Measurement and Analysis of Weather Phenomena with K-Band Rain Radar

Jun-Hyeong Park Dept. of Electrical Engineering

KAIST DaeJeon, Republic of Korea

bdsfh0820@kaist.ac.kr

Ki-Bok Kong Development team Kukdong Telecom

Nonsan, Republic of Korea Kbkong@kdtinc.co.kr

Seong-Ook Park Dept. of Electrical Engineering

KAIST DaeJeon, Republic of Korea

soparky@kaist.ac.kr

Abstract—To overcome blind spots of an ordinary weather radar which scans horizontally at a high altitude, a weather radar which operates vertically, so called an atmospheric profiler, is needed. In this paper, a K-band radar for observing rainfall vertically is introduced, and measurement results of rainfall are shown and discussed. For better performance of the atmospheric profiler, the radar which has high resolution even with low transmitted power is designed. With this radar, a melting layer is detected and some results that show characteristics of the meting layer are measured well.

Keywords—K-band; FMCW; rain radar; low transmitted power; high resolution; rainfall; melting layer

I. INTRODUCTION A weather radar usually measures meteorological

conditions of over a wide area at a high altitude. Because it observes weather phenomena in the area, it is mainly used for weather forecasting. However, blind spots exist because an ordinary weather radar scans horizontally, which results in difficulties in obtaining information on rainfall at higher and lower altitudes than the specific altitude. Therefore, a weather radar that covers the blind spots is required.

A weather radar that scans vertically could solve the problem. This kind of weather radar, so called an atmospheric profiler, points towards the sky and observes meteorological conditions according to the height [1]. Also, because the atmospheric profiler usually operates continuously at a fixed position, it could catch the sudden change of weather in the specific area.

In this paper, K-band rain radar which has low transmitted power and high resolutions of the range and the velocity is introduced. The frequency modulated continuous wave (FMCW) technique is used to achieve high sensitivity and reduce the cost of the system. In addition, meteorological results are discussed. Reflectivity, a fall speed of raindrops and Doppler spectrum measured when it rained are described, and characteristics of the melting layer are analyzed as well.

II. DEVELOPMENT OF K-BAND RAIN RADAR SYSTEM

A. Antenna To suppress side-lobe levels and increase an antenna gain,

offset dual reflector antennas are used [2]. Also, separation

wall exists between the transmitter (Tx) and receiver (Rx) antennas to improve isolation between them. With these methods, leakage power between Tx and Rx could be reduced. Fig. 1 shows manufactured antennas and the separation wall.

B. Design of Tranceiver Fig. 2 shows a block diagram of the K-band rain radar.

Reference signals for all PLLs in the system and clock signals for every digital chip in baseband are generated by four frequency synthesizers. In the Tx baseband module, a field programmable gate array (FPGA) controls a direct digital synthesizer (DDS) to generate an FMCW signal which decreases with time (down-chirp) and has a center frequency of 670 MHz. The sweep bandwidth is 50 MHz which gives the high range resolution of 3 m. Considering the cost, 2.4 GHz signal used as a reference clock input of the DDS is split and used for a local oscillator (LO). the FMCW signal is transmitted toward raindrops with the power of only 100 mW. Beat frequency which has data of the range and the radial velocity of raindrops is carried by 60 MHz and applied to the input of the Rx baseband module. In the Rx baseband module, quadrature demodulation is performed by a digital down converter (DDC). Thus, detectable range can be doubled than usual. Two Dimensional-Fast Fourier Transform (2D-FFT) is performed by two FPGAs. Because the 2D FFT is performed with 1024 beat signals, the radar can have high resolution of the radial velocity. Finally, data of raindrops are transferred to a PC with local LAN via the an UDP protocol. TABLE I. shows main specification of the system.

Fig. 1. Manufactured antenna and separation wall.

2016 URSI Asia-Pacific Radio Science Conference August 21-25, 2016 / Seoul, Korea

1

A Microwave Metamaterial Absorber with Wide Bandwidth

Somak Bhattacharyya Department of Electronics & Communication Engineering

Indian Institute of Information Technology, Allahabad-211012,

India bhattacharyya.somak@gmail.com

Saptarshi Ghosh Department of Electrical

Engineering Indian Institute of Technology,

Kanpur-208016, India joysaptrashi@gmail.com

Kumar Vaibhav Srivastva Department of Electrical

Engineering Indian Institute of Technology,

Kanpur-208016, India kvs@iitk.ac.in

Abstract— A wideband metamaterial absorber has been presented for practical applications in this article. The unit cell of the proposed structure comprises metallic patch of single circular split-ring imprinted on top surface of a metal-backed single layer dielectric substrate. The geometrical dimensions of the structure are suitably optimized so that full width half maxima bandwidth of 11.46 GHz is realized along with wideband absorption of 10.90 GHz from 5.94 GHz to 16.84 GHz with more than 80% absorptivity. Four distinct absorption peaks are observed at 6.46 GHz, 10.24 GHz, 15.26 GHz and 16.66 GHz. The absorption phenomena at these four frequencies are analyzed and the roles of several geometrical parameters of the design are investigated. The proposed structure has been studied under oblique incidence, both for TE and TM polarizations where it shows wideband absorption characteristics upto 45º incident angles for both cases. The proposed structure is very thin (~ λ/8.3 with respect to the centre frequency of the absorption bandwidth) compared to the commercially available microwave absorbers.

Keywords—metamaterials; microwave absorber; wideband

I. INTRODUCTION

Over the last decade, researches on metamaterials are found to be very promising due to their unusual electromagnetic properties, finding their applications in antenna system, cloaking etc [1-2]. Metamaterials based absorbers are also finding its applications in different frequency regimes from low frequency to higher frequencies because of their compactness [3-5]. It is difficult to achieve large absorption using metamaterials absorbers since they are made of high Q resonators [6-7]. Till date, a number of structures have been proposed employing variations of geometrical dimensions of unit cell to achieve multiband as well as bandwidth-enhanced absorbers [8-10]. Structures with more than one layer are also used for bandwidth-enhanced applications [11-12]. Structures using Hilbert curve, high impedance ground plane and lumped elements have also been reported, but all of them are suffering from considerable thickness [13-15].

In this paper, we propose a metamaterial absorber having wide absorption bandwidth for defense applications. The unit cell of the structure contains a circular metallic patch with splits imprinted over a metal-backed single layer dielectric substrate. The full width half maxima (FWHM) bandwidth of 11.46 GHz ranging from 5.60 GHz to 17.06 GHz has been achieved along with 10.90 GHz bandwidth of more than 80% absorptivity ranging from 5.94 GHz to 16.84 GHz. Four distinct absorption maxima has been achieved at 6.46 GHz,

10.24 GHz, 15.26 GHz and 16.66 GHz with peak absorptivities of 99.8%, 99.9%, 99.6% and 92.2% respectively. The field distributions have been studied at these frequencies to explain the absorption phenomena. The roles of several geometrical parameters of the designed structure are investigated. Furthermore, the structure has been studied under oblique incidences for TE and TM polarizations where it is found to act as absorber upto 45° incident angles in both cases.

II. DESIGN OF THE STRUCTURE

Fig. 1(a) Top view of the unit cell of the proposed absorber of wide bandwidth (a = 9 mm, r = 3.95 mm, w = 0.3 mm and ψ = 35.4°) with (b) simulated absorptivity response.

The top view of the proposed structure is shown in Fig. 1(a) along with the directions of electric field, magnetic field and incident electromagnetic wave. The circularly shaped metallic patches with splits imprinted on the top surface of a single layer metal-backed FR-4 dielectric substrate (εr = 4.25 and tanδ = 0.02) of 3.2 mm thickness in a single unit cell are periodically arranged. Copper patches (conductivity of 5.8 x 107 S/m) have been used as metals in the design and are of

(a)

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0.035 mm thickness. The geometrical dimensions of the unit cell of the proposed design are mentioned in Fig. 1(a).

When the electromagnetic wave is incident on the structure, no transmission of electromagnetic wave is possible due to the complete metal backing and thus minimizing the reflection from the top surface results in maximum absorption.

III. SIMULATED RESULTS

The proposed structure whose unit cell shown in Fig. 1(a) has been simulated using periodic boundary conditions in Ansys HFSS. Four distinct reflection minima are observed at 6.46 GHz, 10.24 GHz, 15.26 GHz and 16.66 GHz, resulting in absorption maxima at these frequencies with the respective peak absorptivities of 99.8%, 99.9%, 99.6% and 92.2% as shown in Fig. 1(b). Also, FWHM bandwidth of 11.46 GHz ranging from 5.60 GHz to 17.06 GHz has been realized. Moreover, absorption bandwidth of 10.90 GHz with range from 5.94 GHz to 16.84 GHz with more than 80% absorptivity level has also been achieved. The absorption band covers significant parts of C and Ku bands in addition to the complete coverage of X band.

Fig. 2. Surface current distributions at (a) top and (b) bottom surfaces of the proposed structure whose unit cell shown in Fig. 1(a) at 6.46 GHz, 10.24 GHz, 15.26 GHz and 16.66 GHz.

6.46 GHz 10.24 GHz

15.26 GHz 16.66 GHz

(A)

(B)

(A)

(B)

(A)

(B)

(A)

(B)

Fig. 3. Electric field distributions within the proposed structure with unit cell shown in Fig. 1(a) at 6.46 GHz, 10.24 GHz, 15.26 GHz and 16.66 GHz.

To explain the absorption phenomena, the surface current distributions at top and bottom surfaces of the proposed structure have been studied at the four distinct frequencies with peak absorptions (6.46 GHz, 10.24 GHz, 15.26 GHz and 16.66 GHz) as illustrated in Fig. 2(a) and Fig. 2(b) respectively. It is seen that the directions of surface currents at all these frequencies of peak absorptions are anti-parallel at the two surfaces. This results in a circulating current which is in perpendicular direction to the magnetic field, creating magnetic excitations. The induced electric fields within the structure are also shown in Fig. 3 at these four frequencies, creating electric excitations. At 6.46 GHz and 16.66 GHz, the field concentrations are more at both the regions (A) and (B) while at 10.24 GHz and 15.26 GHz, field concentration is high only at region (A). Also, at 10.24 GHz, the electric field has larger concentration at region (A). The overlapping of these two excitations results in strong electromagnetic absorption.

The constitutive electromagnetic parameters viz., effective permittivity and effective permeability of the system are retrieved [16]. These retrieved parameters are nearly equal over the frequency range of 5.94 GHz to 16.84 GHz. They are very close at 6.46 GHz, 10.24 GHz, 15.26 GHz and 16.66 GHz as listed in Table I, implying impedance matching between input impedance of the medium with the free space impedance, leading to minimum reflection from the surface. This corresponds to maximum absorption. The comparisons of real and imaginary parts of the retrieved effective permittivity (εeff) and effective permeability (μeff) are shown in Fig. 4(a) and Fig. 4(b) respectively.

TABLE I. COMPARISON OF RETRIEVED CONSTITUTIVE ELECTROMAGNETIC PARAMETERS OF THE PROPOSED STRUCTURE WITH

WIDE BANDWIDTH

Frequency (GHz) Real Part Imaginary Part

εeff μeff εeff μeff

6.46 0.79 1.03 4.92 4.13

10.24 0.82 1.18 2.79 2.89

15.26 0.96 1.03 2.17 1.67

16.66 1.98 0.18 1.64 1.38

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(c) Fig. 5. Absorptivity response of the proposed structure with respect to variation of (a) unit cell dimension (a), (b) thickness (t) and (c) real part of permittivity (εr) of the FR-4 substrate.

The absorptivity responses of the structure are studied under variation of the total dimension of the unit cell (a) as shown in Fig. 5(a). When a is varied, the other patch dimensions remain constant. As a increases, the gap between neighbouring metallic patches increases, causing decrease in the equivalent fringing capacitance [17], thus leading to the increase of the frequency of absorption. As seen from Fig. 3, the electric field is significantly concentrated in the region (B), close to the end of the unit cell at 6.46 GHz and 16.66 GHz and partially at 15.26 GHz. Hence, the effect of variation of a is mostly observed as shift of all frequencies except the second peak. The structure is also studied under variation of thickness of the

dielectric substrate. As thickness (t) increases, the fringing field increases, thereby reducing the frequency of absorption [18]. Also, with the increase of the substrate thickness (t = 1.6 mm to t = 3.2 mm), more energy will be trapped within the dielectric substrate providing more loss thereby increasing the bandwidth as observed from Fig. 5(b). With further increase of t, bandwidth decreases. More bandwidth can be achieved by tailoring other geometrical parameters of the unit cell of the structure with t = 4 mm; but the thin nature of the complete structure cannot be fulfilled. The increase in real part of the relative permittivity (εr) of the dielectric substrate leads to rise in effective capacitance of the structure, and thus the complete spectrum undergoes redshift as evident from Fig. 5(c).

Fig. 6. Absorptivity response of the proposed structure under variations of (a) radius of the circle (r) and (b) angle subtended with gap of the split (ψ).

The absorptivity response of the proposed structure has also been studied under several dimensions of the circular patch with splits viz., radius of the circular patch (r) and angle subtended with split gap (ψ). With increase of r, the effective inductance due to the top metallic patch gets increased [19], thereby reducing the lowermost absorption frequency. Since the region (B) gets affected at the lowermost and uppermost frequencies of absorption, increase of r leads to the lowering of lowest and highest frequencies of the absorption band as seen from Fig. 6(a). ψ = 0° implies there is no split in the metallic patch and absorption occurs at a single frequency (near to 19 GHz). As ψ increases, the lowest and highest frequencies with peak absorptivity increase, thus reducing absorption bandwidth. In this case also, the region (B) is primarily affected with the variation of ψ and hence the two aforementioned frequencies get shifted as evident from Fig. 6(b). With increase of ψ, the gap within the split increases. So, effective capacitance decreases leading to the enhancement of lowermost frequency of absorption [19].

IV. RESPONSES UNDER OBLIQUE INCIDENCE AND POLARIZATION ANGLE VARIATIONS

The structure is simulated for different angles of incidence (θ) under TE and TM polarizations as shown in Fig. 7(a)(i) and Fig. 7(a)(ii) respectively. In the first case, the direction of electric field remains constant while the directions of magnetic field and wave vector are changed by an angle θ while in the later one, the direction of magnetic field remains constant while the directions of electric field and wave vector are changed by an angle θ. In both the cases, wide bandwidth has been observed upto 45° incident angles as evident from Fig. 7(b)(i) and Fig. 7(b)(ii) respectively. The structure is further studied for different angles of polarization (φ) as shown in Fig.

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7(a)(iii) while normal incidence takes place. The simulated absorptivity response is shown in Fig. 7(b)(iii), where wide absorption bandwidth response is observed only in the range

150 −=φ and 9075 −=φ . The bandwidth gradually

decreases from 450 −=φ and then again increases from 9045 −=φ .

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Fig. 7(a) Set up for the oblique incidence responses under (i) TE polarization, (ii) TM polarization and (iii) polarization angle variations with (b) simulated absorptivity responses.

V. CONCLUSIONS

A metamaterial absorber with wide bandwidth covering complete X band and significant parts of C and Ku bands with more than 80% absorption bandwidth has been discussed. The unit cell of the proposed structure consists of circular copper patch with splits imprinted over a single layer of 3.2 mm thick FR-4 substrate with complete copper lamination at its bottom plane. FWHM bandwidth of 11.46 GHz ranging from 5.6 GHz to 17.06 GHz has been achieved along with 10.9 GHz bandwidth of more than 80% absorptivity ranging from 5.94 GHz to 16.84 GHz. The absorption phenomena have been discussed using field distributions along with retrieved constitutive electromagnetic parameters. The roles of different geometrical parameters are also investigated. The structure exhibits wide absorption bandwidth upto 45° incident angles under both TE and TM polarizations. The structure is very thin (~ λ/8.3 with respect to the center frequency) providing compactness compared to the existing commercially available electromagnetic absorbers. The only drawback of the structure is its polarization sensitivity.

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