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Hypersonic vehicles oftentimes experience diminished electromagnetic communication and

sensing capabilities due to dense plasma that forms around the vehicle during re-entry.

They can block or attenuate the radio signals, moreover, depending on the electron

density, it will severely affect the performance of the communication antennas including

the Global Position System (GPS) antenna. When operating in a plasma environment, these

antennas’ performance is affected by (i) signal loss / attenuation through the plasma

layer, (ii) polarization mismatch and (iii) impedance mismatch; (ii) and (iii) further reduce

the antenna’s operational bandwidth. While little can be done to reduce signal loss,

proper design of the antenna may mitigate (ii) and (iii). This paper proposes a novel

double-feed dual band GPS antenna with AR bandwidth exceeding 150 MHz in both the L1

and L2 bands, thereby rendering making the antenna polarization highly insensitive to

plasma loading. Besides, to achieve acceptable impedance bandwidth with various plasma

environment, a feedback system including a sensing antenna, controller and tunable

matching network is designed.

Abstract

During the hypersonic re-entry procedure of a vehicle, a continuously

changing plasma layer is formed around the vehicle and block the

electromagnetic communication.

• Continuous plasma layer could be discretized as several lossy dielectric

layers, the relative permittivity for the dielectric is given by the

classical equation

Introduction

Result for the antenna in plasma environment without matching network

Simulation result

In order to mitigate the polarization and impedance mismatch, the

following antenna system is proposed.

• Broadband branch-line coupler

• Dual band antenna with two capacitive feeds

• Tunable matching circuit based on varactor diode

• Single band sensing antenna

Models

Conclusion

• An ultra-wide axial ratio bandwidth dual band GPS antenna is proposed

to compensate the polarization mismatch caused by the dynamic plasma

environment, and a self-matching mechanism is implemented by a

feedback system including tunable matching network and a sensing

antenna.

• Future work will be further optimized the antenna, study the response

time for the self-adjusting process, and apply some alternatives, like

the MEMS varactors to the tunable matching network.

References

[1] Z.H. Qian, R.S. Chen, K.W. Leung, et al. Progress Electromagn Res 52(2005).

[2] L. Zhao, B. W. Bai, W. M. Bao, and X. P. Li, Int. J. of Antennas Propag. (2013) 823626

[3] Li, J-T., and L-X. Guo. J. of Electromagn. Waves and Appl. 26.13 (2012): 1767-1778

[4] K. L. Wong, Compact and Broadband Microstrip Antennas, New York, 2002: Wiley

Dept. of Electrical Engineering and Computer Science, University of Michigan

Xiuzhang Cai a, Eric Michielssen a, Xiaoying Gu a and Amir Mortazawi a

Adaptively matched GPS antenna for plasma environments

ε = (1 −ωp2

ω2 + ν2+ j

ωp2 νω

ω2 + ν2)ε0

(a) 휀𝑟′ for different plasma layers (b) tan 𝛿 for different plasma

Fig. 1: One case of plasma modeling in discretized lossy dielectric at 1.227GHz

1.10 1.13 1.15 1.18 1.20 1.23 1.25 1.28 1.30Freq [GHz]

-50.00

-40.00

-30.00

-20.00

-10.00

0.00

dB

(St(

Cylin

de

r2_

T1

,Cylin

de

r2_

T1

))

HFSSDesign1Return loss ANSOFT

m1

m2

m3

m4

Curve Info

dB(St(Cylinder2_T1,Cylinder2_T1))Setup1 : Sw eep1

Name X Y

m1 1.2200 -9.7155

m2 1.2600 -46.0949

m3 1.2260 -11.4569

m4 1.3000 -24.0920

1.10 1.13 1.15 1.18 1.20 1.23 1.25 1.28 1.30Freq [GHz]

0.00

10.00

20.00

30.00

dB

(Axia

lRa

tio

Va

lue

)

HFSSDesign1Axial Ratio ANSOFT

m2m1

m3

m4

Curve Info

dB(AxialRatioValue)Setup1 : Sw eep1Phi='0deg' Theta='0deg'

Name X Y

m1 1.2760 1.2019

m2 1.2900 3.1073

m3 1.2620 2.9841

m4 1.2260 10.1892

1.10 1.13 1.15 1.18 1.20 1.23 1.25 1.28 1.30Freq [GHz]

-37.50

-25.00

-12.50

0.00

dB

(St(

Cyl

ind

er2

_T

1,C

ylin

de

r2_

T1

))

HFSSDesign1Return loss ANSOFT

m1

m3 m4

Curve Info

dB(St(Cylinder2_T1,Cylinder2_T1))Setup1 : Sw eep1

Name X Y

m1 1.2260 -22.6350

m3 1.1800 -10.2265

m4 1.2900 -10.1453

1.10 1.13 1.15 1.18 1.20 1.23 1.25 1.28 1.30Freq [GHz]

0.00

10.00

20.00

30.00

dB

(Axia

lRa

tio

Va

lue

)

HFSSDesign1Axial ratio ANSOFT

m1m3m2

Curve Info

dB(AxialRatioValue)Setup1 : Sw eep1Phi='0deg' Theta='0deg'

Name X Y

m1 1.2280 0.8512

m2 1.2160 2.9369

m3 1.2420 2.9255

Fig. 2: Comparison of the antenna performance in free space and plasma environment for one wide impedance band GPS antenna

• The performance for the communication antenna will be severely

influenced in the presence of plasma

• Polarization mismatch

• Impedance mismatch

Fig. 3: Block diagram for the antenna system with matching network and feedback circuit

Fig. 4: Top view for the broadband branch-line coupler

Fig. 5: Side view and top view for the dual band GPS antenna

Fig. 6: Tunable LC matching network

(a) Side view with the plasma model (b) Top view

(a) Symbolic representation (b) Schematic

Result of the antenna after applying the matching network

Impedance shifting for the GPS antenna at 1.227GHz and sensing antenna

at 1.3GHz in Smith chart

(a) Total return loss at L2 band (b) Total return at L1 band

(c) Axial ratio at L2 band (d) Axial ratio at L1 band

(e) RHCP realized gain at L2 band (f) RHCP realized gain at L1 band

Fig. 7: Simulation result for the antenna system in different plasma environment without matching network

Plasma # RL not matched(dB) RL matched(dB) Bandwidth(MHz)

1 -11.9 -19.7 35

2 -6.6 -21.5 35

3 -5.6 -21.1 42

4 -5.8 -20.3 55

5 -5.9 -20.7 49

6 -6.3 -20.6 46

7 -8.6 -20.8 43

8 -13.0 -19.7 40

Table 1: The return loss and bandwidth comparison at L2 band for the

matched and not matched GPS antenna with variable plasma environments

From the figure, the shifted impedance for the GPS antenna at 1.227GHz

and that of the sensing antenna at 1.3GHz has a linear relationship, by

linear regression method, their relation could be revealed as:

𝑍𝐺𝑃𝑆 = aZt + b

With a=1.1318+j*0.6696, b=-0.0506-j*0.628, and with mean squared

error(MSE) of 1.134 × 10−5 + j ∗ 5.13 × 10−4 for normalized impedance.

Thereby, the impedance for the GPS antenna could be obtained from the

return loss of the sensing antenna. Then, the required capacitance value

for the tunable matching network and the corresponding bias voltage can

be determined.

Fig. 9: Matching range for the designed tunable matching network

Fig. 8: Impedance shifting for both GPS antenna and the sensing antenna

Fig. 10: Capacitance of the varactordiode with different bias voltage

Fig. 11: Total return loss for the model with and without matching

(a) model With plasma 4 (a) model With plasma 7

(a) In free space (b) In plasma environment