Single-Arm Archimedean Spiral Antenna with Broadband Circular Polarization

In this paper, a single-arm Archimedean spiral (SAAS) antenna with broadband circular polarization is investigated. Unlike traditional single-arm Archimedean spiral antenna, the antenna arm consists of a hybrid meandered strip line and a smooth arc strip line. Especially at low frequencies, the meandered strip line significantly improves the circular polarization performance by extending the antenna surface current path.1e effects of themeandered strip line on the radiation pattern and axial ratio (AR) are studied in detail. To obtain unidirectional radiation, a metallic cavity is added below the SAAS antenna. 1e measurement results show that the voltage standing wave ratio (VSWR) is less than 2 from 0.88GHz to 8.82GHz, which indicates a wide impedance bandwidth of 1 :10 is realized. A wide AR bandwidth of 1 : 5 is available, that the measured AR is less than 3 dB from 1.6GHz to 8GHz.


Introduction
Pantograph arcing, caused by the separation of the pantograph and the contact wire, is very common in high-speed railways. Pantograph arcing can generate wideband electromagnetic interferences, which are mostly random and varies with frequency. Circularly polarized (CP) antennas [1][2][3], which are extensively applied in satellite communication systems, can be applied to pantograph arcing detection. Among circularly polarized antennas, Archimedean spiral antennas have attracted extensive attention because of their broad bandwidth, stable half power beam width (HPBW), and good circular polarization (CP) properties [4][5][6][7]. A traditional Archimedean spiral antenna has two symmetrical arms and requires balanced excitations, which limits its applications in engineering applications. A singlearm Archimedean spiral antenna, which takes one-half the space of a traditional Archimedean spiral antenna and does not need balanced excitations, is preferred in UWB communication applications because of its compact size [8][9][10].
A two-arm Archimedean spiral antenna is excited in an unbalanced mode [11].
is unbalanced mode spiral antenna does not need balanced excitation and exhibits good CP properties. It turns out that an unbalanced mode can be applied in spiral antenna design. In [12], the performance of a single-arm spiral antenna reveals that wide bandwidth and a good axial ratio over a wide frequency band are possible. e relationship between the radiation patterns and arm length for a single-arm rectangular spiral antenna is extensively studied in [13]. A conical-disc-backed single-arm Archimedean spiral antenna is designed to cover the X-band from 8 to 12 GHz [9]. A single-arm hexagonal spiral antenna array having a 30°beam scan without a grating lobe can operate from 2 to 6 GHz [14]. e method of moments (MOM) is applied to analyze the impedance and radiation pattern of the single-arm spiral antenna excited via a vertical probe [15]. e calculated results show that the single-arm spiral antenna has a wide impedance bandwidth and good circular polarization performance over a wide range of arm lengths (from 3.3λ 0 to 4.9λ 0 , where λ 0 is the wavelength of the central frequency). e conformal finite difference time domain (FDTD) method is applied in analyzing the performance of the microstrip muscle-loaded single-arm Archimedean spiral antenna [16]. e abovementioned numerical calculation results provide that the single-arm spiral antenna has broad bandwidth and good CP performance. A wideband antenna array consisting of single-arm spiral antennas is designed in [17]. An excellent AR of less than 0.74 dB is realized by rotating each single-arm spiral antenna sequentially together while imposing a proper phase shift. e research results from [18] demonstrate that the radiating direction of a single-arm spiral antenna depends on the arm length. A tilted-beam spiral antenna with a single-arm is used in a multiband multipolarization sharedaperture antenna [8]. A single-arm rectangular spiral antenna that utilizes a set of microelectromechanical system (MEMS) switches to obtain a reconfigurable scan beam is discussed in [19]. An adaptive beam is realized by introducing variations in current distributions by using switches. A single-arm rectangular spiral antenna, which can realize reconfigurable radiation patterns, is realized by using four switches over a high-impedance surface [20]. e meandered line is demonstrated as an effective way to reduce the antenna size [21,22]. A reflector plane with wideband balun instead of absorber and cavity is employed to realize a lowprofile spiral antenna [23]. Coplanar waveguide transmission line fed spiral antenna without using an external balun or matching network is available to realize compact antenna structure [24]. Broadband circular polarization is an important aspect of the spiral antenna design. A circular reflector with one circular slot at the junction of two orthogonal rectangular slots is designed to improve the performance of AR [25]. e effect of the meandered line on the circular polarization performance has not been studied in single-arm spiral antenna design until now, so we will discuss the meandered line in improving the AR of the single-arm spiral antenna. is paper presents a printed low-cost single-arm Archimedean spiral (SAAS) antenna. e antenna arm employs a meandered strip line to enhance the circular polarization performance at low frequencies.
e meandered strip line SAAS antenna is studied and compared with a classical SAAS antenna to demonstrate its good circular polarization performance. Simulation results show that the AR of the proposed SAAS antenna is lower than that of the classical SAAS antenna. e meandered strip line effectively extends the antenna surface current path that improves the AR of the classical SAAS antenna. Moreover, a metallic cavity is employed to transform the bidirectional beam into the unidirectional beam. e simulated and measured results indicate that the proposed SAAS antenna with a metallic cavity has a wide impedance bandwidth and AR bandwidth. Figure 1 shows the configuration of the proposed SAAS antenna. As shown in Figure 1(a), the SAAS antenna is composed of a single-arm, a substrate, a conducting disc ground (acting as the ground plane) and a coaxial feeding probe. e SAAS antenna arm is defined by the Archimedean function r � aφ, where a � 0.75 mm/rad is the spiral constant and φ is the winding angle. Unlike classical singlearm spiral antenna, this SAAS antenna arm exploits a hybrid strip line rather than a smooth arc line [12]. A smooth arc strip line and a meandered strip line are employed in the inner loops and outer loops, respectively. ey are directly connected together at point A, as shown in Figure 1(c). Because the meandered strip line is helpful in extending the current path, it is used to improve the antenna circular polarization performance at low frequencies. e effects of the meandered strip line on the antenna performance are studied in the following section.

Antenna Design and Configuration
Traditional spiral antennas with two spiral arms need balanced excitations. A balun is needed to provide balanced excitations [22]. Benefiting from the unbalanced structure, the SAAS antenna can be excited by unbalanced feeding, e.g., a coaxial probe, as shown in Figure 1(a). As shown in Figure 1(b), the outer conductor and inner conductor of the coaxial probe are connected to the disc ground (radius r g � 25 mm) and the antenna arm, respectively. Without balun, the SAAS antenna gets a compact structure via unbalanced feeding. An FR4 substrate, whose dielectric constant is 4.2, thickness h � 2 mm, and radius r s � 68 mm, is exploited in the SAAS antenna. e detailed SAAS antenna arm configuration is exhibited in Figure 1

Comparisons between Antennas Using Meandered Strip
Line and Smooth Arc Strip Line. Two Archimedean spiral antennas are simulated, as shown in Figure 2. In Figure 2(a), the winding angles are φ s � 0.5π, φ e � 13.5π and φ m � 27π. In Figure 2(b), the winding angles are φ s � 0.5π and φ m � 27π. e simulated reflection coefficient of both antennas is illustrated in Figure 3. It is noticed in Figure 3 that the proposed SAAS antenna has a lower reflection coefficient (0.24 GHz to 0.76 GHz) at low frequencies. Because the electrical current flowing along the antenna arm is extended on the meandered strip line. Generally, the lowest working frequency of a traditional Archimedean antenna with two arms can be estimated by 2πr ≈ λ L (r is the radius of the Archimedean antenna and λ L is the wavelength of the lowest working frequency). is approach is also applicable for a single-arm Archimedean antenna [12]. However, exactly one wavelength is not adequate to obtain a good refection coefficient at the lowest working frequency. e circumference is chosen to be approximately two wavelengths at the lowest frequency [11]. For this SAAS antenna, compact size is expectable. Notice that in Figure 3, the reflection coefficient is less than −10 dB from 0.7 GHz. e radius of antenna r s is 68 mm, and the circumference is exactly one wavelength of the lowest working frequency, 0.7 GHz. e axial ratios (ARs) at 0.8 GHz and 1 GHz for both antennas are shown in Figure 4. As shown in Figure 4(a), the AR of the classical SAAS antenna exceeds 3 dB at 0.8 GHz, so the circular polarization is not well performed. e AR of the proposed SAAS antenna is less than 3 dB from −50°to 50°, and its minimum is less than 1 dB. is result means that  International Journal of Antennas and Propagation good circular polarization is available. Similarly, as can be observed from Figure 4(b), good CP is realized at 1 GHz for the proposed SAAS antenna. e AR is within 3 dB from −40°to 40°, which is lower than that of the classical SAAS antenna. e AR improvement of the proposed SAAS antenna can be explained by the behavior of the surface current along the antenna arms. We draw a spiral line along the center of the antenna arm. en the current data can be derived from the current simulation, which is described in Figure 5. Figure 5 shows the absolute value of the surface current along the spiral line. At the end of the antenna arm, the current of the proposed SAAS antenna is much less than that of the classical antenna. is indicates that the meandered strip line can better attenuate the surface current. e current reflected from the end of the proposed SAAS antenna arm is reduced. e reflected current can deteriorate the AR, which is harmful to the circular polarization performance. Hence, we can infer that the meandered strip line is effective in improving the CP performance of the SAAS antenna. e radiation patterns of both antennas at 0.8 GHz are shown in Figure 6. As observed from Figures 6(a) and 6(b), the proposed and the classical SAAS antennas show similar radiation performance. e difference between the antenna gains is less than 0.11 dB, and the difference between the 3 dB beam width is less than 6°, which means the proposed SAAS antenna has similar radiation patterns with the classical SAAS antenna, and stable half power beam width (HPBW) is available for the proposed SAAS antenna.   International Journal of Antennas and Propagation

Study of Meandered Strip
Line. It is illustrated in the above section that the SAAS antenna has better circular polarization using a hybrid strip line than a traditional smooth arc line. e parameter φ e , which determines the position of the connection point A, will affect the circular polarization performance. e ARs of the SAAS antenna with different φ e are shown in Figure 7. e AR is below 3 dB from −40°to 40°at 1 GHz when φ e is 13.5π, which still holds as the frequency increasing to 6 GHz in xoz plane. e circular polarization at a lower frequency is the key factor that determines the configuration of the SAAS antenna.
us, we set the connection point A at φ e � 13.5π. e width of the antenna arm is an important factor in antenna design. We study the influence of the parameter w a on the antenna performance. e simulated reflection coefficient of the SAAS antenna with different w a is shown in Figure 8. When the frequency is below 1 GHz, the S 11 is better as w a is 2.4 mm. e S 11 is similar with different w a when the frequency is higher than 5 GHz. us, we get that the influence of w a on the S 11 performance is not significant at higher frequencies. e minimum of S 11 is between 2 GHz and 5 GHz, which shifts to a higher frequency as width increases.
e simulated ARs of the SAAS antenna with different w a at 1 GHz are shown in Figure 9. e AR is less than 2 dB from −49°to 46°in xoz plane when the w a is 2.4 mm, which is better than others. It is less than 3 dB from −47°to 47°in yoz plane. e antenna can realize a better circular polarization when the width of the antenna arm is set to 2.4 mm. e width of the meandered strip line w m is also studied. e simulated reflection coefficient of the SAAS antenna with different w m is shown in Figure 10. S 11 is better as International Journal of Antennas and Propagation w m � 3.0 mm when the frequency is below 1 GHz and higher than 5 GHz. e performance of S 11 is similar between 2 GHz and 5 GHz with different w m , while the minimum of S 11 is different. e simulated ARs of the SAAS antenna with different w m at 1 GHz are shown in Figure 11. As shown in Figure 11, the AR is less than 3 dB from −47°to 47°in both xoz and yoz planes when w m is 3.0 mm, which is better than others. us, we set the value of w m to 3.0 mm to get a better circular polarization at a lower frequency.

Study of Disc Ground.
e disc ground, etched on the back side of the SAAS antenna, has a crucial influence on the reflection coefficient. Simulated results of the reflection coefficient with different disc ground radii are shown in Figure 12. Notice that the reflection coefficient deteriorates as the disc ground radius increases. Especially in a lower frequency band, the reflection coefficient with a radius of 5 mm is much worse than that with a radius of 25 mm. e input impedance values of the proposed SAAS antennas with different disc radii are shown in Figure 13. e SAAS with a small disc ground (radius of 5 mm) has a small resistance from 0.23 GHz to 1 GHz, and its reactance fluctuates dramatically compared with the others. It follows that the disc ground has an intensive influence on the antenna performance in lower frequency band. us, the radius of the disc ground is set to 25 mm in the following simulations.
e simulated ARs at different frequencies for SAAS antennas with different disc ground radii are shown in Figure 14. As shown in Figure 14, the ARs exhibit similar trends.
at is, the AR deteriorates as the frequency     increases. Most of the ARs are less than 3 dB when the frequency is below 6 GHz. An antenna with a small disc ground (15 mm) has better performance when the frequency is below 5 GHz. is is because the disc ground act as a reflector will cause distortion of the current on the antenna arm, which degrades the AR performance. us, a small disc ground is preferred to get good circular polarization.

e SAAS Antenna with Metallic Back Cavity.
To realize unidirectional radiation, the proposed SAAS antenna backed by a metallic cavity is simulated, as shown in Figure 15. e height of the metallic cavity is 100 mm, which is 0.26λ L (λ L is the lowest working frequency, 0.80 GHz). e metallic cavity is filled with absorbing material (dielectric constant 4.1, dielectric loss tan δ � 0.7) to improve the reflection coefficient in lower frequency band. e simulated reflection coefficient of the SAAS antenna with and without a cavity is illustrated in Figure 16. It can be observed from Figure 16 that the reflection coefficient of the SAAS antennas with and without a cavity has a similar performance. e simulated ARs at different frequencies of the SAAS antenna with and without a cavity are shown in Figure 17.
e AR of the SAAS antenna with a cavity is higher than 3 dB when the frequency is below 1.4 GHz, which is worse than that obtained without a cavity. e AR performance    International Journal of Antennas and Propagation degradation in lower frequency is caused by the metallic cavity. e radiated electromagnetic waves from the antenna arm are reflected by the cavity, which can change the surface current distribution on the antenna arm, thus leading to AR degradation. e reflected electromagnetic waves cannot be completely absorbed in a wide frequency band. e wavelength of the lower frequency is longer. us the AR is more susceptible to be influenced by the cavity. e simulated ARs at different frequencies of the SAAS antenna with different heights of the cavity are shown in Figure 18. e AR has a wider bandwidth when the height of the cavity is 100 mm compared with other values. at means a higher height of the cavity has less influence on the AR. As the height of the cavity increases, the AR becomes lower, and the circular polarization becomes better.

Experiment and Discussion
As shown in Figure 19, the SAAS antenna prototype with the parameters shown in Figure 1 is fabricated and measured with an Agilent E8363B vector network analyzer. e simulated and measured results of the reflection coefficient S 11 are presented in Figure 20. e measurement results of the antenna prototype show that the S 11 is below −10 dB from 0.89 GHz to 8.58 GHz. Also, we can get that the voltage standing wave ratio (VSWR) is less than 2 from  International Journal of Antennas and Propagation 0.88 GHz to 8.82 GHz, which illustrates a bandwidth of 1 : 10. e difference between the measured and simulated results might be attributed to the following reasons. Firstly, to decrease the antenna model complexity and save the simulation time, the subminiature version A connector is replaced by an ideal port to feed the antenna in the simulation. It may cause undesirable reflection in the antenna prototype and lead to the difference between the measured and simulated results of S 11 . Secondly, the antenna arms and the ground are set to be a perfect conductor in the simulation. ey are made of copper and silver-plated, whose surface roughness can change the current distribution, which might lead to the current reflection.
irdly, the inconsistency of the dielectric constant and the thickness of the substrate may cause the S 11 degradation in the antenna prototype.
e SAAS antenna prototype shown in Figure 19 has bidirectional radiation. To realize unidirectional radiation, a metallic cavity filled with an absorber is employed, as shown in Figure 21. e height of the cavity is 100 mm, which is 0.29λ L (λ L is the lowest working frequency, 0.88 GHz). e VSWR of the SAAS antenna with a back cavity is exhibited in Figure 22. It is demonstrated that the cavity with an absorber   International Journal of Antennas and Propagation has little influence on VSWR in the low-frequency band and makes no difference in the high-frequency band. Moreover, it exhibits good agreement with the simulated results. e measured normalized radiation patterns of the SAAS antenna with metallic cavity and without metallic cavity are exhibited in Figure 23. It can be seen that the radiation is bidirectional without the metallic cavity. After the metallic cavity is employed, the radiation became unidirectional.
e main polarization (right-handed circular polarization, RHCP) is approximately 10 dB higher than the cross polarization (left-handed circular polarization, LHCP). e measured results are in good agreement with the simulated results. e measured 3 dB beam width of the SAAS antenna at different frequencies is as follows: it is 108°a t 1 GHz, 54°at 4 GHz, and 65°at 8 GHz. It also can be seen in Figure 23 that the maximum radiation direction changes with frequency. e antenna is designed for pantograph arcing detection in high-speed railways. Generally, the direction of the interference caused by pantograph arcing is unknown and random. In this case, an omnidirectional radiation pattern is preferred. However, the bandwidth of the omnidirectional antenna is usually narrow. us, the SAAS antenna is a good candidate for pantograph arcing detection, although its maximum radiation direction changes with frequency. e measured and simulated gains of the SAAS antenna with metallic cavity are exhibited in Figure 24. It is seen that the measured antenna is not very high, which is below 3 dBi. e SAAS antenna is a bidirectional radiating antenna. e radiated electromagnetic waves towards the metallic back cavity are absorbed by the absorbing material that affects the antenna gain. However, the absorbing material is necessary for improving the S 11 performance in the lower frequency     International Journal of Antennas and Propagation band. We make a trade-off between the antenna gain and the S 11 performance. It is also seen in Figure 24 that the measured and simulated antenna gain has the same trend as frequency varying. e measured result is lower than the simulated result.
is may be caused by the loss of the substrate which is not considered in the antenna simulation. e measured and simulated ARs at different frequencies are illustrated in Figure 25. e simulated result of the AR without metallic cavity is below 3 dB from 1.2 GHz to 8 GHz, which is better than that with metallic cavity, especially at lower frequencies. It may be attributed to the metallic cavity under the substrate. Due to longer wavelengths at lower  frequencies, the current distributions on antenna arms are more susceptible to be influenced by the metallic cavity, which leads to degraded AR performance. e measured AR with a cavity is less than 3 dB from 1.6 GHz to 8 GHz; that is, the AR bandwidth is 1 : 5. e measured AR is not as good as simulated AR in lower frequencies, which may be caused by the nonideal absorbing material.
e measured ARs at different angles are shown in Figure 26. We can see that the measured ARs are below 3 dB from −50°to 50°as the frequency below 5 GHz. e ARs are approximate below 3 dB from −20°to 20°as the frequency above 6 GHz. is degradation is related to the disc ground. e disc ground acts as a reflector, which will cause distortion of the current on the antenna arm. When the working frequency increases, the active radiating area concentrates towards the antenna center.
us, the AR performances at higher frequencies are more susceptible to be influenced by the disc ground.
We make a comparison between the proposed antenna and antennas in other literature. Compared with the singlearm spiral antennas in other references, the proposed antenna has a wider impedance bandwidth 1 : 10.0 and AR bandwidth 1 : 5.0, as reported in Table 1. However, the cavity height of the proposed antenna is 0.29λ L which is not as good as that in other literature. Because traditional metallic cavity is employed in our antenna design, technology that is beneficial to reduce the cavity height should be considered to decrease the antenna profile in the future.

Conclusions
An SAAS antenna is proposed and studied in this paper. e performance of the proposed antenna, including the reflection coefficient, input impedance, current distribution, radiation pattern, and AR, are studied. It is illustrated that the meandered arm structure is effective in improving the circular polarization at low-frequency. Moreover, it is also an effective way to reduce the antenna size. With a metallic cavity, the proposed antenna can realize unidirectional radiation. e measurement results indicate that the proposed SAAS antenna can realize a wide AR bandwidth of 1 : 5.
Data Availability e data that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of Interest
e authors declare that there are no conflicts of interest regarding the publication of this study.