Design Example: A Cylindrical Coplanar Waveguide Leaky-wave

The dielectric constant of the substrate ε_r is 2.2, and the thickness h is 0.508mm. The outer radius b is 80 mm, the center strip width w is 10 mm, and the slot width is 5mm.

Due to the odd-symmetry of longitudinal currents, two sets of inverted balanced lines [23] are fed into the CPW, as shown in Fig. 3.14. The length of the CPW antenna is 150 mm.

Fig. 3.15 plots the normalized propagation constants. Its radiation region starts at 9.5 GHz and extend to 16.0 GHz. The measured return loss is shown in Fig. 3.16. At 6, 7, and 8 GHz, the measured antenna gains are 6.8, 9.2, and 7.3 dBi, respectively. In addition, the measured antenna gains are 9.1, 9.4, and 8.5 dBi at 9, 10, and 11 GHz, respectively. In Fig. 3.17 and 3.18, the antenna mainbeams are fixed at the endfire direction. Both of CPW leaky-wave antennas and single-conductor leaky-wave antennas [23] have fixed mainbeams because their similar structures and current symmetries.

6 8 10 12 14 16

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

Frequency (GHz)

Fig. 3.15 The normalized phase constants and attenuation constants.

Fig. 3.16 The measured return loss of the proposed cylindrical coplanar waveguide leaky-wave antenna.

-15 -10 -5 0 5 10

Fig. 3.18 The measured copolarization radiation patterns at 9, 10, and 11 GHz in the xz-plane.

Fig. 3.17 The measured copolarization radiation patterns at 6, 7, and 8 GHz in the xz-plane.

(a) (b)

Fig. 4.1 (a) The single-conductor strip line with the virtual PEC.

(b) The slotline with the virtual PMC.

Chapter 4

Broadband Leaky-Wave Antennas

This chapter mainly discusses planar broadband leaky-wave antennas. The slotline antenna is a complementary structure of the single-conductor strip antenna, whereas the inverted-T antenna is derived from the single-conductor strip antenna. All of these three types of structures can be very wideband antennas, due to their surface-wave-like leaky modes. A wideband slotline leaky-wave antenna with a microstrip-to-CPW feeding structure and an inverted-T leaky-wave antenna with simple ground plate are presented.

4.1 Slotline Leaky-Wave Antennas

The cross-sectional views of single-conductor strip leaky-wave antennas [23] and slotline leaky-wave antennas [36] and are plotted in Fig. 4.1. It is obvious that they are complementary structures. The center PEC and PMC represent odd-symmetry and even-symmetry for longitudinal currents, respectively. In this section, we focus on the slotline leaky mode and its corresponding feeding structure.

Frequency (GHz)

Fig. 4.2 The normalized phase constants and attenuation constants of slotlines and TM1 surface wave mode of grounded dielectric slab (w=0 case).

By using the spectral domain approach, we calculate the normalized phase constants and attenuation constants of the first higher leaky-mode of slotlines.

Moreover, slotlines belong to the structure plotted in Fig. 3(f) in Oliner's paper [37], which indicates the existence of leaky modes. The propagation constants of TM1

surface wave of grounded dielectric slabs [33] are also computed. When we compare these two different propagation constants in Fig. 4.2, it can be found that as the slot width decreases, the propagation constants of slotlines are approaching those of grounded dielectric slabs. Therefore, the first higher order leaky mode of slotlines may be treated as a guided surface wave propagating along the longitudinal direction of the slot. Since single-conductor strips are complementary structures of slotlines, the surface wave mode and the first higher leaky mode should have the similar relationship in single-conductor strips.

(a) (b)

Fig. 4.3 The proposed slotline leaky-wave antenna. (a) The top view . (b) The bottom view.

Since the radiation region of the slotline leaky mode is very wide, a broadband feeding circuit should be utilized to provide the required bandwidth. The broadband microstrip-to-CPW transition [24] is adopted to excite the first higher leaky mode of slotlines. Besiders, a section of tapered line is also combined with the microstrip-to-CPW transition.

Frequency (GHz)

5 10 15 20 25 30

Return Loss (dB)

-25 -20 -15 -10 -5 0

Fig. 4.4 The measured return loss of the slotline leaky-wave antenna.

The proposed slotline leaky-wave antenna shown in Fig. 4.3 has the following structural parameters: slot width is 10 mm, antenna length is 90 mm, the dielectric constant is 2.2, and the substrate thickness is 0.508 mm. Fig. 4.4 plots the measured return loss, and this antenna has a bandwidth about 14.7 GHz, starting from 11.8 to 26.5 GHz. The measured copolarization radiation patterns at 14 and 16 GHz are illustrated in Fig. 4.5, with antenna gains 8.9 and 10.1 dBi, respectively. In Fig. 4.6, the measured radiation patterns at 18 and 20 GHz are, with antenna gains 11.5 and 12.3 dBi, respectively. In Fig. 4.7, antenna gains at 22 and 24 GHz are 13.3 and 12.3 dBi.

The two mainbeams of upper and lower half-space are closer to each other as the frequency increases.

-15 -10 -5 0 5 10 15

Fig. 4.5 The measured copolariztion radiation patterns at 14 and 16 GHz.

-15 -10 -5 0 5 10 15

Fig. 4.6 The measured copolariztion radiation patterns at 18 and 20 GHz.

-15 -10 -5 0 5 10 15

Fig. 4.7 The measured copolariztion radiation patterns at 22 and 24 GHz.

4.2 Broadband Inverted-T Leaky-Wave Antenna

For the first higher order leaky mode in single-conductor strip leaky-wave antenna plotted in Fig 4.8(a), an infinite virtual PEC boundary is assumed at the center of the strip, in which the longitudinal currents are odd-symmetric and transverse currents are even-symmetric with respect to the center. From the image theory, the inverted-T leaky-wave antenna with infinite PEC boundary in Fig. 4.8(b) will have the same radiation characteristics for the y > 0 plane with that of the single-conductor strip leaky-wave antenna shown in Fig. 4.8 (a).

(a) (b)

Fig. 4.8 (a) The single-conductor strip leaky-wave antenna.

(b) The inverted-T leaky-wave antenna.

Since the inverted-T and the single-conductor strip leaky-wave antenna are equivalent in the half space, we first calculate the normalized propagation constants of the single-conductor strip leaky-wave antenna with the spectral domain approach.

Shown in Fig. 4.9 are the normalized phase constants and attenuation constants for the single-conductor strip leaky-wave antenna with the following structural parameters:

antenna width w_s=14 mm, the dielectric constant of the substrate ε_r=2.2 and the substrate thickness h=0.508 mm. The radiation region is from 6.6 GHz to 18.0 GHz, with the normalized phase constant remains almost a constant, which implies that the mainbeam will be fixed around the endfire direction.

To excite the first higher order leaky mode of single-conductor strip antenna in Fig. 4.8(a), a broadband feeding structure consists of two out-of-phase balanced microstrip lines that utilizes a broadband phase inverter [23]. However, only a finite conductor plate is needed for the inverted-T leaky-wave antenna, and the difficulty in design of the broadband phase inverter is removed.

We implement an inverted-T antenna shown in Fig. 4.10 with the antenna width w_a=7 mm, the antenna length L_a=100 mm, a substrate of dielectric constant ε_r=2.2 and the substrate thickness h = 0.508 mm. The half-width strip is mounted vertically over the conductor plate. A signal line is fed at the edge opposite to the plate, and the conductor plate is the ground. The measured return loss is plotted in Fig. 4.11. The bandwidth extends from 10.9 to 18.0 GHz, for a bandwidth ratio about 1.65:1.

Themeasured co-polarization radiation patterns in the xz-plane at 13, 14, and 15 GHz are shown in Fig. 4.12, with the measured gains 7.44, 9.48, and 10.58dBi, respectively. The mainbeam directions at these three frequencies are all around 66°

from the broadside direction. Fig. 4.13 plots the measured co-polarization radiation patterns in the xz-plane at 16, 17, and 18GHz, while the measured gains are 10.94,

11.71, and 10.94 dBi, respectively. The mainbeam directions at these three frequencies are close to 68° from the broadside direction.

The patterns of this antenna vary slightly from 13 to 18 GHz, and the shapes of the mainbeams are almost the same. For a single-conductor strip leaky-wave antenna, its mainbeam always keeps at the endfire direction. But the mainbeam of an inverted-T leaky-wave antenna is tilted from the endfire direction since the ground plate is not infinite.

Frequency (GHz)

3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 0.00

0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00

β/k₀ α/k0

Fig. 4.9 The computed normalized phase constants and attenuation constants

Frequency (GHz)

2 4 6 8 10 12 14 16 18

Return Loss (dB)

-25 -20 -15 -10 -5 0

Fig. 4.11 The measured return loss.

Fig. 4.10 The proposed inverted-T leaky-wave antenna.

-15 Fig. 4.12 Measured co-polarization radiation patterns at 13, 14 and 15 GHz in the

xz-plane. Fig. 4.13 Measured co-polarization radiation patterns at 16, 17 and 18 GHz in the

xz-plane.

Chapter 5

Conclusion and Future Work

This thesis studies the propagation characteristics of leaky modes on cylindrical substrates and broadband planar leaky-wave antennas. Two kinds of leaky-mode symmetries represented with PEC and PMC on both structures are analyzed by the full-wave method.

Three cylindrical types of transmission lines: microstrip lines, slotted coaxial lines, and coplanar waveguides are investigated. The effects on propagation constants under different structural parameters are presented. All of these leaky modes on cylindrical substrates have wide radiation regions. Feeding structures used in planar leaky-wave antennas, such as aperture-coupling and inverted balanced microstrip lines, are applied to excite the first higher leaky mode on cylindrical leaky-wave antennas successfully.

A broadband feeding structure, microstrip-to-CPW transition, is utilized for slotline leaky-wave antennas. From the measured radiation patterns, slotline leaky-wave antennas can have high gains in a wide bandwidth. Besides, a novel inverted-T leaky-wave antenna, which is modified from a single-conductor strip leaky-wave antenna, is developed and implemented. The feeding circuits are simplified by replacing the phase inverter by a finite ground plate.

However, there are still many issues that need to be further investigated in this thesis. These topics are listed in the following:

1. The creeping waves, the second or higher leaky modes on cylindrical substrates . 2. Mode-coupling between transmission lines on cylindrical substrates.

3. Implementation of slotted leaky-wave antenna on standard coaxial lines like

4. Better impedance matching circuits for planar and cylindrical CPW leaky-wave antennas.

5. More relationships between surface waves and leaky modes on slotlines and single-conductor strip lines.

6. Design of feeding structure of slotline leaky-wave antennas to obtain a leaky mode purity and reduce the sidelobes.

Appendix

The Green’s functions used in cylindrical microstrip lines are listed as following:

1

( )

The Green’s functions used in slotted coaxial lines are listed as following:

1

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1981.

博士候選人資料

姓 名 ：林烈全

性 別 ： 男

出生年月日 ： 民國 69 年 4 月 5 日

籍 貫 ： 台灣屏東

學 歷 ：交通大學電信工程系學士 (民國 87 年 9 月~91 年 6 月) 交通大學電信工程系碩士 (民國 91 年 9 月~93 年 6 月) 交通大學電信工程系博士班(民國 93 年 9 月~)

論文題目 ：圓柱形洩漏波天線與寬頻平面式洩漏波天線

著作目錄:

期刊論文:

1. L.-C. Lin, H. Miyagawa, T. Kitazawa, R. B. Hwang,. and Y.-D. Lin,

“Characterization and design of cylindrical microstrip leaky-wave antennas,”

Antennas and Propagation, IEEE Transactions on, vol. 56, issue 7, pp. 1853-1859,

July 2008.

會議論文:

1. L.-C. Lin, Y.-D. Lin and T. Kitazawa, “Leaky-mode propagation characteristics of slotted coaxial lines,” Antennas and Propagation International Symposium, 2007 IEEE, pp. 3233 – 3236, June 2007.

2. L.-C. Lin, Y.-D. Lin and T. Kitazawa, “Propagation characteristics of leaky coplanar waveguides on cylindrical substrates,” Asia-Pacific Microwave Conference 2007, pp.1-3, Dec. 2007.

3. L.-C. Lin, Y.-D. Lin and T. Kitazawa, “Broadband inverted-T leaky-wave

antennas,” Antennas and Propagation International Symposium, 2008 IEEE, July 2008

4. L.-C. Lin, Y.-S. Cheng, R. B. Hwang, T. Kitazawa, and Y.-D. Lin, “Slotted conductor-backed coplanar waveguide antennas,” International Symposium on Antennas and Propagation 2008, Oct. 2008.

在文檔中 圓柱形洩漏波天線與寬頻平面式洩漏波天線 (頁 46-0)