Broadband Circularly Polarized Rectenna With Wide Dynamic-Power-Range for Efficient Wireless Power Transfer

This paper presents a circularly polarized (CP) rectenna with the advantage of achieving high efficiency in both wide operating power and frequency ranges. The proposed rectenna is composed of a high-efficiency rectifier and broadband CP antenna. In the proposed rectifier, a novel wideband resistance compression technique is presented, highly improving the matching performance of the circuit in a wide range of input power and frequency. The technique is achieved by an impedance manipulation network and a coupled-lines-based resistance compression network (RCN). Theoretical analysis is carried out and closed-form equations are derived for the design of the rectifier. Simulated and measured results show that over 60% (up to 76%) conversion efficiency is achieved for the input power range of 5–17 dBm and frequency band of 1.7–2.9 GHz (mobile, Wi-Fi and ISM bands). Subsequently, unlike the conventional half-wavelength slot antenna, a compact CP antenna with a side length of quarter wavelength is designed by using a 1-wavelength loop slot and a coupled-line-based phase shifter. It has wide axial-ratio bandwidth and good radiation pattern. At last, by integrating the proposed rectifier together with the CP slot antenna, a broadband CP rectenna is completed with higher efficiency by about 18.6% than that without wideband RCN.


I. INTRODUCTION
Recently, wireless power transfer (WPT) has gained increasing attention. It is very significant in many applications, such as internet of things (IoT) devices [1], medical devices [2], low-power electronics [3], and simultaneous wireless information and power transfer (SWIPT) system [4], [5]. At the receiver side of both WPT and SWIPT system, the efficiency improvement of the RF rectennas is an important work.
On the one hand, achieving high efficiency in wide or multiple frequency bands is beneficial for many applications, including increasing the output power of the rectenna for energy harvesting [6]- [12] and using the high peak-toaverage power ratio (PAPR) signals [13]- [16]. Broadband The associate editor coordinating the review of this manuscript and approving it for publication was Tariq Masood . rectennas are necessary for the PAPR signals such as UWB signals and chaotic signals [17], [18]. Moreover, broadband design also improves the applicability of WPT for the applications at different countries and regions with different requirements about working frequency band. In [19], a multiband rectenna with shorting pins eliminating the matching network is introduced, the frequency bands cannot be tuned independently. Besides, circular-polarization also enables the antenna to receive more energy from the complex environment with multipath reflection and refraction. Nevertheless, it is challenging to design circularly-polarized rectennas in wide bandwidth.
On the other hand, it is also important to maintain high efficiency within a wide dynamic power range. The variation of the power levels leads to diode impedance change due to the characteristic of non-linearity, and then causes impedance VOLUME 8, 2020 This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/ mismatch and rectifier performance degradation. In order to solve this problem, high-efficiency rectifiers within wide power range are implemented [20]- [25]. Among these works, some of the rectifiers with wide operating power range are designed by connecting several sub-rectifiers optimized at different incident power levels together in parallel [20]- [22]. But extra power management systems are needed which increases the circuit complexity. Varactor diodes [23] and GaAs pHEMT [24] are used to improve the matching performance and extend the maximum breakdown power. A branchline coupler is designed to re-inject the reflected power due to impedance mismatch back into the sub-rectifiers, which improves the conversion efficiency [25]. However, these methods cause additional energy loss.
To reduce the cost for extending the operating power range, resistance compression networks (RCNs) are applied to rectifier designs [26]- [30]. In the previous RCN-based rectifiers [26]- [30], the RCN compresses the variation range of the rectifier input resistance which changes with input power, consequently improving the matching and conversion efficiency. However, most of the reported RCN-based rectifiers have limited operating frequency bandwidth since the performance of RCNs heavily relies on the working frequency of the network, including the auxiliary circuit that transforms the diode impedance to be resistive, and the transmission line RCN in [29]. When the frequency varies, the RCN cannot maintain its function since it is frequency-dependent. Therefore, a wideband impedance manipulation and resistance compression technique is necessary to match the rectifying circuit with the broadband CP antenna. In [31], a complex impedance compression network is developed and applied to the singleand dual-band rectifiers, but has narrow operating frequency bandwidth. In [32], a wideband rectifier is designed by using hybrid resistance compression technique. However, the operating power range is not enhanced.
In this paper, a broadband circularly polarized (CP) rectenna with wide operating power range is proposed. It consists of a wideband RCN-based rectifier and a CP antenna. Unlike the previous frequency-dependent RCN which can only maintain its function in a fixed frequency band, a novel wideband resistance compression technique is proposed in this paper. This technique can simultaneously improve the matching of the rectifier within wide input power and frequency ranges. It is realized by an impedance manipulation network and a coupled-lines-based resistance compression network (RCN). By using this technique, the diode impedance changing with the input power and frequency can be compressed, and the conversion efficiency of the rectifier can be enhanced. A compact broadband CP antenna is designed by feeding a single 1 − λ loop-shaped radiating slot through a wideband coupled-line-based phase shifter. Thus the proposed CP antenna has wide axial-ratio bandwidth and good radiation pattern with a very compact size. At last, the rectifier is co-designed with the CP antenna to complete a CP rectenna with the advantage of high efficiency in both wide operating power and frequency ranges. The proposed CP rectenna can fulfil the requirements of different countries in different frequency bands. Experimental verification using a WPT experiment system shows that the rectenna has good performance.

II. WIDEBAND RESISTANCE COMPRESSION TECHNIQUE FOR RECTIFIERS
A high-efficiency rectifier based on a novel wideband RCN is designed in this section. As shown in Fig. 1, the proposed rectifier is composed of an impedance transformer (TL7), a coupled-lines-based wideband RCN, two identical impedance manipulation networks, diodes and filtering capacitors. The impedance manipulation networks firstly convert the complex impedance to resistance in a wide band. Then the wideband RCN reduces the variation range of the resistance and TL7 converts the resistance range to be around 50 . The configuration of the impedance manipulation network and wideband RCN are described and the design formulas are derived to guide the design. The paralleled capacitor in each branch is used to redirect the RF signals back to the diode, as well as transmitting the DC power to the load R Load . In this design, the Schottky diode HSMS-2860 with the SOT-23 Package is used. The filtering capacitor C from Murata has a value of 100 pF, and the load resistance is 510 . The simulated and measured results including the return loss and conversion efficiency of the proposed rectifier are discussed and compared with some counterparts, as well as other related works.

A. IMPEDANCE MANIPULATION NETWORK
The calculation procedure of the parameters of both two impedance manipulation networks are described in this part. The impedance manipulation networks on the two branches are exactly the same. Each one consists of a microstrip line (TL1 or TL3) and a short-circuited shunt stub (TL2 or TL4). It is used to transform the input impedance of the rectifying circuit to be nearly resistive within the operating input power 80562 VOLUME 8, 2020 and frequency range. Because the two impedance manipulation networks are the same, only the one with TL1 and TL2 is analyzed here as an example.

1) SERIES TRANSMISSION LINE TL1
Firstly, the characteristic impedance Z 1 and electrical length θ 1 of TL1 are calculated. TL1 is used to make the imaginary part of the diode impedance Z L odd-symmetrical with respect to the center frequency, meanwhile maintaining the real part nearly constant. In the operating frequency band from f 1 to f 2 , the diode input impedances Z d at the boundary frequency f 1 and f 2 are denoted as R d1 + jX d1 and R d2 + jX d2 , respectively. The characteristic impedance and electrical length of TL1 are Z 1 and θ(f 1 ) (or θ 1 ) at f 1 . By using TL1, the input impedance at f 1 and f 2 can be expressed as where θ(f 2 ) is the electrical length at f 2 . By introducing the frequency ratio k = f 2 /f 1 , we have θ(f 2 ) = kθ(f 1 ) = kθ 1 . Moreover, TL1 transforms the input impedances Z in1 (f 1 ) and Therefore, Z 1 and θ 1 can be calculated by The diode input impedance Z d can be obtained by circuit simulation. Then the characteristic impedance Z 1 and electrical length θ 1 of TL1 can be calculated according to (3) and (4). In this design, the working band of the rectifier is expected to be [1.7 GHz, 2.7 GHz]. Then Z 1 and θ 1 are calculated to be 140. 4 and 45.5 • , respectively. Fig. 2(a) shows the input impedance Z in1 of the rectifying circuit in the frequency band of 1.7-2.7 GHz. As observed, the real part of Z in1 is around 39 and the imaginary part is nearly odd-symmetrical with respect to the frequency of 2.2 GHz at the input power level of 12 dBm. Moreover, the impedance Z in1 at 0 dBm and 6 dBm varies in a similar trend with that at 12 dBm, and the curves are quite close to each other, benefiting from TL1. Thus TL1 still works, although the diode impedance changes with input power.

2) SHUNT SHORTED-CIRCUITED LINE TL2
Secondly, the characteristic impedance Z 2 and electrical length θ 2 of TL2 are calculated. TL2 is used to offset the imaginary part of Y in1 maintaining the real part nearly unchanged. Y in1 is the inverse of Z in1 . The characteristic impedance and electrical length of TL2 are Z 2 and θ(f 1 ) (or θ 2 ) at f 1 . Thus the input admittance of the short-circuited shunt stub is Y in2 (f ) = 1/(jZ 2 tan θ(f )). By properly designing TL2, its admittance is also odd-symmetrical with respect to the center frequency of 2.2 GHz, and varies inversely as compared to the imaginary part of Y in1 . It can make the imaginary part close to zero. As analyzed above, the admittances after adding TL1 are conjugated at f 1 and f 2 , which can be expressed as G−jB and G+jB, respectively. In order to offset the imaginary part, the admittance of the short-circuited shunt stub should be Combining (5) and (6), we have the following relationship Therefore, the characteristic impedance Z 2 and electrical length θ 2 of TL2 can be calculated. In this design, Z 2 and θ 2 are 30.6 and 69.6 • , respectively. Fig. 2(b) depicts the input impedance Z R of the rectifying circuit with the impedance manipulation network. As observed, the input impedance varying with frequency and power lies beside the real axis on Smith Chart. Therefore, the parameters Z 1 , Z 2 , θ 1 , θ 2 for both two impedance manipulation networks can be calculated according to (3), (4), (7), and (8).

B. WIDEBAND RESISTANCE COMPRESSION NETWORK
In this part, a wideband RCN is proposed and applied to the rectifier design. In a RCN, there are two transmission lines in the two branches of the RCN, with the electrical length of 90 0 + θ and 90 0 − θ, respectively [27], and characteristic impedance Z T . In order to develop it to wide band operation, firstly, the load of the RCN should be resistive in a wide band. This has been achieved by the impedance manipulation network. As analyzed above, by using the impedance manipulation network, the imaginary part of the rectifier input impedance Z R is close to zero, especially at the boundary frequencies f 1 and f 2 . Thus it is negligible, and the real part of the input impedance is considered in the following analysis. Since the input resistance of the rectifier varies with input power, the common resistance range over the operating frequency band is denoted as [Z R1 , Z R2 ]. According to the concept of RCN [29], we have In the conventional RCN, the electrical length of the two branches cannot remain complementary as frequency varies, resulting in a narrow frequency band. In this work, two coupled microstrip lines TL5 and TL6 are specifically designed to implement the wideband RCN as shown in Fig. 1 and 3. The characteristic impedance of TL5 is Z ce for the even mode and Z co for the odd mode, and its electrical length is θ C1 at f 1 and θ C2 at f 2 . TL5 is equivalent to a transmission line with the characteristic impedance Z T and electrical lengths θ T1 at f 1 and θ T2 at f 2 . θ T1 is equal to (90 0 + θ). It is necessary to determine θ T1 and θ T2 to realize wideband resistance compression. They are expected to satisfy the following relationship In this case, TL6 is expected to be equivalent to a transmission line with the characteristic impedance Z T , electrical length (180 0 − θ T1 ) at f 1 , and electrical length θ T1 at f 2 . Therefore, the equivalent electrical lengths of TL5 and TL6 remain complementary at f 1 and f 2 , which satisfies the requirement of RCN.
The characteristic impedance and electrical length of the couple lines TL5 and TL6 are calculated based on θ T1 , θ T2 and Z T , which are analyzed below.
The ABCD matrices of the coupled line TL5 and its equivalent transmission line satisfy the following relationship at Then we have Likewise, the related expressions at f 2 can be obtained. Moreover, in order to satisfy (10), θ T1 and θ T2 are supposed to have the following relationship Combining (12) and (15), as well as (13) and (16), we have Thus the odd-and even-mode characteristic impedance of TL5 can be calculated by combining (13), (14) and (17) After substituting (18) and (19) into (12), the electrical length of TL5 can be obtained by where θ C2 = f 2 /f 1 × θ C1 for TL5 with fixed physical length. Therefore, by solving (18)-(20), the characteristic impedance and electrical length of the coupled line TL5 are obtained. Subsequently, TL6 can also be designed according to the above equations, which has the equivalent characteristic impedance Z T and electrical length (180 0 − θ T1 ) at f 1 . In this way, the requirement of RCN can be satisfied at both the lower and upper frequencies f 1 and f 2 . Moreover, the equivalent electrical length of the couple line TL5 changes from θ T1 to (180 0 − θ T1 ), while that of TL6 varies from (180 0 − θ T1 ) to θ T1 as the frequency increases from 1.7 to 2.7 GHz. In this case, the equivalent electrical length of TL5 and TL6 remains complementary to a certain extent over 1.7-2.7 GHz. Furthermore, because the input impedance of the rectifier changing with input power has similar variation range within the operating frequency band by using the impedance manipulation network, the resistance compression can be realized over the whole operating frequency band. Finally, TL7 is used to match the compressed impedance variation range to the antenna, which slightly improves the impedance matching performance. Note that if the proposed WRCN is applied to other rectifiers with extremely large or small diode impedance, such as the diode SMS7630, TL7 would be more useful.

C. CIRCUIT PERFORMANCE
Based on the above analysis and equations, a wideband RCNbased rectifier can be obtained. The rectifier is designed to work from 1.7 GHz to 2.7 GHz on the Rogers RO 4003 substrate, as shown in Fig. 4. The circuit parameters are optimized as: L 1 = 2.6 mm, L 2 = 14.9 mm, L 3 = 7.6 mm, L 4 = 17.8 mm, L 5 = 5.85 mm, L 6 = 20.8 mm, L 7 = 10.7 mm, S = 1 mm, W 1 = 3.15 mm, W 2 = 2.1 mm, W 3 = 1 mm, W 4 = 1.7 mm, W 5 = 2.65 mm, W 6 = 5.4 mm, W 7 = 0.15 mm. The 50 -transmission line with the length of L5 is used to connect TL2 and TL5 (TL4 and TL6) in the layout of the rectifier, which slightly affects the performance. To highlight the advantage of wide operating power and frequency ranges of the proposed rectifier, three reference rectifiers as shown in Fig. 5 are compared, i.e., Rec1: a previous single-band rectifier with RCN by using the method in [29], Rec2: a simple single-band rectifier without the RCN, Rec3: a wideband rectifier without the RCN by using the method in [33]. Rec1 is realized by using a RCN [29]. Rec2 is completed by replacing the asymmetric RCN with a symmetric T-junction combiner. Rec3 is implemented by using two identical coupled lines and two impedance manipulation networks. Note that the function and design formulas of the couple lines in [33] is total different from the proposed wideband RCN. All the diodes used in the rectifiers are Schottky diode HSMS-2860, the capacitance value of the filtering capacitors is 100 pF, and the resistance value of loads is 510 . Fig. 6 shows the simulated S 11 of the proposed wideband RCN-based rectifier, as well as the Rec1, Rec2, and Rec3. As observed, the proposed wideband RCN-based rectifier has a wider 15-dB impedance bandwidth of 1.5-2.7 GHz (57.1%) than Rec1 with a bandwidth of 1.9-2.45 GHz (25.3%). The frequency bandwidth of the proposed rectifier is similar with that of Rec3. Fig. 7(a), (b) and (c) plot the S 11 versus input power of all the rectifiers at 1.7 GHz, 2 GHz and 2.7 GHz, respectively. Note that Rec1 and Rec2 achieve the best matching at 2 GHz. As shown in Fig. 7 (b), the 15-dB return-loss power range  of the proposed rectifier is similar with that of Rec1 at 2 GHz. Moreover, both Rec1 and the proposed one with RCNs achieve better impedance matching than Rec2 and Rec3 without RCNs. When the frequency varies, the matching performance of Rec1 and Rec2 is greatly degraded, as shown in Fig. 7 (a) and (c). However, the proposed rectifier still maintains good matching performance at 1.7 GHz and 2.7 GHz. Compared with Rec3, the proposed rectifier has wider operating power ranges, which means that the proposed rectifier achieves impedance compression in a wider frequency band.
For validation, the fabricated proposed rectifier is measured. The RF-DC conversion efficiency is quantified by P in is the input power. V out1 and V out2 are the output voltages of the rectifier, which are measured by two multimeters. The simulated and measured efficiency of the proposed rectifier versus frequency is plotted in Fig. 8. The input power/frequency range for the conversion efficiency that is larger than 70% of peak efficiency is defined as the optimal power/frequency range. As seen, the optimal frequency  range includes the working frequency band of the broadband CP antenna is from 1.7 GHz to 2.7 GHz. The measured results agree well with the simulated ones. The slight difference is due to the fabrication tolerance and diode model inaccuracy.
Moreover, the comparison of the measured efficiency between the proposed rectifier and the Rec1, Rec2 and Rec3 is shown in Fig. 9. As shown in Fig. 9(a), four rectifiers have nearly equal peak efficiency for the input power of 14 dBm. But the proposed RCN-based rectifier and the Rec3 maintain high conversion efficiency in a much wider frequency band. When the input power decreases, as shown in Fig. 9(b) and (c), the measured efficiency of Rec2 and Rec3 without RCNs decreases substantially. When the input power is reduced to −10 dBm, the maximum efficiency of Rec2 and Rec3 is less than 5%, and that of the proposed rectifier is about 20%. This is because the input power variation results in not only the decrease of diode efficiency, but also the degradation of matching performance. The proposed RCN improves the impedance matching of the rectifier with a wide input power range, thereby increasing the efficiency. The proposed rectifier realizes impedance compression over a wide frequency range, thus improves the efficiency in a wide dynamic power and frequency range. The efficiency fluctuation of the rectifier is mainly due to the variation of impedance matching performance in the operating frequency band and the measurement error.
Furthermore, Fig. 10 illustrates the simulated and measured efficiency of the proposed rectifier and three reference ones versus input power at 2 GHz, which is the working frequency of Rec1 with RCN and Rec2 without RCN. The efficiency of Rec1 and the proposed rectifier with RCNs is higher than that of Rec2 and Rec3 without RCNs before the breakdown voltage is reached. Besides, the proposed rectifier has a comparable performance with Rec1, which indicates that the proposed method improves the bandwidth without scarifying the peak efficiency. It also can be seen that efficiency improvement of rectifiers at lower input power level (from −10 to 0 dBm) is larger than that with higher power level (from 0 to 14 dBm), which is caused by more serious impedance mismatch of the rectifier without RCN as input power decreases from the optimum power.
It should be noted that two branches in Fig. 4 can be combined into one output by connecting them in parallel with the load resistance of RL/2. In this case, the conversion efficiency remains almost unchanged.

D. COMPARISON
A comparison of the proposed rectifier with the wideband RCN with other related works reported in the literatures is given in Table 1. As observed, compared to other related works that extends the operating power range, the proposed rectifier achieves a wide operating power range and a much wider bandwidth. Moreover, compared with the ultrawideband rectifier in [19], the proposed rectifier improves not only the frequency bandwidth, but also the operating power range since the proposed method improves the impedance matching when the input power varies. The proposed rectifier also has a comparable rectifier area with other works, except [32]. The reason for compact circuit area in [32] is that lumped components are used, which introduces much insertion loss at the same time. In addition, the proposed method also provides closed-form equations to clearly demonstrate the design of the rectifier. In brief, the rectifier using the proposed method can achieve high efficiency over wide dynamic power and frequency ranges.
Note that the WRCN-based rectifier with HSMS-2860 diodes in this work is designed as an example to prove the feasibility of the proposed method. The WRCN-based rectifier can work at lower or higher input power level by using appropriate diodes, such as the SMS-7630 diode with  low breakdown voltage (2 V) or HSMS-2820 diode with high breakdown voltage (15 V).

III. COMPACT BROADBAND CIRCULARLY POLARIZED ANTENNA DESIGN
The design of a compact and broadband CP antenna is proposed in this section. Based on [35], we employ a coupledline-based phase shifter for wider operating bandwidth. It is used to receive RF energy for the rectifier in a wide frequency band. Simulated and measured results are also discussed, including S 11 , axial ratio, gain and radiation patterns.
A. ANTENNA DESIGN Fig. 11 shows the configuration of the proposed CP antenna which consists of a one-wavelength square-loop slot on the bottom layer of the substrate and a wideband feeding network on the top layer. The 1 − λ loop-shaped radiating  slot with a side-length of about λ/4 has more compact size and wider bandwidth than the conventional λ/2 rectangular radiating slot. It is very convenient to generate two orthogonal electric-field components by feeding the single loopshaped radiating slot in orthogonal directions. The feeding network is composed of a microstrip feeding line, a Wilkinson power divider, a coupled-line-based 90 0 phase shifter and two Y-shaped feeding stubs. The characteristic impedance of the microstrip feeding line is Z 0 = 50 . The power divider is implemented according to the procedure in [36]. The Wilkinson power divider have two λ g /4-arms (λ g is the guided wavelength in the center frequency) with a characteristic VOLUME 8, 2020 impedance of 1.414Z 0 for impedance transformation. There is a 100-resistor palced across the outputs of the power divider. The coupled line with a gap and a uniform microstrip line without a gap together form a 90 • -phase shifter. The coupled line can be used to enhance the operating bandwidth of the 90 • -phase shifter according to [37]. The input RF signal is divided into two portions with the same magnitude and 90 • -phase difference by the Wilkinson power divider and the 90 • -phase shifter. They are fed to bottom corners of the loop slot by the corresponding Y-shaped feeding stubs. In this way, two orthogonal electric-field components with a phase difference of 90 • are generated, and the proposed antenna can generate RHCP signal on one side and LHCP signal on the other side.  Fig. 12 depicts the measured and simulated S 11 , axial-ratio (AR), and realized gain of the antenna. Fig. 12(a) shows that the antenna has a relative impedance bandwidth of 50.0% (1.68-2.8 GHz) for S 11 < −10 dB. Fig. 12(b) shows that the AR bandwidth for AR < 3 dB is 48.4% (1.66-2.72 GHz). So the operating frequency bandwidth of the antenna is 1.68-2.72 GHz. Fig. 12(b) also shows the gain of the proposed antenna within the operating frequency band of 1.68-2.72 GHz ranges from 2.9 to 4.5 dBi. The gain variations of both simulation and measured results are less than 3 dB in the whole operating band, which is very desirable. Fig. 13 shows the normalized measured and simulated radiation patterns in the yz and xz planes at 1.7 GHz, 2.2 GHz and 2.7 GHz, whose radial scale of the normalized pattern is 10 dB. The proposed antenna radiates right-hand CP (RHCP) fields in the +z axis direction, which is stronger than the left-hand CP (LHCP) fields (cross-polarization) by more than 10 dB across the whole band. Meanwhile, the antenna radiates LHCP fields in the −z axis direction with 10 dB stronger than the RHCP fields (cross-polarization). The measurement results agree well with the simulaiton ones.
The CP antenna in [38] has much larger size than the proposed antenna. The antennas presented in [39], [40] are not suitable to be integrated with the proposed RCN, although they have wider operating bandwidth than the proposed one in this paper. It is because that the CP performance of the antennas in [39]- [41] is substantially affected by the current on the ground planes. If they are integrated with the RCN, the shapes and size of their ground planes will be changed a lot, and the current on the ground will also be changed deteriorating the CP performance. Therefore, the proposed  antenna has comparable bandwidth and total size, and is suitable to be used to design a rectenna.

IV. BROADBAND RECTENNA BASED ON WIDEBAND RESISTANCE COMPRESSION TECHNIQUE A. DESIGN OF THE RECTENNA
A broadband rectenna with wide dynamic operating power range is designed based on the above-mentioned CP antenna and rectifier. The designs of the CP antenna and rectifier were simulated by Driven Model in HFSS and harmonic balance (HB) simulation in ADS. Subsequently, the cosimulation between antenna and rectifier was performed in ADS by harmonic balance (HB) simulation. The proposed broadband CP antenna has a working frequency band of 1.68 -2.72 GHz, and the wideband RCN-based rectifier  with a wide dynamic power range achieves more than 60% efficiency (S 11 < −15 dB) in the frequency range of 1.7 -2.7 GHz. The rectenna can be designed by slight optimization of the matching network between the antenna and rectifier. The rectifier is placed on the bottom layer of the antenna. Fig. 14 shows the photograph of the broadband rectenna with a wide dynamic power range. A rectangular-loop slot lies on the top layer of the rectenna as the radiator. A small metal pad is used to connect the ground of the antenna and that of the rectifier together to ensure that the radiation performance of the slot antenna is affected little. Besides, a feeding network and a rectifier based on the wideband RCN are located on the bottom layer.
The diagram and photograph of the measurement WPT system for the rectenna are shown in Fig. 15 and Fig. 16. The RF signal is generated by the signal source, then amplified by the power amplifier with DC power supply and radiated by the horn antenna with left-handed circular polarization (HD-1040SGACPH7HL). Subsequently, it is harvested and rectified by the proposed rectenna, as well as being transmitted to the load. Finally, the output voltage is measured by a multimeter. The rectenna is located in the far field of the transmitting antenna with a transmission distance of 3 m. The RF-DC conversion efficiency of the rectenna is calculated by where V out1 and V out2 are the measured output voltages on the DC loads. P d is the input power density and A eff is the effective aperture size of the proposed rectenna. P d is calculated by the Friss transmission equation as presented  below.
where P t is the transmitting power, G t is the horn antenna gain, and D is the distance between the horn antenna and the center of the rectenna. The attenuation of the system can also be calculated.

B. EXPERIMENTAL RESULTS
The measured efficiency of the broadband rectenna with and without the wideband RCN for the power density of 5 µW/cm 2 , 15 µW/cm 2 and 30 µW/cm 2 is shown in Fig. 17. It can be seen that the RF-DC conversion of the rectenna is increased over a wide frequency band by using the proposed wideband RCN. Within the power density from 5 to 30 µW/cm 2 , the efficiency improvement is up to about 24.5%. On the other hand, the efficiency of the rectenna increases with the power density. The fluctuations in efficiency are mainly due to slight instability of the measurement WPT system. VOLUME 8, 2020

V. CONCLUSION
A novel wideband resistance compression technique has been proposed to improve the impedance matching performance between the rectifier and antenna, consequently increasing the conversion efficiency in wide operating power and frequency ranges. The proposed rectifier has achieved over 60% (up to 76%) conversion efficiency for the input power from 5 to 17 dBm and frequency bandwidth of 1.7-2.9 GHz. Compared with the previous RCN-based rectifiers, the proposed rectifier has achieved impedance compression and efficiency improvement in a wider frequency range. The proposed rectifiers have better conversion efficiency. The rectifier has also been co-designed with a broadband CP antenna working from 1.68 to 2.72 GHz to construct a broadband CP rectenna with dynamic-power-range. The proposed rectenna is of good industrial value as it can overcome the efficiency degradation of the WPT applications caused by the transmission distance and environmental changes to some extent. Also, it is able to be used among different countries and areas due to its wide operating band. In addition, the feeding network of the antenna and the matching network of the rectifier will be co-designed to miniaturize the rectenna in our future research. Moreover, antennas with higher gain will be designed to improve the received power of the rectenna.