing good increase in MIMO capacity if channel correlation is sufficiently low.As number of compact printed MIMO antenna with low mutual coupling has been proposed consisting up to six ports this includes design based on PIFA's, patch and slot antennas [3].
One of the effective method of decoupling or reducing the mutual coupling is to use Defected Ground Structure (DGS) technique.In DGS technique a compact geometrical slot is embedded on ground plane of the antenna.A single defect or a no of periodic and aperiodic defects configuration may be comprised in DGS.This periodic or aperiodic defect etched on the ground plane of the microwave circuit is referred to as DGS.The defect on the ground plane affects the current distribution and current distribution directly affects the characteristics of transmission line by including some parameters such as slot resistance, slot capacitance, slot inductance.Thus changing the effective resistance, capacitance and inductance, which influence the characteristics of the antenna.
In the literature, Reito et.al [4] proposed a spiral MIMO antenna resonating at 2.4GHz frequency of dimension 20x24 mm 2 is implemented and its characteristics were studied.In [5] study on frequency tunable dual band inverted L spiral antenna is made.It consists of L-shape and spiral shape which are combined to get dual band.Varacter diode is used to select the two band frequency.This antenna is design for the frequency band from 1900 MHz -3100MHz lower band and 3300 MHz -3700 MHz for upper band.The lower band has gain of 2.83dB and upper band has gain of 3.28dB.The dimension of this antenna is 120mm x 60mm.The substrate used is FR-4.In [6] a spiral type monopole antenna is also proposed which consist of two spiral structure which covers WLAN application at 2.4 GHz.The size of this antenna is 30mmx12mm with substrate size of 0.8mm.In [7] planar multiband MIMO antenna system operating in GSM 1800, LTE2300, LTE2500, WLAN and WiMax bands consist of inverted Lshape monopole antenna with capacitively coupled rectangular open ring resonator and spiral structure, two protruded ground batches and M shaped neutralization line to improve bandwidth and isolation were proposed.The overall size of the antenna is 97mm x 60mm.
In [8] a two-element MIMO antenna system is designed in form on 2×1 meander line array structure to cover 2570-2620 MHz frequency using FR-4 substrate of dielectric constant 4.3 and height of 1.6mm.Isolation is compared by adding parasitic slits and divider between two antenna element and it is seen that better isolation is obtained using divider slits.
In [9] defected Ground Structure for Isolation Enhancement in a Printed MIMO Antenna System is proposed.By changing dumbbell shaped DGS length, width and dimensions of auxiliary and primary rectangles high band centre frequency is controlled, low band centre frequency is controlled by changing dimensions of spiral.
Implementation of microstrip patch antenna array [10] at 3.8GHz for WiMax and UAV applications, a single element of 2x1 and 2x2 microstrip rectangular patch antenna was designed and simulated which covers UAV application having bandwidth from 3.7693GHz -3.8413GHz and return loss for this resonance was equal to 18dB.

Design and Analysis of MIMO Spiral Antenna for GPS, WLAN and UAV Applications
This paper is organized as, in chapter II proposed antenna design is discussed, followed by analysis and results in chapter III, and chapter 4 concludes the paper.

II. ANTENNA DESIGN
The proposed antenna is designed for 1.62GHz, 2.42GHz and 3.79 GHz for GPS, WLAN and UAV(LTE) applications respectively by reducing overall size of antenna.The most important factor of the antenna is to choose the best suitable substrate.In our case FR-4 substrate is used of thickness 1.6 mm.The dimension of the antenna is 28x20.55mm 2 which is less than the conventional antenna by 84% at lowest frequency i.e. at 1.62 GHz frequency.The geometry of proposed antenna is shown in figure1and all the dimensions are given in table1.The specification of the designed antenna is given in table.The antenna was designed and simulated using CST (Computer Studio Technology) software.Based on the above analysis, antenna is designed and fabricated on FR4 substrate with a relative permittivity of 4.3, thickness of 1.6mm and loss tangent of 0.002.The Fabricated antenna is shown in figure 2. The measurements are done using vector network analyzer ZVB 14 which operates at a range of 20MHz-14GHz.

B. Gain
Gain of the antenna is also important result.The simulated gain result is shown in figure 5 and it shows better gain at resonance frequency i.e. at 1.62 GHz gain is 3.41dB, at 2.45 GHz gain is 3.53 dB and at 3.79 GHz gain is 1.2 dB.

D. Envelope Correlation Coefficient
Initially the design started with MIMO antenna with uniform ground was studied, but ECC for this was high so in order to increase the decoupling so that ECC value is nearly equal to zero, the introduction of complementary two turn spiral resonator CSR2 on ground plane is which is also the factor in reducing the overall size of the antenna.ECC value can be calculated from S parameters.The simulated ECC plot result is shown in figure 3. ECC value for 1.62 GHz is 0.2, for 2.42 GHz is 0.25 and for 3.69 GHz is 0.08 which is less than 0.5 is the required ECC value for MIMO antenna.His research interest on design of microstrip antennas Wireless applications.

Fig. 1 .
Geometry of Proposed MIMO Antenna a) Front View, b) Back View Fig. a Fig. b Fig.2.Fabricated Printed Multiband MIMO Antenna (a) Front View, (b) Back View

Fig. 3 .
Fig. 3. Simulated and Measured S11 of Proposed Antenna Simulated and measured isolation loss is shown in figure 4.And at resonance frequency 1.62 GHz, 2.45 GHz and 3.79 GHz isolation loss is almost at -8 dB, -10 dB and -15 dB respectively.The S21 and S12 graph is same for both measured and simulated results.On introduction of DGS structure which causes asymmetricity in the design, there is difference in S11 and S22.

Fig. 4 .
Fig. 4. Simulated and Measured Isolation Loss of Proposed Antenna

Fig. 6 .Fig. 8 .
Fig.6.Simulated Radiation Pattern of the Antenna at 1.62 GHz For First Element, (b) For Second Element