Design of Optically Transparent Antenna with Directional Radiation Patterns

An optically transparent antenna with directional radiation pattern is proposed in this paper. This antenna consists of a radiation patch above the transparent glass substrate and a frequency selective surface acting as the ground of microstrip antenna on the back of the glass. The frequency selective surface (FSS) performs well as ground in the working frequency range to achieve directional radiation pattern. A SMA connector is used to feed the antenna. The antenna can operate at the on-board dedicated short-range communication band from 5.725GHz to 5.85GHz. Simulations andmeasurements are carried out to study the antenna performances in terms of impedance matching, gain, and radiation patterns. Both of the simulation and measurement results are shown to illustrate the good performance of the proposed antenna. It is very suitable for car network communication scene.


Introduction
At the end of the last century, NASA scientists pioneered the idea of using an optically transparent film to create an antenna [1].In 2000, the Outaleb research team at Rennes First University in France first proposed the idea of using the ITO thin films to produce optically transparent antennas [2].So far, optically transparent antennas have been studied for more than a decade.As we know, an optically transparent antenna can be integrated on the surface of other items with almost completely transparent characteristic and will not affect the appearance of goods, which is the unique advantage of optically transparent antennas [3].
At present, there are four kinds of materials that can be used to make optically transparent antennas: the first kind is a transparent metal oxide film, for example, indium tin oxide film (ITO film).In the antenna design, a compromise must be made between the light transmittance and surface impedance [4][5][6].The second is the AgHT series multilayer thin film.Compared with the ITO film, the AgHT film has better conductivity and worse transmittance [7].The third is ultrathin metal film made of copper, silver, gold, and other metal conductors.Antennas made of this material have high radiation efficiency.But the light transmittance is pretty low [8].
The fourth is metal mesh antenna, which has higher radiation efficiency, whose light transmittance is higher than that of the ultrathin metal film.But the requirements for the processing technology of this material are very high [9][10][11].Considering the light transmittance, conductivity, and processing technology of the four materials, ITO film is the best choice.ITO film is also the most widely used material in the research and design of optically transparent antennas.
During a ten-year development, the study of materials that can be used to make transparent antennas has been very comprehensive and extensive.The basic principles of optically transparent antennas had been mastered.Nowadays, the applications of optically transparent antennas in actual scenes attract more attention.There are already some papers that proposed optically transparent antenna structures for a variety of practical scenarios [12][13][14][15].An optically transparent GPS antenna integrated in a car's windshield was proposed by Laser Antenna in Australia [16].The optically transparent RFID reader antenna for smart fitting room application was proposed by the same authors of this paper [17].But all of the optically transparent antennas cannot achieve directional radiation [18], which makes these antennas have low effective utilization of radiated power, and hence confidentiality is not good.This paper proposed a structure to realize the directional radiation of optically transparent antennas and designed an optically transparent antenna with directional radiation patterns for car network application scenarios.The proposed antenna is a microstrip antenna with the radiation patch made of transparent ITO film, the substrate made of transparent glass, and the ground of the antenna made of a square ring band resistance frequency selection surface (FSS).The FSS ground has good light transmittance and can reflect the electromagnetic waves radiated from the antenna totally and achieve directional radiation.The antenna works at the on-board dedicated short-range communication band (5.725-5.850GHz).Simulation and measurement results are shown to illustrate the good performance of the antenna.The results showed that the performance of the proposed antenna is pretty good and it is suitable for car network application scenarios.

FSS Design
In this proposed antenna design, the square ring FSS structure is used to replace the metal ground of the microstrip antenna, so that the ground of the antenna can reflect the electromagnetic waves in the antenna operating frequency band and has good light transmittance at the same time.
Figure 1 shows the structure of the FSS unit.The main structure parameters of the square ring FSS are the length of the square (wfs), the width of the square ring (), and the distance between the FSS units (2 * dw).These three parameters together determine the performance of FSS.The main factor determining its total reflection band is the length of the square (wfs); when the circumference of the square ring FSS unit is close to one wavelength of the received electromagnetic wave, the frequency selective surface will reflect the electromagnetic wave back.
The FSS is simulated and optimized by CST electromagnetic simulation software.The square ring unit is simulated under the periodic boundary condition, which means the simulated FSS has an infinite number of FSS units.The FSS is excited by an incident wave which is one of the excitation ports in CST software.11 of the square ring band stop FSS under the periodic boundary condition is shown in Figure 2. When wfs = 8.4 mm,  = 0.2 mm, and dw = 2 mm, the FSS has good total reflection characteristic in the short-range communication band.Since the width of the square ring is very small, the metal part in the FSS structure is relatively less, which means the FSS has good light transmittance.

Antenna Design
The structure of the proposed antenna is shown in Figures 3  and 4. Figure 3 shows the overall view of the whole antenna and Figure 4 shows the side view of the proposed antenna.The top layer is the radiation patch made of ITO film, whose surface impedance is 7 Ω/sq.The length of the patch is lp and the width of the patch is wp.The middle layer is the transparent glass substrate, whose permittivity is 4.6, loss tangent is 0.0036, and thickness is 1 mm.The length of the glass substrate is Lg and the width of the glass substrate is Wg.The bottom layer is the 5 × 5 square ring FSS structure, which is metal sputtered onto the glass substrate.The feed point is put at the midpoint of the lower side of the patch.A metal sheet was put on the FSS to achieve better feeding,   whose length is lf and width is wf.The designed antenna is expected to work at the on-board dedicated short-range communication band from 5.725 GHz to 5.85 GHz.Figures 5 and 6 show the fabricated proposed antenna.The antenna is fed by a SMA connecter with 50 Ω impedance.The finally chosen dimensions of the proposed antenna are illustrated in Table 1.It is obvious that the antenna has magnificent light transmittance.We have measured the light transmittance of the antenna, which is about 80% in the area of the patch and 84% around the patch.

Parameter Study of the Antenna
Parametric studies were carried out to characterize the proposed antenna, where, unless otherwise stated, only a single parameter was changed, while the others were kept constant as in Table 1.In this antenna design, the length and width of the radiation patch determine the resonant point and impedance bandwidth.Figure 7 shows that the resonant point of the antenna decreases as the length of the radiating patch (lp) increases.When lp = 8.85 mm, the antenna resonates at 5.8 GHz, and the working frequency band whose 11 is below −10 dB is 5.725-5.85GHz. Figure 8 shows that, with the width of the patch (wp) increasing, the antenna resonant frequency is declining.When wp = 12 mm, the antenna resonates at 5.8 GHz and can completely cover the on-board short-range communication frequency band.

Square Ring FSS Design (Main Parameters: wfs, 𝑤, and dw).
In this design, the square ring frequency selective surface is used to replace the ground of the microstrip antenna, which can reflect the electromagnetic wave back in the antenna operating frequency band.Figure 9 shows that as the length of the square (wfs) increases, the resonant frequency band of the antenna decreases.When the circumference of the square ring FSS unit is about the resonant wavelength of the microstrip antenna on board, that is, when wfs = 8.4 mm, the antenna just resonates at 5.8 GHz.11 are less than −10 dB in the on-board short-range communication band (5.725-5.85GHz).
Figure 10 shows that, with the increase of the width of the square ring (), the resonant frequency is increasing.When  = 0.2 mm, the antenna just resonates at 5.8 GHz, and the frequency band is in the range 5.725-5.85GHz and 11 are less than −10 dB, which meet the requirements of car network communication.
Figure 11 shows that the distance between the FSS units (2 * dw) the antenna's impedance bandwidth becomes narrow, and the resonant frequency of the antenna increases.When dw = 2.0 mm, the antenna resonates at 5.8 GHz and 11 are less than −10 dB over the on-board shortrange communication band (5.725-5.85GHz).

Simulation and Measurement Results
The measurements were carried out in a chamber.

𝑆-Parameter.
Figure 12 shows the simulated and measured 11 of the proposed antenna.The solid line is the simulation result, and the dashed line is the measurement result.It can be seen that the measured result agrees well with the simulation result, and the trends of the two are the same.In the on-board short-range communication band (5.725-5.85GHz), 11 are less than −10 dB, which means the antenna has good impedance matching characteristic and can meet the requirement of car network communication.

Gain.
Figure 13 shows the gain of the designed antenna.The solid line is the actual measurement result, and the dotted line is the simulation result.The overall trends of the two lines are basically the same; the difference between them is due to fabrication error and ITO model accuracy in simulation software.It can be seen that, in the on-board short-range communication band (5.725-5.85GHz), the radiation gain of the antenna is around −6 dBi.This is because the patch of the antenna is made of indium tin oxide film, and it has a high surface impedance, which means the conductivity is not very good.Thus, the radiation gain of the antenna produced by this material is low.However, the −6 dBi gain is also enough for our specific application, which is on-board short-range communication.

Radiation
Patterns.Figures 14,15,16,17,18,and 19 show the radiation patterns of the designed antenna in XOZ plane and YOZ plane in the on-board short-range communication band (5.725-5.85GHz).The solid line is the measurement result and the dashed line is the simulation result.It can be seen that, in the working frequency band 5.725-5.85GHz, the antenna has good directional radiation performance.The maximum radiation direction is + direction, and it is almost perpendicular to the plane where the antenna is located.these differences.First, the ITO model in CST is not very accurate, and thus there are some differences between the simulation and the actual prototype.Second, the fabrication error can also bring about differences.

Conclusion
In this paper, an optically transparent antenna with directional radiation pattern is designed.The patch of the antenna is made of indium tin oxide film whose surface impedance is 7 Ω/sq.The substrate is transparent glass with a dielectric constant of 4.6 and thickness of 1 mm.In this antenna structure, a 5 * 5 square ring FSS is selected to replace the ground of a microstrip antenna, so as to realize the optical transparency and directional radiation characteristics.The simulation and measurement results show that the antenna structure has good 11, gain, and radiation performances in the onboard short-range communication band (5.725-5.85GHz).The directional radiation direction is perpendicular to the antenna.When the antenna is applied to the windows of a car, it is very suitable for the complex and changeable electromagnetic environment.Thus, the proposed antenna is very suitable for car networking scenes.

Figure 1 :
Figure 1: The structure of the unit of the square ring FSS.

Figure 2 :Figure 3 :
Figure 2: 11 of the square ring FSS under the periodic boundary condition.

Figure 4 :
Figure 4: The side view of the proposed antenna.

Figure 5 :
Figure 5: Overall view of the fabricated proposed antenna.

Figure 6 :Figure 7 :
Figure 6: Side view of the fabricated proposed antenna.

4. 1 .
Radiation Patch Design (Main Parameters: lp and wp).In order to verify the influence of the parameters of the designed antenna on the overall characteristics of the antenna, we studied the parameters of the antenna in CST electromagnetic simulation software.

Figure 8 :Figure 9 :
Figure 8: The optimization of width of the radiation patch.

Figure 10 :Figure 11 :
Figure 10: The optimization of width of the square ring.

Figure 19 :
Figure 19: Radiation pattern of the antenna in YOZ plane at 5.85 GHz.

Table 1 :
Parameters of the proposed antenna (units: mm).