Study on FR4 substrate backed flaring antenna structure for wideband applications

. A class of planar antenna which offers better performance over broad band of frequencies that fits itself in wide range of applications are in need of research. The license free band from 3.1 GHz to 10.6 GHz that do not cause interference to narrowband signals is chosen as operating range of frequencies for the chosen Vivaldi antenna. The novelty of the proposed work consists in the remodeling of a mathematical equations for elliptic antenna structure and the study of the Vivaldi antenna operating over the frequency range between 2.63 GHz to 14.58 GHz that lies in Super High Frequency (SHF) bands – S, C and X bands. The designed antenna is fabricated and its radiation characteristics are studied and analyzed. Purpose. Analysis of the designed antenna on the basis of an elliptical curve mathematical model will allow the antenna to operate over wide band of frequencies thus extending its applications over the areas of application in the areas of satellite communication, military, weather radar, surface ship radar. Methods. The antenna is being designed using HFSS software and fabricated antenna is being tested at anechoic chamber. Results. A mathematical model of a structure of Vivaldi antenna was developed and studied. The performance measures of designed antenna such as return loss, gain, VSWR and radiation pattern are simulated and analyzed. The antenna is being fabricated and tested using vector network analyzer and anechoic chamber. Practical value. The analysis of designed antenna over wide band of frequencies is demonstrated. The further research directions of the designed antenna can be extended for the subsequent implementation of the results in suitable applications.


Introduction
Vivaldi antenna belongs to a class of aperiodic continuously scaled planar antenna structure which is aimed to offer theoretically unlimited instantaneous bandwidth.It can be further referred as fixed radius broadband slot antenna or tapered slot antenna.It gains its name as its structure resembles structure of music instrument such as violin, cello or cross section of a trumpet.The broadband characteristics of Vivaldi antenna make them appropriate for Ultra-Wideband signals.Some of the characteristics of the Vivaldi antenna are low profile due to flat structure, scaled at any design frequency, better impedance matching.Vivaldi antenna can be best fit for broadband directional communication system.Some of the applications are satellite communication, military, weather radar, surface ship radar etc. Ultra-Wideband technology offers high bandwidth over large area of spectrum.UWB offers high data rate, increased immunity to interference, low power spectral density.The band of operation of UWB signals range from 3.1 GHz to 10.6 GHz.This band is license free and offers limitations on radiated power level.Thus, UWB signals do not cause interference to narrowband signals.
Vivaldi antenna is introduced with resonant parallel strip to remove unwanted narrow band.Interference band can be removed by properly tuning resonant parallel strip [1].Antipodal Vivaldi antenna is analyzed for frequency range of 3.1 GHz to 10.6 GHz suitable for wideband applications [2].Vivaldi antenna along with hemicylindrical slots and directors finds its application in microwave imaging [3].The gain and radiation characteristics are improved over wide bandwidth.Reduced size antipodal Vivaldi antenna with a slot resonator acta as effective candidate for Wireless baseband transmission for short range communication [4].Circular slots on the structure of radiator are introduced to reduce Radar Cross Section over wide frequency band of 7 GHz to 11 GHz [5].At the edge of radiating slots, cross shaped resonator and abridged rectangular slots are introduced at the edge of radiating arms to enable the antenna to operate for mm-wave and THz applications.The designed antenna finds its applications in imaging systems, detection of drugs, security applications and wireless communication [6].By introducing slots and making rounds on the edges of the patch, the broadband characteristics of antenna can be achieved with increase in antenna beamwidth and reduction in antenna size.
The designed antenna [7] has potential in the fields of mobile communication and missile borne field.A new dielectric substrate ASTRA MT77 material has been used for the design of Antipodal Vivaldi structure [8].The chosen substrate offers good electrical and nonelectrical characteristics of the antenna.Maximum gain of 9 dBi is achieved over the frequency range of 1 GHz to 13 GHz.Microstrip to Slot line transition [9] is introduced for Tapered Vivaldi slot antenna.The performance of the antenna is analyzed for various transition shapes.By varying types of feeds, integrating various structure of slots, choosing proper substrates, modifying the shape of radiator, the performance of the Vivaldi can be further enhanced [10].Further simulation of Vivaldi antenna for Ultra-wideband radar applications [11] had been performed for frequency range between 3GHz and 11GHz.Additional rectangular slots are further introduced to improve its gain.Various performance Enhancement techniques are discussed such as choosing of substrate, structure of flares, introducing slots and corrugations, balanced and array of antipodal Vivaldi antenna, introducing computational intelligence [12].Proper technique can be chosen based on specific application.
The goal of the paper is to study the basic characteristics of Vivaldi antenna by designing its structure and simulating various parameters such as Return loss, VSWR, Radiation pattern.The antenna is further fabricated to verify the obtained results over wide band of frequencies.This paper carries out a comprehensive study of the characteristics of Vivaldi antenna by defining its structure with mathematical remodeling.The design equations are used for proper design of flaring structure and needed feeding.Return loss, VSWR, gain and radiation pattern are simulated to characterize the designed antenna performance over broad range of frequencies.The designed antenna is fabricated and tested in anechoic chamber.The designed antenna operated over wide band of frequencies that finds its application in the areas of satellite communication, military, weather radar, surface ship radar.

Calculation of relationships and assumptions of the designed antenna.
The radiation flares and feed line are main parts of the designed antenna.The smooth transition is achieved between radiation flares and feed line is achieved using elliptical configuration.Flares take shape of optimum elliptical curves as observed from Fig. 1.This shape offers good broadband characteristics.
x = 1 e Rz + 2 Where R denotes the rate of tapering; C1 and C2 are tapering; Tapering length ~ λ/2; The upper frequency limit is taken to be infinity; The lower frequency limit is computed as follows, Where W-Width of the designed antenna; -Effective dielectric constant which could be computed as,  Assuming the characteristic impedance of the feed line Zo = 50 Ω, the width of the feedline can be calculated using below formula, The designed antenna is of dimension 95 mm x 100 mm.The top layer is made of conducting copper plate of thickness 0.01 mm as seen from Fig. 2. The middle layer is FR4 (Fire Redundant) substrate of dielectric constant 4.4 and thickness of 1.6 mm.The bottom layer is of ground copper plate of thickness 0.01 mm as seen from Fig. 2.

Return Loss Measurement
The designed antenna maintains a reference level of -15 dB over the operating frequency band as observed from Fig. 3.The antenna structure supports -15 dB impedance bandwidth from 2.63 GHz to 14.58 GHz.The range of bandwidth covers Super High Frequency (SHF) bands -S, C and X bands.Wide -15 dB impedance bandwidth is achieved from 2.63 GHz to 14.58 GHz.It denotes loss of power in a signal that is returned in a transmission line due to discontinuity.

Radiation Pattern Measurement
Good Radiation pattern at each operating frequency is observed in Fig. 5. E-plane corresponds to Phi=0 deg (Red Lines) and H-plane corresponds to Phi=90 deg (Green Lines).The copolar components of radiation fields of antenna are analyzed by varying the value of Phi from 0 deg and 90 deg.The radiation pattern is plotted for frequencies 5.6 GHz, 7.3 GHz, 7.8 GHz, 9.1 GHz, 9.8 GHz, 10.7 GHz, 11.5 GHz, 12.2 GHz, 13.1 GHz, 14.08 GHz lies within the frequency range between 2.63 GHz to 14.58 GHz.The maximum increase in gain of 7.0 dB is observed at desired band of frequencies.The figure 6 depicts the peak value of gain in decibels along the direction of antenna main lobe or maximum effective power transmitted in the direction of peak radiation.The fabricated antenna and its experimental setup are shown in figure 7. The fabricated antenna is connected to Vector Network Analyzer for measurement of return loss between 2.63 GHz to 14.58 GHz.The proposed design is validated by fabricating and testing the designed antenna between 2.63 GHz to 14.58 GHz.The frequencies over which the fabricated antenna operates finds its application at various fields of study such as satellite communication, military, weather radar, surface ship radar etc.The measured return loss of fabricated antenna operates on wideband of frequencies but with some deviations in resonant frequency behavior.There are deviations in measured and simulated results which may occur due to fabrication tolerance and experimental errors as observed in Fig 9 .But as it operates over desired bandwidth, it acts as suitable candidate for wideband applications.

Conclusions
Good broadband characteristics is achieved by the shape of optimum elliptical curves which aid smooth transition between radiation flares and feed line.The antenna is designed using HFSS and various parameters of antenna such as return loss, maximum gain, radiation pattern, VSWR are measured over broad range of frequencies.The designed antenna is fabricated and tested in anechoic chamber.Fabricated antenna may show some deviations in result as compared to simulated antenna due to fabrication tolerance and experimental errors.The research can be further extended by introducing additional slots, by using metamaterials, by changing type of feeds and by using arrays.

Figure 2 .
Figure 2. Top Layer Patch (Orange) and Bottom Layer Ground (Green) of the simulated antenna

Figure 3 .
Figure 3.Return loss obtained for the simulated antenna over wide range of frequencies3.2VSWR MeasurementVSWR accounts for transmission line imperfections which cause power loss and reflected energy.The VSWR of the designed Vivaldi antenna in Fig.4decreases obviously in the range of 2.63 GHz to 14.58 GHz.The VSWR is less than 1.40 in the operating frequency band.

Figure 4 .Figure 5 .
Figure 4. VSWR obtained for the simulated antenna over wide range of frequencies

Figure 6 .
Figure 6.Maximum gain at desired band of frequencies

Figure 7 .
Figure 7. Experimental setup to analyse antenna parameters at Vector Network Analyzer.

Figure 8 .
Figure 8.(a) Front view of the fabricated Antenna, (b) Rear view of the fabricated Antenna The fabricated antenna is of dimension 95 mm x 100 mm.The front view and rear view of the fabricated antenna is depicted in Fig.8 (a) and Fig.8 (b).

Figure 9 .
Figure 9. Measured Return loss of the fabricated Antenna

Figure 10 .
Figure 10.Measured VSWR of the fabricated Antenna over wideband of frequencies The 2-D far field simulation pattern is measured at 13 GHz where maximum gain is obtained.The E-plane and H-plane radiation pattern as observed from Fig. 11 (a) and (b) are measured in anechoic chamber with step angle of 9o where the far field distance is marked as 3 m.The strength of waves radiated from the fabricated antenna at different angles are observed.The transmitting antenna is fixed Broadband Horn antenna operating over 800 MHz to 18 GHz while the positioner of the Antenna under test is rotated over the frequency range of 2.63 GHz to 14.58 GHz based on the step angle of 9o to obtain the radiation pattern over desired band of frequencies.

Figure 11 .Figure 12 .
Figure 11.(a), (b) E-plane and H-plane radiation pattern measured at anechoic chamber at step angle of 9 o

Table 1 .
Parameters of the designed Vivaldi Antenna.

Table 2 .
Thickness of each layer of the designed Vivaldi Antenna