The Ninth International Conference on Electronic Measurement & Instruments ICEMI’2009

RCS Reduction of Missile-borne Quasi-traveling Wave Microstrip Antenna

YiMin Li 1,2 HongYi Liu 2 1 Beijing University of Aeronautics and Astronautics, Beijing city, China, Postal Code: 100083

2The 63850 Troop of the Chinese People’s Liberation Army, Phone: +86-0436-3261760, Email: Liuhy197429@sina.com.cn

Abstract–The reduction of a missile-borne microstrip antenna’s radar cross section (RCS) is very important for improving the stealth performance of the missile. In this paper, the principle of antenna’s RCS is firstly analyzed. By taking a kind of missile-borne quasi-traveling wave microstrip antenna as example, a new method is then presented to reduce its RCS significantly and improve its performance as well. The main point of this method is to cut two H-shape slots and install two short-circuit pins in each microstrip unit of the antenna, which reduces the RCS significantly in a wide frequency band, expends the bandwidth, and remains the gain of the antenna unchanged at the same time. Keywords – quasi-traveling wave microstrip antenna, radar cross section RCS , slot, short-circuit pin.

INTRODUCTION

In order to improve the missile breakthrough capabilities, measures are often taken to enhance its stealth performance. Since missile-borne antenna acts as a strong electromagnetic scatter in its operating frequency band, it is very important to reduce the antenna’s radar cross section (RCS). Although the RCS of a missile-borne microstrip antenna can be effectively reduced by increasing the dielectric loss of the substrate and coating the antenna patch to absorb wave, the radiation efficiency and gain of the antenna are decreased, which might even lead to the malfunction of the antenna. The RCS can be reduced in a wide frequency band by loading distributed resistance, for example, loading resistance at the four corners of a microstrip antenna, but it is achieved at the expense of sacrificing the antenna’s radiation efficiency. Although the slots on patch can reduce RCS in a very broad frequency band, they narrow the antenna gain badly [1]. In this paper, the principle of antenna RCS is analyzed firstly. By taking a missile-borne quasi-traveling wave microstrip antenna as an example, a new method is presented to reduce its RCS significantly and improve its performance as well.

THE PRINCIPLE FOR RCS REDUCTION

a. The Analysis of Antenna Scattering

According to the scattering theory, the antenna scattering field can be expressed as

0( ) ( )1

s s m tlc il

alE z E z b E�� �

�� � 1

where ( )slE z is antenna total scattering electric

field; ( )scE z is the scattering electric field when the

antenna is loaded with matched terminal resistance; l�

is the load reflection coefficient; a� is antenna reflection

coefficient; 0mb is the receiving electric field amplitude

when the antenna is in matched state tiE is the antenna

radiation electric field excited by unit input. In (1) Antenna scattering field includes two parts: ( )s

cE z is the antenna structure-mode scattering field which has nothing to do with the antenna load it is the scattering field when the antenna is connected with matched load and its scattering mechanism is as same as that of a

common scattering object 01m tl

ial

b E��� �

is the

antenna-mode scattering field, it results from unmatched terminal load which reflects incident energy. When the antenna load is matched, , 0l c lZ Z� � � incident wave is fully absorbed and no reflection occurs. In this situation, the scattering field only includes structure-mode, it is decided by the antenna type, structure, material, size and some other physical parameters, is also decided by the frequency, polarization and direction of the incident wave.

If antenna is connected with unmatched load, when the incident wave reaches to the antenna port, part of the energy will be reflected back to the load [2]. This process will repeat many times. The process can be expressed as:

20 0

1[1 ( ) ( ) ]1

m m ml a l a l a

l a

b b�� � � � � � �� � � � � ��� �

(2) The magnitude of the scattering wave results from

load reflection is 0 1m l

l a

b�

� � �.Being connected with

arbitrary load if the incident electric field is tiE the

radiation electric field at antenna port is the second part in (1) that is, the antenna-mode scattering field. Rewrite 1 as

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The Ninth International Conference on Electronic Measurement & Instruments ICEMI’2009

( ) ( ) ( )s s al c lE Z E Z E Z� � 3

Equation (3) shows that the antenna total RCS is the sum of the structure-mode RCS and the antenna-mode RCS in according to the superposition phase, that is:

js e e �� � �� � 4

If antenna is connected with matched load, the antenna-mode RCS 0e� � so the antenna RCS is

only decided by structure-mode RCS s� .This paper focuses on how to reduce antenna structure-mode RCS.

b. The Principle of RCS Reduction

Microstrip antenna RCS is

2 2 21 1 2 2 0, ,uv s s r u vR Z e G G� � � � � ��

5 where uv� is the microstripe antenna RCS,

incident wave is in 1 1,� � direction and u polarization,

scattering wave is in 2 2,� � direction, v polarization

0 is the incident wave length sZ is the scattering

impedance; sR is the real component of sZ ; 1 1,uG � �

is the u polarization gain; 2 2,vG � � is the v

polarization gain; re is antenna radiation efficiency. According to 5 , microstripe antenna RCS reduction

can be achieved by reducing the radiation efficiency,

gain and 2s sR Z .Although the microstrip antenna

RCS reduction can be obtained by coating loss dielectric, increasing substrate loss and loading distributed resistance, these methods will badly affect the antenna gain, bandwidth and radiation efficiency[2].Thus, the reasonable means to reduce the antenna RCS is to

minimize 2s sR Z .Slots on the patch of microstrip

antenna can introduce current mutation and the effect is equal to impedance loading, which can adjust

2s sR Z in 5 .After slotting, because of cutting off

the current path, the current will become curved around these slots[3], and the phases of some currents are opposite, which can offset some scattering. Slotting also introduces a reflection wave source when the phase is opposite between the scattering wave and the reflecting wave. The installation short-circuit pins can provide susceptance, which can increase the imaginary

component of sZ and decrease 2s sR Z , so the

antenna RCS can be reduced.

AN EXAMPLE OF RCS REDUCTION

Fig.1 shows the current distribution of a missile-bone quasi-traveling wave microstrip antenna. The antenna is made up of ten rectangle patches and the center operating frequency is 35GHz. The substrate of the antenna is RT/duroid5880,the permittivity 2.2r� � ,the

thickness 0.254h mm� ,each rectangle patch width 3.4W mm� and length 2.74L mm� .To make the

antenna maximum gain direction 30 , 0� �� � � � �the microstrip lines that link all rectangle patches has the length 1.1d mm� width 1 0.5w mm� [4] .

Fig.1. The current distribution of the original quasi-traveling wave

microstrip antenna

The quasi-traveling wave microstrip antenna is an antenna array. According to the principle of antenna pattern product, firstly we study how to reduce the RCS of a single rectangle patch microstrip antenna, and then adjust the size of the microstrip line between patches to make the antenna pattern similar to that of the original antenna. For a single rectangle patch microstrip antenna, we can reduce RCS by slotting on patch and adding short-circuits pins [5]. But it is very difficult to find out the location and shape of the slots and the locations of the short-circuit pins. Theoretically we can apply cavity-mode method to calculate, but in fact deduction of the analytical solution is very difficult, in most cases it is impossible. Therefore, to solve the problem, application of numerical calculation and experiment is more convenient.

Fig.2. The improved patch

As shown in Fig.2 in order to reduce the RCS on condition that the antenna gain and VSWR worsen, we cut six slots and install two short-circuit pins. It is found

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that y-direction slotting can lower the resonant frequency, the longer the slot the more RCS reduction and the more frequency decrease; x-direction slots have no effect on the resonant frequency, but can improve the antenna gain; when the location of the two short-circuit pins move along the x-axis, the antenna resonant frequency will decrease, but the gain will increase; when the distance between the two pins increases, the antenna resonant frequency does not change, but the RCS significantly reduces. After repeated tests, the best locations for the slots and the pins are found out. The center locations of two x-direction slots are (3.05,0.2), (3.05, -0.2), the length 0.9mm, the width 0.2mm; the central locations of four y-direction slots are (2.7,0.8), (2.7, -0.8), ( 3.4, 0.8), (3.4, -0.8), the length 1mm, the width 0.2mm; the locations of two short-circuit pins are (2.3,0.8), (2.3, -0.8) .On other nine patches of the antenna, we cut slots and install short-circuit pins in the same way. In order to keep the antenna pattern unchanged in operating direction ( 30 , 0� �� � � � � ), the length of microstrip lines are adjusted to 1.2mm. The current distribution of the improved antenna is showed in fig.3. Comparing Figure 1 and Figure 3, we can see that the current of the improved antenna mostly concentrates around the pins and slots; it shows that the current has more turns than the original antenna.

Fig.3. Current distribution of improved antenna

The monostatic RCS is shown in Fig.4 and Fig.5.Shown as Fig.4, when the plane wave incidence angle is 30� �� � 45� � � ,the transmitter and receiver are � polarized, the monostatic RCS is greatly reduced in 33GHz to 35GHz band, especially at the 33.2GHz with the reduction up to 27dB; However, the RCS remained essentially unchanged above 35GHz. Shown as Fig.5, when the plane wave incidence angle is

30� �� � 45� � � , the transmitter and the receiver are � polarized, and the RCS reduction is 7.44dB at 35GHz.The RCS is also significantly reduced from 33.3GHz to 34.5GHz and 35GHz to 35.8GHz.

33.00 33.50 34.00 34.50 35.00 35.50 36.0036.50 37.00Freq [GHz]

-80.00

-70.00

-60.00

-50.00

-40.00

-30.00

-20.00

The original antenna

The improved antenna

dB(MonostaticRC

STheta)

Fig.4. Monostatic RCS curve 0� � �

33.00 33.50 34.00 34.50 35.00 35.50 36.00 36.50 37.00Freq

-70.00

-65.00

-60.00

-55.00

-50.00

-45.00

-40.00

-35.00

-30.00

dB(MonostaticRC

S)

The improved antenna The original antenna

Fig.5. Monostatic RCS curve 45� � �

As can be seen in Fig.6 and Fig.7, the improved

antenna performance is significantly enhanced. This shows that the antenna impedance characteristics is changed by the short-circuit pins and the slots, and they match the microstrip lines better.

33.00 33.50 34.00 34.50 35.00 35.50 36.00 36.50 37.00Freq [GHz]

-30.00

-25.00

-20.00

-15.00

-10.00

-5.00The improved antenna

The original antenna

dB(S11)

Fig.6. Antenna S11 curve

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The Ninth International Conference on Electronic Measurement & Instruments ICEMI’2009

33.00 33.50 34.00 34.50 35.00 35.50 36.00 36.50 37.00Freq [GHz]

1.00

1.10

1.20

1.30

1.40

1.50 1.60

1.70

1.80

1.90

2.00

2.10

The improved antenna

The original antenna

VSWR

Fig.7. VSWR curve

Fig.8 and Fig.9 show the antenna radiation pattern. As can be seen in the figures, the improved antenna radiation pattern remains essentially unchanged.

-28.00

-16.00

-4.00

8.00

90

60

30 0

-30

-60

-90

-120

-150 -180

150

120

The improved antenna

The original antenna

Fig.8 The radiation pattern of E plane

-13.00 -11.00 -9.00 -7.00 -5.00 -3.00 -1.00 1.00

90

60

30 0

-30

-60

-90

-120

-150 -180

150

120

The improved antenna

The original antenna

Fig.9. The radiation pattern of H plane

CONCLUSION

The theoretical analysis and experimental results have proved that the slots [5] and the short-circuit pins can significantly reduce the RCS of traveling-wave microstrip antenna in a particular direction at the premise of keeping the antenna performance basically unchanged. Since double H-shaped slotting has several adjustable parameters and makes current turning many times [6], cutting double H-shaped slots is a good way to reduce RCS.

Theoretical analysis and experimental results show that symmetrically installing two short-circuit pins on both sides of the X-axis not only make it easy to adjust the resonant frequency but also reduce the RCS when the pins are separated further from the X-axis.

It is found in the experiments that the RCS reduction and gain change trend of the improved traveling wave microstrip antenna is similar to that of the units which constitute the traveling wave microstrip antenna. It results from the pattern product theory. Thereforetrying to reduce the RCS of a quasi-traveling wave microstrip antenna which is composed of many uniform unites, we can first adjust one unit, then adjust the sizes of the microstrip lines which connects the units.

ACKNOWLEDGMENT

I am very grateful to Mr. Feng Wei for his revising my paper.

REFERENCES

[1] Ding Jun, Cheng Chunxia, Guo ChenJiang. A Method for Microstrip Antenna RCS Reduction [J]. Computer Simulation2008 9 130-133.

[2] Fang DaGang. Antenna Theory and Microstrip Antenna [M].Beijing: Science Press, 2006.

[3] Zhong ShunShi. Microstrip Antenna Theory and Applications [M].Xi An Xi’an Electronic Science and Technology University Press 1991.

[4] Liu JingPing,Wu DaJun. The Design of Conformal Antenna for Millimeter Wave Fuze [J]. Guidance and Fuze 2008 (2) 14-17.

[5] YangYi, Gong ShuXi, He XiuLian.The of Gap Loaded Methods for Microstrip Antenna RCS Reduction [J]. Fire Control Radar Technology, 2006, 3(35): 1-3.

[6] DR Jackson. The RCS of a Rectangular Microstrip Patch in Substrate-Superstrate Geometry [J]. IEEE Trans on AP (S0018-926X). 1990, 38(1): 2-8.

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