**introduction**

Coplanar stripline (CPS) is a coplanar transmission line method proposed in the 1970s. Due to its simple structure, it is easy to bridge between active and passive two-port devices, avoiding the process of perforation. trouble. At the same time, CPS is insensitive to dielectric thickness, has small parasitic effects caused by discontinuous structures, and has low loss during high-frequency electromagnetic wave propagation. Therefore, it is widely used in feed networks and microwave circuits, such as printed dipole antennas and filters. , couplers, resonators and amplifiers.

In the rectenna system, the low-pass filter requires the fundamental wave to pass, which can effectively prevent the second and third harmonics to improve the conversion efficiency of the rectenna system. Therefore, in a rectenna system, a low-pass filter with a wide stop band and low loss is more practical. CPS bandpass filter and bandstop filter used in rectenna system, the insertion loss simulation at operating frequency is -0.3dB, and the stopband bandwidth of attenuation less than -10dB is about 6GHz, which effectively suppresses the second harmonic, but The third harmonic suppression performance is greater than -5dB and the size is large.

According to the analysis of the discontinuous structural characteristics of CPS, the half-wavelength T-shaped open-circuit branch and the split-loop resonant circuit are equivalent to the series-connected LC circuit, which generates transmission zero points and realizes the stop-band characteristics. In order to reduce the volume of the filter, the groove on the microstrip line also has a stop band characteristic. Therefore, by adding T-section and T-slot resonance inside and outside the transmission line, high-order harmonics can be well suppressed, and the design of the low-pass filter wide stop band is realized.

In this paper, a novel open-loop CPS resonator is proposed, and the equivalent circuit diagram of its lumped element is analyzed. Based on the open-loop resonator, a third-order CPS low-pass filter is designed, which has the characteristics of small size, low loss in the passband and wide band width. It effectively suppresses the second and third harmonics and can be applied to the RF front-end and In a rectifying antenna system.

**Coplanar stripline (CPS) structure**

The CPS structure has a finite size dielectric plate as shown in FIG. According to the CPS transmission line theory, as the gap S continues to increase, the total loss of the CPS becomes smaller and smaller, and the characteristic impedance becomes larger and larger. In other words, a CPS structure with a high characteristic impedance corresponds to a small transmission loss. In order to obtain higher diode conversion efficiency, CPS is required to have low loss and high characteristic impedance.

Figure 1 Coplanar stripline structure

The dielectric plate used in the CPS structure low-pass filter designed in this paper has a relative dielectric constant of 2.55, a thickness of 0.8 mm, and a copper-clad thickness of 0.035 mm. According to the full-wave simulation software IE3D, when the line width W and the spacing S of the parallel transmission line are 0.6 mm and 0.4 mm, respectively, the characteristic impedance Z0 of the CPS is 172 ohm.

**CPS low pass filter design**

**1 CPS open loop resonator**

The structure and equivalent circuit of the open-loop CPS resonator proposed in this paper are shown in Fig. 2. It is realized by a resonant open loop with a length of Î»g/2. According to the analysis of the discontinuous structural characteristics, the perimeter of the Î»g/2 open loop is equivalent to the inductance Lp, the gap g1 between the open loop and the transmission line is equivalent to the coupling capacitor cp1, and the symmetric open loop pitch g2 is equivalent to the coupling capacitor cp2. Then, the open loop structure shown in FIG. 2 can be equivalent to a series LC circuit. An equivalent series resonant circuit is connected in parallel between the transmission lines to produce a transmission zero point to achieve stop-band characteristics.

In Fig. 2, the length of the open loop is 7.4 mm, the line width is 0.4 mm, the gap pitches g1 and g2 are both 0.4 mm, and the resonance frequency is 9.4 GHz. The frequency characteristics are as shown in Fig. 3(a). The S21 in the passband is -0.068dB minimum, and the S21 reaches -31.8dB at 9.4GHz. According to the analysis of Fig. 3(b) of the IE3D simulation curve, when the slits S, g1, and g2 increase, the insertion loss in the pass band gradually decreases, and the resonance frequency shifts to the high frequency. Mainly because the coupling capacitance between the ring and the transmission line and the symmetrical ring is reduced, the resonance frequency is increased, and the resonance Q value is increased.

Figure 2 CPS open-loop resonator

Figure 3 (a) Frequency response characteristics of CPS open-loop structure

Figure 3(b) Trend of frequency response of CPS open-loop structure with g2

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