Dual‐curl antennas for broadband circular polarisation

A novel dual-wire antenna to obtain wideband circular polarisation is proposed. The antenna consists of curl elements with a balanced feed above a ground plane. Using the moment method, an antenna with bal- anced feedlines having two sources is analysed ﬁrst. Analysis reveals that the antenna shows a 3 dB axial-ratio bandwidth of 36%. Next, an antenna with quasi-balanced feedlines having one source to simplify the feed system is investigated. It is found both numerically and experimen- tally that the axial-ratio bandwidth remains unchanged, and a VSWR of less than two is obtained.

Antenna with balanced feedlines: Figure 1 shows the antenna made of wires of the radius ρ. The antenna consists of two curl elements at height h above the ground plane. The curl ends F n (n = 1 and 2) are connected to feedlines F n -F n ' of spacing S, and the bottom ends F n ' are excited with two sources having the same amplitudes with a phase difference of 180°, as shown in Figure 1(c). To compensate for the phase difference, we rotate the curl element on the +y side by 180°with respect to the other. Note that the curl element is specified by circumference C, adjacent wires at distance d, and straight segment length L f , as shown in Figure 1(e).
The curl parameters (C, d, L f ) are selected for CP radiation using the moment method [9], where the ground plane size is assumed to be infinite. The curl height is chosen to be the same as that (h = λ 0 /4) of other dual-wire antennas [1][2][3][4][5], where λ 0 is the free-space wavelength at a test frequency f 0 . The feedline spacing and wire radius are S = 0.025λ 0 [10] and ρ = λ 0 /200 [11].
The simulated axial ratio and gain versus frequency are shown with solid lines in Figure 2. The curl parameters are (C, d, L f ) = (1.25λ 0 , 0.052λ 0 , 0.125λ 0 ). It is found that a 3 dB axil-ratio bandwidth is 36%, where the gain is more than 9.3 dBi. For reference, dotted lines show those of an isolated curl element (see the inset) for (C, d, L f ) = (1.22λ 0 , 0.046λ 0 , 0.122λ 0 ) at the same height h. The bandwidth is 25%, where the gain is more than 6.0 dBi. It can be said that the present antenna's bandwidth is 1.4 times as wide as that of the isolated element. Note that the curl element's bandwidth is comparable to that (24%) of a two-wire spiral element at h = λ 0 /4 with a circumference of 1.6λ 0 [11]. Figure 3 shows the simulated radiation patterns at the test frequency f 0 . The dotted and solid lines show right and left-hand CP wave components, respectively. It is seen that the antenna radiates a left-hand CP beam (E L ) in a direction normal to the antenna plane, in the +z-axis di- short circuit y z h y ground plane rection. The half-power beamwidths (HPBW's) are 79°and 57°in the φ = 0°and 90°planes, respectively. The gain is evaluated to be 9.6 dBi. Note that the principal polarisation (E L ) depends on the curl winding direction. In other words, the opposite polarisation (E R ) becomes principal one for a curl with a winding direction opposite to that shown in Figure 1.
Antenna with quasi-balanced feedlines: So far, we have analysed an antenna with two sources. In this section, we investigate another antenna with one source to simplify the feed system. Figure 1(d) shows the simplified feed system with one source. The source excites a bottom end F 1 ' of the feedlines F n -F n ', and the other end F 2 ' is short-circuited to the ground plane. The short circuit in place of the source is the only difference compared to the antenna in the previous section.
We hold the configuration parameters the same as those in the previous section and analyse the antenna. Solid lines in Figure 4 show the simulated frequency responses of the axial ratio, gain, and VSWR. It is found that the axial-ratio bandwidth is the same as that (36%) for two sources. The gain and VSWR in the bandwidth are more than 8.5 dBi and less than 2, respectively. Note that the VSWR is evaluated based on a 75 coaxial line.
The solid and dotted lines in Figure 5 show the simulated radiation patterns at f 0 . It is found that the antenna radiates a CP beam similar to that for two sources (see Figure 3). The HPBW's are 80°(79°for two  sources) and 54°(57°) in the φ = 0°and 90°planes, respectively. The gain is 9.3 dBi (9.6 dBi).
Using current distribution, we explain why the radiation characteristics are similar to those for two sources. Figure 6(a-d) show the currents for one and two sources, respectively. A comparison between the currents reveals that the antenna for one source has a traveling-wave tape current distribution, similar to that for two sources. The amplitude de-  cays from curl end F n toward the centre o n, and the phase varies almost linearly along the arm length. It is also observed that the currents along feedlines F n -F n ' for one source have almost the same amplitudes with a phase difference of 180°as the corresponding currents for two sources. In summary, the similar radiation characteristics for one source are attributed to induced currents appropriate for CP radiation.
To validate the simulated results, we perform experiments. An antenna is fabricated at f 0 = 3 GHz using the ground plane of 5λ 0 × 5λ 0 .   Table 1. It is emphasized that the present antenna has the widest CP wave bandwidth. The bandwidth is 1.4 times as wide as that of a dual-spiral antenna [5]. Note that the spiral arm is closely wound with more turns than the curl.

Conclusion:
A curl antenna has been used in a two-element array at the height of λ 0 /4 above the ground plane. It is found that the isolated element has a 3 dB axial-ratio bandwidth of 25%, which is comparable to that of a spiral element at the same height with a circumference of less than 2λ 0 . It is also found that the bandwidth further increases to 36% in a two-element array with balanced feedlines having two sources. We reveal that quasi-balanced feedlines having one source lead to a VSWR of less than two, not deteriorating the axial-ratio bandwidth.