Design of Low-Profile Dual-Band Printed Quadrifilar Helix Antenna With Wide Beamwidth for UAV GPS Applications

In the paper, a low-profile dual-band printed quadrifilar helix antenna (PQHA) with wide beamwidth is presented for UAV GPS applications. The antenna is composed of a hollow dielectric cylinder with four arms of dual-helix metal strips and a hollow dielectric ring with four pairs of circular metal strips. To provide dual-band operation, different lengths of the dual-helix metal strips are applied. By loading the circular metal strips on the dual-helix metal strips, size reduction of the antenna height is realized. Moreover, wide beamwidth is obtained. A miniaturized quad-feed network is also designed to provide equal amplitude signals with sequentially quadrature phases. To validate the proposed structure, a prototype is fabricated. The height of the antenna is 18.5 mm. The experimental results show that good impedance matching ( $\vert S_{11}\vert < -18$ dB) and axial ratio (< 2 dB) are obtained at GPS L1/ L2 bands. At 1.227 GHz, the measured 3-dB axial ratio beamwidths (ARBWs) are 186° and 187° at xoz and yoz planes, respectively. While the values are 167° and 163° at 1.575 GHz. The measured half-power beamwidths (HPBWs) are both more than 120° at the two bands. The results indicate that the proposed PQHA is a good candidate for UAV GPS applications.


I. INTRODUCTION
Unmanned aerial vehicles (UAVs) are widely used for exploration, surveillance, data and multimedia communications, and telemetry applications. Recent advances in UAVrelated technologies have led to an increasing demand of small, lightweight antennas for communication and navigation [1], [2]. During the operation of UAVs, the position information of the UAV can be obtained by GPS positioning. To receive signals from GPS orbiting satellites, the GPS antenna is required to have right-hand circular polarization (RHCP) and a uniform mode covering the entire upper hemisphere.
In previous researches on GPS antenna, the quadrifilar helix antennas (QHA) are well studied [3]- [6], which usually The associate editor coordinating the review of this manuscript and approving it for publication was Venkata Ratnam Devanaboyina .
have good circular polarization (CP) performance. Improved from QHA, printed quadrifilar helix antennas (PQHA) are developed with four radiation elements printed on flexible dielectric substrate [7]- [10]. Generally, the PQHA is composed of four parallel helix-shaped quadrifilar radiation elements, and each radiating element is fed in phase quadrature at the endpoint close to the bottom ground.
Although the structure of PQHA is already small, further size reduction is required to meet the space limitations and the high overload for unmanned platform application. Several achievements to reduce antenna size have already been developed including inserting dielectric loading [11], [12], applying different helix turn angle [13], folding the helix antenna arms [14], using the impedance matching network [15] and the power divider networks [16]. Recently, methods for miniaturizing dual-band or triple-band antenna have also been proposed. In [17] and [18], dual-band PQHAs 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/ with stepped-width arms and dual-faced slot radiation structure are proposed. In [19], by meandering and turning the helix arms into the form of square spirals, size reduction of 43 % is obtained. More recently, a miniaturized dual-band spiral PQHA for UAV application is proposed by surface and inner dielectric loading [20]. Although the techniques described above would reduce the antenna's size, each of these techniques has some specific advantages and limitations with respect to impedance bandwidth, radiation pattern, size and structural strength. In GPS systems, the positioning needs receive signals from at least four satellites. Thus, CP antennas with 3-dB axial ratio beamwidth (ARBW) of larger than 120 • and high gain at low elevations are more preferred for better reception of signals no matter whether the satellites are in low or high elevations. However, the features of wide 3-dB ARBW and high gain at low elevations are often ignored by most of the PQHAs.
In the paper, a low-profile dual-band PQHA with wide beamwidth is proposed for GPS L1/L2 applications. It consists of a hollow dielectric ring and a hollow cylinder with metal strips etched on the external surfaces. By loading the hollow dielectric ring on the hollow cylinder as the metal strip supporter, size reduction of the PQHA is realized. Moreover, the beamwidth is also broadened. For verification, a prototype is fabricated and measured. The experimental results show that the advantages of compact structure, good AR, broad beamwidth, high cross-polarization discrimination and low profile are exhibited by the proposed antenna. This paper is organized as follows. Section II presents the proposed PQHA. Section III introduces the implementations followed by a conclusion in Section IV.

II. DESIGN OF THE PROPOSED PQHA A. ANTENNA CONFIGURATION
The structure of the proposed PQHA is shown in Figure 1. It is composed of a hollow dielectric supporting ring with four pairs of circular metal strips, a hollow dielectric supporting cylinder, a cylinder substrate etching by four pairs of dual-helix metal strips, four grounding lines, four feeding lines, a ground plane, and a quad-feed network. The hollow dielectric supporting ring and the hollow supporting cylinder are fabricated using 3-D printed technique with the material of nylon. The measured permittivity of the nylon is 3 in the experimental environment. The hollow dielectric supporting ring is mounted on the top of the hollow dielectric supporting cylinder. The F4B (ε r = 3.5, tanδ = 0.003, h = 0.2 mm) is selected as the substrates for supporting the dual-helix metal strips and the feed network. In the design, a miniaturized quad-feed network is utilized to provide equal amplitude signals with sequentially quadrature phases. The feed network is composed of three Wilkinson power dividers, a 180 • phase shifter, and two 90 • phase shifters. Since the space is limited on the unmanned platform, the BP2G1+ chip working in the range of 1.2 GHz ∼ 2.0 GHz is utilized as the power divider for size reduction of the feed network. Modified phase shifters with shortened stubs are also proposed for further miniaturization. Figure 2 shows the expanded view of one pair of the radiation elements. The arms in Part I are the dual-helix metal strips etched on the external surface of the cylinder substrate. The arms in Part II are the circular metal strips supported by the hollow dielectric ring. By mounting the hollow dielectric ring on the top of the hollow dielectric cylinder, the arms in Part II are connected with the arms of Part I, resulting in the formation of two radiators. For dual-band GPS application, the longer arm is designed at 1.227 GHz and the shorter arm is designed at 1.575 GHz.

B. EVOLUTIONS OF THE PROPOSED PQHA
The evolutions of the proposed PQHA are shown in Figure 3. Firstly, a traditional PQHA resonated at 1.575 GHz  is designed, as shown in Figure 3(a) (named as PQHA-I). Each helix arm is designed to be λ/4 and 1/2 turn. As a result, the width (w 1 ) and length (l 2 + l 3 ) of the arm are optimized as 1.6 mm and 40 mm, respectively, with a radius (R 2 ) of 10 mm. The overall height of the dielectric cylinder (H a ) is 30 mm. Figure 4 shows the simulated |S 11 | and a resonate frequency is observed at 1.575 GHz. However, the impedance matching is not good. To adjust the impedance, four grounding lines are added to each of the helix arm, as shown in Figure 3 (b) (named as PQHA-II). It can be seen from Figure 4 that the |S 11 | at 1.575 GHz is changed from −5.5 dB to −18 dB by inserting the grounding lines. For PQHA-II, the optimized overall height of the dielectric cylinder (H a ) is reduced to 25 mm with (l 2 + l 3 ) of 36.4 mm. The values of w 1 and R 2 are unchanged. Except of better impedance matching, size reduction is also obtained.
Next, another arm operating at 1.227 GHz is added for dual-band operation, as shown in Figure 3(c) (named as PQHA-III). By tuning the length (l 1 + l 3 ) of the added arm, dual-resonated frequencies can be observed, as shown in  A hollow dielectric supporting ring is mounted on the top of the hollow dielectric supporting cylinder. By etching the circular metal strips on the dielectric ring, the height of the dielectric cylinder is reduced to 18.5 mm for resonating at the same two frequencies. Besides size reduction, wide 3-dB ARBW can also be obtained by using the circular metal strip. Figure 5 shows the simulated ARBW and half-power beamwidth (HPBW) of the four evolution structures at 1.575 GHz. It is observed that by using the circular metal strips, the 3-dB ARBW is enhanced while maintain the same HPBW.

C. PARAMETRIC STUDY OF THE PROPOSED PQHA
Since our design makes use of the circular metal strips to reduce height and enhance the 3-dB ARBW, a parametric study is presented in this section to illustrate the effects. Firstly, the radius of the circular metal strip is investigated. Figure 6 shows the different cases of the circular metal strips. Since the length of the circular metal strip directly affects the resonant frequency, the arm radius R 3 and the subtended angles (θ 1 and θ 2 ) in each case are chosen in such a way VOLUME 8, 2020 that all of the different cases resonate at 1.575 GHz and 1.227 GHz. The optimized values of the arm radius R 3 and the subtended angles (θ 1 and θ 2 ) for each case are also provided in Figure 6. It is observed that to resonate at the same frequency, the values of θ 1 and θ 2 are decreased with the increasing of R 3 . Figure 7 shows the simulated gain and AR in the ϕ = 0 • elevation plane for the different cases. It is observed that with the increase of θ 1 and θ 2 , the 3-dB ARBW at the two resonate frequencies are both enhanced. While the changing of the HPBWs are not obvious. In detail, at 1. However, it should be mentioned that θ 1 can't be larger than 180 • for avoiding the connection with the dual-helix metal strip.
Secondly, the influence of the distance between the dualhelix metal strip (named as the connecting line L 4 ) is discussed. Since the connecting line (L 4 ) is etched on a cylinder, the rotated angle θ 3 of the connecting line is utilized as the variable. Figure 8 shows the simulated ARBWs of the PQHA when the value of θ 3 is varied in the range of 35 • to 55 • . The corresponding values of L 4 are also provided. It is observed from Figure 8(a) that the 3-dB ARBW is nearly constant at 1.227 GHz, which indicates that the distance between the dual-helix metal strips shows less influence on the ARBW at GPS L2 band. While it is found from Figure 8(b) that the 3-dB ARBW at 1.575 GHz is slightly changed with the distance. Widest 3-dB ARBW is obtained at θ 3 = 45 • . When θ 3 is less than or larger than 45 • , the arm resonated at 1.575 GHz is more closed to the arm resonated at 1.227 GHz. Since small distance enhances the mutual coupling, the radiation at 1.575 GHz will be affected by the adjacent radiation arm. Finally, the value of θ 3 is chosen as 45 • , and the corresponding value of L 4 is 5.5 mm. Other parameters are optimized using the HFSS, and the optimal dimensions of the PQHA are listed in Table 1.

D. DESIGN OF THE MINIATURIZED FEED NETWORK
In order to provide equal amplitude signals with sequentially phases of 0 • , −90 • , −180 • , and −270 • , a quad-feed network is designed, as shown in Figure 1(d). It is composed of three Wilkinson power dividers, a 180 • phase shifter, and two 90 • phase shifters. For UAV Application, the dimensions of the feed network should be as small as possible. Thus, methods of size reduction are applied. Firstly, the BP2G1+ chip working in the range of 1.2 GHz ∼ 2.0 GHz is utilized as the power divider [21]. Secondly, a miniaturized wideband phase shifter is proposed to provide quadrature phases. In general, the topology in [22] is a simple one, which is composed of a main line and a reference line. The main line is realized by a combination of shunt λ/8 open-/short-ended stubs, while the reference line is a simple straight transmission line. However, the λ/8 open-/short-ended stubs are still large for limited design area. For further miniaturization, a modified structure of [22] is proposed, as shown in Figure 9.
The main line of the proposed phase shifter is composed of two transmission lines with characteristic impedance of Z 1 and electrical length of θ 1 , an open-ended stub with characteristic impedance of Z 2 and electrical length of θ 2 , and a short-ended stub with characteristic impedance of Z 3 and electrical length of θ 3 . The reference line has a characteristic impedance of Z 4 and an electrical length of θ 4 . In the following, the design formulas of the proposed phase shifter are derived. Firstly, according to [23], the ABCD matrix of the reference line can be expressed by While, the ABCD matrix of the main line is (2a) and (2b), as shown at the bottom of the page.
By converting the ABCD matrices into the scattering matrices, the transmission phase shift of the main line is calculated to be and that of the reference line is Therefore, the transmission phase difference of the two paths is To keep ideal impedance matching, the following relation should be satisfied, which is derived from |S p11 | = 0 Then, substitute (6) into (5), derives (7), as shown at the bottom of the next page.
Using equations (2b), (6) and (7), the circuit parameters of the phase shifter can be obtained. The design procedures are listed below: [A] M =    cos 2θ 1 − Z 1 sin 2θ 2 2 a 1 jZ 1 sin 2θ 1 − Z 1 sin 2 θ 1 a 1 j Z 1 sin 2θ 1 + Z 1 cos 2 θ 1 a 1 cos 2θ 1 − Z 1 sin 2θ 2 1) According to (7), the value of Z 1 can be calculated with predetermined θ 1 , θ 4 and ϕ. 2) Under the condition of (2b) equals with (6), the values of θ 2 , θ 3 , Z 2 and Z 3 can be chosen arbitrarily. Based on the above design procedure, the 90 • phase shifter ( ϕ = 90 • ) is firstly designed. The values of θ 1 and θ 4 are predetermined to be 12 • and 100 • , respectively. The value of Z 1 is calculated to be 20.6 . Then the values of θ 2 , θ 3 , Z 2 and Z 3 can be chosen. When choosing, two limitations should be considered. First is the electrical length. Since the area of the feed network is limited, the electrical lengths of the open-/short-ended stubs should be as short as possible. Second is the characteristic impedance. High impedance should not to be chosen for avoiding high fabrication difficulties. Finally, the values of θ 2 , θ 3 , Z 2 and Z 3 are chosen as 25 • , 31 • , 70 , and 70 , respectively. For the 180 • phase shifter ( ϕ = 180 • ), the values of θ 1 and θ 4 are predetermined to be 73 • and 280 • , respectively. The value of Z 1 is calculated to be 18.2 . The values of θ 2 , θ 3 , Z 2 and Z 3 are chosen as 19 • , 28 • , 62 , and 14 , respectively. Figure 10 shows the simulated results of the 90 • and 180 • phase shifters. The amplitude and phase characteristics in the desired frequency band are good enough for the feeding network design.

III. FABRICATION AND MEASUREMENTS
The fabricated photograph of the proposed PQHA is shown in Figure 11. The performance of the fabricated PQHA is measured by PNA network analyzer N5230A and anechoic chamber. Figure 12 shows the measured |S 11 | of the fabricated PQHA integrated with the feed network. It is seen that the measured |S 11 | is less than −10 dB in the range of 1.1 ∼ 1.65 GHz. The measured AR and peak gain versus the frequency are shown in Figure 13. It is observed that the AR is less than 3 dB in the ranges of 1.15 GHz ∼ 1.30 GHz and 1.52 GHz ∼ 1.60 GHz. At 1.227 GHz, the measured   gain is 2.9 dBic, and the value is 3.9 dBic at 1.575 GHz. Figure 14 shows the measured radiation efficiency of the fabricated PQHA. It is observed that from 1.15 GHz to 1.3 GHz, the radiation efficiency is more than 60 %, while from 1.52 GHz to 1.6 GHz, the values are more than 80 %. At 1.227 GHz and 1.575 GHz, the efficiencies are 75.8 % and 89.6 %, respectively.
In Figure 15, the measured radiation patterns compared to the simulated ones are shown for GPS L2 (1.227 GHz) ϕ = θ 4 − arctan 2Z 1 Z 0 sin 2θ 1 Z 2 0 2 cos 2θ 1 cos θ 2 1 + sin 2 2θ 1 + Z 2 1 2 cos 2θ 1 sin θ 2 1 − sin 2 2θ 1 (7) 157546 VOLUME 8, 2020   Table 2 shows the comparisons between the proposed and previous reported PQHAs. It is observed that by loading the circular metal strips, the proposed antenna shows the lowest height. Besides, wide 3-dB ARBW is also obtained which is usually ignored by other researches. Based on the advantages of low profile and wide beamwidth, the proposed antenna can be a good candidate for UAV GPS applications.

IV. CONCLUSION
In the paper, the design of a low-profile dual-band PQHA with wide beamwidth is presented. The dual-band operation is realized by using the dual-helix metal strips with different lengths. The height of the proposed PQHA is shortened by using the circular metal strips. Moreover, wide beamwidth is also obtained. To provide equal amplitude signals with sequentially quadrature phases, a miniaturized quad-feed network is designed, which is composed of the chip-based power divider and the proposed modified phase shifter. To validate, a prototype working at GPS L1/L2 bands is designed, fabricated and measured.