Analysis of equivalent antenna based on FDTD method

2014-02-15YunxingYANGHuichangZHAOCuiDI

Defence Technology 2014年3期

Yun-xing YANG,Hui-chang ZHAO*,Cui DI

School of Electronic and Optical Engineering,Nanjing University of Science and Technology,Nanjing 210094,China

Yun-xing YANG,Hui-chang ZHAO*,Cui DI

School of Electronic and Optical Engineering,Nanjing University of Science and Technology,Nanjing 210094,China

An equivalent microstrip antenna used in radio proximity fuse is presented.The design of this antenna is based on multilayer multipermittivity dielectric substrate which is analyzed by fnite difference time domain(FDTD)method.Equivalent iterative formula is modifed in the condition of cylindrical coordinate system.The mixed substrate which contains two kinds of media(one of them is air)takes the place of original single substrate.The results of equivalent antenna simulation show that the resonant frequency of equivalent antenna is similar to that of the original antenna.The validity of analysis can be validated by means of antenna resonant frequency formula.Two antennas have same radiation pattern and similar gain.This method can be used to reduce the weight of antenna,which is signifcant to the design of missile-borne antenna.

Equivalent microstrip antenna;FDTD;Multi-permittivity dielectric

1.Introduction

In recent years,antenna has drawn more and more attention with the increase in demand for military antennas[1,7]. Especially for missile-borne antenna,it is required to be small in size and light in weight.In 1953,Deschamps proposed the conception of microstrip antenna[2].The microstrip antenna has many advantages,such as light weight, small size,and planar structure.Lots of work have been done on its miniaturization[3,4].But few research focused on decreasing the weight of antenna and leaving its basic parameters unchanged under the conditions of same size and thickness.

In this work,another model is proposed.The aims of this work are:a)to preserve the shape of antenna;b)to preserve the of antenna electromagnetic parameters;and c)to decrease the weight of antenna.For this,a substrate with low permittivity and high permittivity media is taken the place of the original substrate,in which air is used as a lower permittivity media(high permittivity media can be any media as its permittivity is higher than original media).The introduction of the air media can effectively decrease the weight of antenna.The analysis of equivalent antenna is based on FDTD.

2.Equivalent antenna structure and FDTD analysis

In1966,K.S.Yee proposed the theory of FDTD[5].FDTD is a novel method which can be used to intuitively and succinctly describe Maxwell's equations.In this method,the electric feld and magnetic feld are included in a threedimensional model(Yee cell).It is used for solving all the problem of electromagnetic feld.A circular antenna is easily integrated into a warhead of bomb compared to rectangle antenna.Therefore the formulae in Refs.[6,7] should be amended in the cylindrical coordinate system(see Fig.1).

Two medium exist in Yee grid(the permittivities are εr1，εr2). From Ampere circuital theorem,we have,

Assuming that the magnetic conductivities of two media are the same.It can be known from the boundary condition of electromagnetic feld that the normal component of magnetic feld on the Interface between the two media is continuous,In cylindrical coordinate system,Eq.(3)could be rewritten as

Fig.1.Yee cell.

In FDTD iteration.

Fig.2.Structure of substrate.

The right side of Eq(5)can be extended to Substituting Eqs.(4),(6)and(7)into Eq.(3),we have,

Fig.3.Resonant frequencies of three antennas.

After the simplifcation,the equivalent permittivity can be obtained

Through the analysis mentioned above,the structure of antenna substrate can be designed,as shown in Fig.2.

3.Discussion of simulation

Through the above analysis,FR4 is chosen for original substrate.The basic parameters of antenna are εr=4.4, R=15 mm,h=2 mm,r=13 mm.Eq(10)shows the relationship between two medium.Table 1 lists the parameters of equivalent antennas

Table 1Parameters of equivalent antennas.

Fig.4.S11 with different feed feint.

Fig.5.Original radiation pattern.

Fig.6.Equivalent radiation pattern.

Fig.3 shows that the resonant frequency of original antenna is 3.10 GHz,and the resonant frequencies of the equivalent antennas A and B are also remain unchanged.It can be concluded from the resonant frequency formula that the permittivity is the only factor which has effect on resonant frequency when the radius of radiation patch and the working mode remain the same.So two media can be treated as a single medium which has a same permittivity as that of original medium.The results prove the accuracy of FDTD analysis. Because the position of feed point was not changed in simulation,the S11 parameters ofthe equivalentantenna deteriorated compared to the original antenna.So the equivalent antenna is not exactly equal to the original antenna.The parameters can be improved by changing the position of feed point.Fig.4 shows the S11 parameters after changing the position of feed point.

Fig.5 and 6 show the radiation patterns of equivalent antenna and original antenna.It can be seen from Fig.5 and 6 that the direction and gain of equivalent antenna are similar to those of the original one.This shows that the equivalent antenna can replace the original antenna.

4.Conclusions

A method for decreasing the weight of antenna was proposed in the paper.By means of analysis based on FDTD,air medium and a higher permittivity medium are introduced to take the place of original substrate.The simulation results show that resonant frequency,radiation pattern and gain of equivalent antenna did not change.The air media can be used to reduce the weight of antenna.

Acknowledgement

The author would like to thank the National Natural Science Foundation of China(Grant No.6111168)for the support.

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Received 14 January 2014;revised 17 February 2014;accepted 19 March 2014 Available online 31 July 2014

*Corresponding author.

E-mail address:zhaohch@mail.njust.edu.cn(H.C.ZHAO).

Peer review under responsibility of China Ordnance Society.

Defence Technology

2014年3期