Design and Analysis of Frequency Sweep Antenna Array Based on Coupling Line and Left-handed Composit

1. Introduction

Left hand material, also known as double negative medium or negative refractive index material, is a kind of material with negative dielectric constant and negative permeability at a certain frequency band. The idea of left-handed materials was first proposed by v.g. Veselago, a former Soviet. When electromagnetic waves propagate in left-handed materials, the electric field, magnetic field and wave vector meet the left-handed spiral relationship, and the phase velocity is opposite to the direction of energy flow. With the in-depth research in recent years, left-handed materials have developed rapidly and have been applied in many fields. In 2002, Professor Itoh of UCLA and others put forward the theory of left-handed composite transmission line (CRLH TL), which uses microwave components to make artificial left-handed composite transmission line. This structure has low insertion loss, wide bandwidth, phase advance and other characteristics, and has great application prospects in engineering.

Phased array radar is a very important direction of radar technology development, and frequency scanning antenna technology is one of the most key parts of phased array radar technology. Conventional frequency scanning antennas mostly use slow wave structure to provide phase change for scanning, but the microstrip line loss is large, which greatly affects the gain of the antenna. Previous studies have proved that using left-handed composite transmission line instead of slow wave line to realize serial power divider has the advantages of wide bandwidth, small volume and low insertion loss. However, the conventional left-handed and right-handed composite transmission lines using interdigital capacitance structure are difficult to increase the interdigital capacitance, which limits the use of this transmission line in the low-frequency part. In 2009, AMR M. E. safwat proposed a left-handed composite transmission line (cl-crlh TL) based on microstrip coupling line. This transmission line has a wider frequency range, can provide more phase advance, less loss, simple structure and easy to implement. In this paper, a new frequency scanning antenna array is proposed. The serial feed network is composed of coupling lines and left-handed composite transmission lines. It can expand the frequency band range of the antenna array, reduce the loss, improve the gain of the antenna array, and has a large angle scanning range. Each element of the antenna array adopts non-uniform feeding mode, which can reduce the sidelobe level of the antenna. The eight element frequency sweep antenna array designed in this paper can reach the scanning angle of - 40 ° 10 ° in the working frequency band, the gain is 14dB, and the sidelobe level is lower than - 20dB.

2. Frequency sweep antenna array design

2.1 design of left and right hand composite transmission line of coupling line

The unit structure of the left-hand and right-hand composite transmission line of the coupling line is shown in Fig. 1 [5], which is obtained by short circuiting and deforming the two ports of the coupling microstrip line, and the unit structure shown in Fig. 1 can be connected periodically to form the left-hand and right-hand composite transmission line. In this design, seven element structures are periodically connected and connected with a section of microstrip line to form a left-hand and right-hand composite transmission line, as shown in Figure 2.

Fig. 1 unit structure of left and right hand composite transmission line of coupling line

Fig. 2 left and right hand composite transmission line of coupling line

The dielectric constant of the dielectric substrate is 2.65 and the thickness is 1.5mm. The simulation results are shown in Fig. 3. Figure 3 (a) shows the return loss characteristic curve of the left-hand and right-hand composite transmission line of the coupling line. It can be seen from the figure that the transmission line has a wide working frequency band, and the loss at low frequency is also very small, and the return loss is less than - 10dB in the frequency range of 0.5ghz 2.79ghz. Figure 3 (b) shows the phase characteristic curve. It can be seen from the figure that it has the characteristic of phase advance in its working frequency band and has a large phase change rate. In the frequency range of 1.25GHz 1.4GHz, the phase variation range is 129 ° - 29.8 °.

Figure 3 cl-crlh TL simulation results

2.2 serial feed network design

The antenna array uses the serial power divider as the feed network. The trunk of the serial power divider is a left-hand and right-hand composite transmission line, which is matched to 50 Ω at all levels and output ports of the power divider. Its structure is shown in Figure 4.

The serial power divider can be considered as a cascade of T-junction power divider. The structure of single section T-junction power divider is shown in Figure 5. The T-junction power divider meets the following relationship:

;

Where is the characteristic impedance of the input port, the characteristic impedance of the two output ports and the output power of the two output ports.

Fig. 4 feed network structure

Fig. 5 structure of T-junction power divider

In the serial feed network, the transverse impedance transformation adopts the impedance converter, and the longitudinal impedance transformation adopts the single stub matching branch for matching. Each array element adopts non-uniform feed, and the power distribution of the output port of the power divider meets the Chebyshev distribution. According to the port power distribution and the relationship between the power of the T-junction power divider and the characteristic impedance, the parameters of each impedance conversion part in the feed network can be calculated.

2.3 antenna array design

The serial power divider is used as the feed network of the antenna array, and the unit antenna is connected with the feed network through SMA connector. The element antenna adopts microstrip fed quasi Yagi antenna. In order to ensure that the unit antenna has sufficient gain during large angle scanning, the H-plane of the unit antenna is used for scanning. The plane of the unit antenna is perpendicular to the plane of the feed network. The real object of the antenna array is shown in Figure 6. The dielectric constant of the dielectric substrate is 2.65 and the thickness is 1.55mm.

Fig. 6 physical diagram of eight element frequency sweep antenna array

The simulation results of the eight element frequency sweep antenna array are shown in Figure 7. Figure (a) shows the return loss characteristic curve of the antenna array. It can be seen from the figure that the return loss is less than - 10dB in the frequency band of 1.22ghz 1.48ghz. Figure (b) shows the frequency scanning characteristic curve of the antenna array. It can be seen from the figure that the scanning angle of the antenna array can reach - 40 ° 10 °, the gain of the antenna array is 13dB 14.7db, and the sidelobe level can reach - 24dB within the scanning angle range.

(a) Simulation results of antenna array return loss

(b) Frequency sweep characteristic curve of antenna array

Fig. 7 simulation results of eight element antenna array

3 test results

The eight element frequency sweep antenna array is tested, and the return loss and pattern of the antenna array are measured.

Use Agilent e8363b vector network analyzer to measure the return loss of antenna array, and the measurement results are shown in Figure 8. It can be seen from the figure that the return loss in the 1.2Ghz 1.5GHz frequency band is less than - 10dB, and the return loss in the 1.25GHz 1.4GHz frequency band is less than - 18.5db.

The test results of antenna array pattern are shown in Figure 9. According to the test results, the frequency changes from 1.25GHz to 1.4GHz, and the scanning angle changes from - 40 ° to 10 °.

The test results of the antenna array are in good agreement with the simulation results. In the working frequency band, the antenna array has good frequency scanning characteristics, large scanning angle range and small loss.

Fig. 8 return loss measurement results of antenna array

Fig. 9 frequency sweep characteristics of antenna array

4. Conclusion

In this paper, a frequency sweep antenna array based on coupling line left-hand and right-hand composite transmission lines is proposed. The feed network of the antenna array is composed of coupling line left-hand and right-hand composite transmission lines. The simulation and physical test results show that the antenna array has good frequency sweep characteristics. Compared with the conventional slow wave line structure frequency scanning antenna, it has the advantages of small volume, simple structure, wider working frequency band, smaller loss and larger scanning angle range. Each element of the antenna array adopts non-uniform feed, which can effectively reduce the sidelobe level.

Design and Analysis of Frequency Sweep Antenna Array Based on Coupling Line and Left-handed Composit 1

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