Comparative study of stripline dipole antenna on semiinfinite and lens ...

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Abstract—The input impedance and radiation characteristics of a stripline dipole antenna on different substrate schemes are presented. Two radiation ...
The 8th European Conference on Antennas and Propagation (EuCAP 2014)

Comparative Study of Stripline Dipole Antenna on Semiinfinite and Lens Substrates at Terahertz Frequency Truong Khang Nguyen1, 2 and Ikmo Park2* 1

Division of Energy Systems Research, Ajou University 5 Woncheon-dong, Youngtong-gu, Suwon 443-749, Republic of Korea 2

Department of Electrical and Computer Engineering, Ajou University 5 Woncheon-dong, Youngtong-gu, Suwon 443-749, Republic of Korea * [email protected]

Abstract—The input impedance and radiation characteristics of a stripline dipole antenna on different substrate schemes are presented. Two radiation environments, a semi-infinite galliumarsenide substrate (hereafter called a “semi-infinite substrate”) and a thin gallium-arsenide substrate backed by a silicon lens (hereafter a “lens substrate”), were examined, and the antenna performances were analyzed in a broad frequency range up to 5.0 THz. The semi-infinite substrate approach is useful in the initial investigation of the antenna characteristic itself while saving a significant amount of computational time. The study provides good guidelines for the design of a terahertz photoconductive antenna in that the antenna effect and the lens substrate effect can be individually characterized and optimized for the best possible performance. Index Terms—Terahertz antenna, stripline dipole antenna, semi-infinite substrate, extended hemispherical lens, highpermittivity material.

I.

INTRODUCTION

The photoconductive (PC) antenna, which generates and detects terahertz pulses by transient photocarriers induced by ultrafast laser pulses, is one of the most commonly used components for terahertz wave generation as well as detection [1, 2]. A PC antenna is typically patterned on a high-resistivity silicon lens substrate and used in a terahertz time-domain spectroscopy (THz-TDS) system. The performance of a PC antenna depends mainly on the substrate material, geometry of the active area, geometry of the antenna, and the excitation laser pulse [3–8]. Consequently, the overall output of a THzTDS using a PC antenna can be considered as a spectral convolution of the responses of (i) antenna, (ii) lens substrate, and (iii) photoconductive material to the optical sources. The first two responses are related to the coupling efficiency of THz waves between the antenna and the free space, and these could be improved by shaping the antenna and lens substrate [9, 10]. The last one is related to the laser-to-electrical powerconversion efficiency, which is important but very difficult to estimate accurately [11]. This efficiency is very small and typically depends on factors such as photoconductive material, bias condition, excitation power, geometry of the active area, etc. In our study, we only consider the responses of the antenna and lens; accordingly, a constant source, instead of a time-

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varying source, is used to drive the signal in the excitation gap of the studied antenna. The properties of antennas on semi-infinite substrates have been extensively studied for many years [12–15]. However, there have been no detailed studies analyzing a traveling-wave antenna on a semi-infinite substrate made of a highpermittivity material. In this paper, we examine a travelingwave stripline dipole antenna designed on a semi-infinite substrate and a lens substrate, i.e., made of high-permittivity materials of gallium arsenide and silicon. The antenna characteristics are then fully analyzed and compared by employing the EM simulators. The study of an antenna on a semi-infinite substrate allows one to consider the antenna characteristic itself and can be used in the initial investigation of the antenna’s property, which saves a substantial amount of computational time. The study of an antenna on a lens substrate involves the frequency-dependent radiation property of the lens substrate and requires enormous computational resources for investigating radiation properties in a broad frequency range up to 5.0 THz. The numerical results showed that the antenna exhibited similar input impedance characteristics for both the semi-infinite and lens substrates; however, the radiation characteristics of the antenna were completely different. It is expected that once the responses of the antenna and the lens substrate are fully characterized, the response of the photoconductive material to the optical sources, which is very difficult to estimate accurately, can be inferred from the measurement result. II.

ANTENNA GEOMETRY AND MODELING

Figure 1 depicts the schematics of a stripline dipole antenna on the semi-infinite and lens substrates. The width and length of the stripline dipole are w and L, respectively. The feeding gap for applying the current source and the square pad at the dipole termination for applying the DC bias voltage are designated as g and p, respectively. In the semi-infinite substrate model, a high-permittivity material of GaAs, whose dielectric constant is 12.9, was used. In the lens substrate model, an additional high-permittivity material of Si (İr = 11.7) was used to form a model of a thin GaAs substrate backed by a Si lens. The GaAs substrate was selected with a thickness of

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The 8th European Conference on Antennas and Propagation (EuCAP 2014)

schemes, the antenna was driven by a wire port at the gap with a constant 1V source. With the same conditions of frequency range, discretization, and simulator of MWS, the input impedance calculation of the semi-infinite substrate model needed only 4 hours, whereas the lens substrate model needed as many as 102 hours for one simulation run when using a parallelization of up to 18 threads in the Server-Xeon E5-2660V2 workstation with a clock speed of 2.2 GHz. III.

ANTENNA CHARACTERISTICS

A. Input Impedance Characteristics Figure 2 shows the input impedance of the stripline dipole on a semi-infinite substrate and a lens substrate. It is obvious that the antenna exhibited similar input impedance characteristics for both the semi-infinite and lens substrates. This is consistent with the observation that an antenna exhibits similar impedance characteristics on either a lens substrate or a

(a)

(b) Fig. 1. Schematics of stripline dipole antenna on (a) semi-infinite GaAs substrate and (b) extended hemispherical Si lens substrate.

TGaAs and a width of A. The Si lens has an extended hemispherical shape whose extension length and radius are denoted as Text and R, respectively. Some design parameters are fixed throughout the study as follows: L = 1500 ȝm, w = 10 ȝm, g = 5 ȝm, p = 100 ȝm, TGaAs = 300 ȝm, TExt = 240 ȝm, R = 1500 ȝm, and A = 3000 ȝm. The selections of TGaAs and TExt provide a ratio T/R = 0.36 which is the optimal ratio for the best possible antenna gain in the frequency range of interest. The antenna on semi-infinite and lens substrates was numerically simulated by the full-wave simulator Microwave Studio (MWS) by CST based on the finite-integration timedomain (FIT) method. This method allows for monitoring radiation properties of the antenna in a wide frequency range with only one simulation run. However, MWS by CST has a limitation in determining the far-field radiation of the antenna on a semi-infinite substrate. For this reason, another simulator, FEKO, based on the method of moments (MoMs), was employed to investigate the radiation property of the stripline dipole antenna on the semi-infinite substrate. In FEKO, the semi-infinite substrate was approximated by Green’s function layer, and the antenna gain was calculated with the assumption of a perfect impedance matching condition. In both substrate

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(a)

(b) Fig. 2. Input impedance characteristics of the stripline dipole antenna on different substrate schemes: (a) resistance and (b) reactance.

The 8th European Conference on Antennas and Propagation (EuCAP 2014)

(a)

(b) Fig. 3. (a) Gain and (b) radiation efficiency of the stripline dipole antenna on different substrate schemes where R = 1.5 mm and T/R = 0.36 for the lens substrate case.

semi-infinite substrate except for the radiation patterns [16]. A small fluctuation in the antenna input impedance at low frequencies could be eliminated by using a longer stripline dipole [6, 7]. The resistance of the antenna showed a flat and stable variation over the entire frequency of interest and thus supports a wideband impedance characteristic. The imaginary parts of the input impedance of the antenna was capacitive in the low-frequency range, i.e., 0-2.0 THz, but changed to inductive in the high-frequency range, i.e., 2.0-5.0 THz. The inductive reactance of the antenna was progressively increased with the increases of frequency. Generally, the antenna exhibited a traveling-wave behavior with respect to the input impedance. B. Radiation Characteristics A comparison of gain of the main beam on the z-axis (ș = 180°) for the stripline dipole on the semi-infinite and lens

substrates is presented in Fig. 3(a). The stripline dipole on the semi-infinite substrate produced a flat gain with a very small variation and thus provided a frequency-independent gain profile over the entire frequency range. There are two benefits from this behavior. The first one is that the stripline dipole supported wideband spectral characteristics with stable radiation patterns over the entire frequency range of interest. The second one is that such a frequency-independent gain response would support the elucidation of radiation property of the antenna on a lens substrate. The gain of the antenna on the lens substrate increased gradually, reaching a maximum of 37.3 dB at 4.2 THz, before decreasing with further increases in the operating frequency. We observed from the simulation (not shown here) that the optimal ratio T/R of 0.36 was the same for the smaller lenses—for instance, 2-mm or 1-mm diameter lenses. More importantly, the gain response of the antenna on a substrate lens exhibited an increased level of sensitivity to the lens shape when increasing the lens diameter, and this is particularly important in optimizing large substrate lenses. The radiation efficiency behaviors of the antenna on the semi-infinite and lens substrates are illustrated in Fig. 3(b). Similar trends were observed where the radiation efficiency was low in the low limit of the frequency range, increased rapidly, and reached saturation in a wide frequency range. The saturation frequency and magnitude level in the radiation efficiency of the antenna on the semi-infinite substrate were 0.5 THz and 90%, whereas that of the antenna on the lens substrate were 2.5 THz and 65%, respectively. The low radiation efficiency of the lens substrate case resulted primarily from the effects of surface-wave loss and internal reflection inside the lens that did not exist in the semi-infinite substrate. The radiation patterns of the stripline dipole antenna on the semi-infinite substrate at various frequencies, i.e., from 0.5 to 3.5 THz with 1 THz increments, are shown in Fig. 4. The antenna produced stable radiation patterns with a small variation in both the xz- and yz-planes as the frequency increased. The spreading pattern phenomenon to the substrate/air interface at the high frequencies of the antenna resulted from the leakage of current along the stripline, i.e., the x-direction, thus making the antenna exhibit a high side-lobe level when used with a lens substrate. We also plotted the radiation patterns of the stripline dipole antenna on the substrate lens where R = 1500 ȝm and T/R = 0.36, seen in Fig. 5. The radiation patterns exhibited an increased number of side lobes with respect to the increase of the operating frequency, particularly in the xz-plane patterns. This behavior is attributed to the phenomenon that the more traveling wave period is experienced along the stripline dipole when increasing the frequency. It is possible to infer these resulting radiation patterns from the spreading pattern to substrate/air interface of the antenna, which was observed in the semi-infinite substrate case. In addition to the sidelobe behavior, the antenna presented the trend of narrowing the main beam and of increasing the gain level with respect to the increases of frequency. This behavior resulted from increasing the effective aperture size of the lens and thus increasing the effect of beam collimation.

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20 10 0 -10 -20 xz-plane yz-plane

-30 -40

0

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180

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0

Theta (degree) (a)

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(b)

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(d) Fig. 4. Radiation patterns at (a) 0.5 THz, (b) 1.5 THz, (c) 2.5 THz, and (d) 3.5 THz of the antenna on thin GaAs substrate backed by Si lens (R = 1.5 mm, and T/R = 0.36).

Fig. 4. Radiation patterns at (a) 0.5 THz, (b) 1.5 THz, (c) 2.5 THz, and (d) 3.5 THz of the antenna on semi-infinite GaAs substrate.

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The 8th European Conference on Antennas and Propagation (EuCAP 2014)

IV.

CONCLUSIONS

The characteristics of a stripline dipole antenna on two radiation environments, i.e., a semi-infinite substrate and a thin GaAs substrate backed by a Si lens, were presented and compared. The approach of a semi-infinite substrate study allows one to characterize the antenna response itself without the influence of the lens substrate while saving a significant amount of computational time. The antenna on a semi-infinite substrate exhibited a frequency-independent gain profile, whereas it exhibited a frequency-dependent gain profile for the lens substrate case. The frequency-dependent radiation property of the lens substrate is thus described and confirmed. The study provides for a PC antenna design guideline in that the antenna effect and the lens substrate effect can be individually characterized and optimized for the best possible performance.

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[9]

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[11]

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