A Loop-Line Antenna with Enhanced Bandwidth of Circular Polarization

We investigate a circularly polarized (CP) transmission-line antenna composed of loop radiation elements, called a loop-line antenna. To enhance the CP wave bandwidth, an isolated loop radiation element is first analyzed using the method of moments. It is found that a CP wave bandwidth for a 3 dB axial-ratio criterion has a maximum value of 8%. On the basis of the analysis results, we design the loop-line antenna. It is found that the antenna shows a CP wave bandwidth of 6%, which is twice as wide as that of a conventional loop-line antenna.,e analysis results are verified with experimental ones. In addition, a condition for wideband CP radiation is discussed and found to be a composite radiation element consisting of two CP loops.


Introduction
A transmission-line antenna (line antenna) is in a class with a series-fed linear array and can radiate a circularly polarized (CP) wave [1,2]. e antenna is characterized by a simple feed system in comparison with that of a conventional array, since the feed line itself contributes to CP wave radiation. So far, the line antennas have been designed using several kinds of radiation elements above a ground plane, such as rampart [3], crank [4], and loop elements [5,6].
A recent study using a loop element has shown that the CP wave bandwidth of a line antenna can be increased [7].
is has become possible with a study of an isolated loop element without an array environment. e loop height above the ground plane has been selected for wideband CP radiation.
is paper is an extended version of the preliminary conference one [7] written by the present authors. e study of the isolated loop element is newly focused on the reason why the wideband CP radiation is obtained. For this, a more in-depth analysis than the preliminary one [7] is performed in Section 3 using the method of moments [8]. Even in Section 2 showing a need for selection of loop height above the ground plane, we add new results (shown with dotted lines in Figures 1 and 2), which play an important role of finding a condition for wideband CP radiation in Section 3.
In Section 4, we replace all the simulated results of the line antenna in [7] with new ones that can be validated by experimental work.
is is based on an evolution of the antenna design, by which input impedance matching a 50-Ω coaxial line is realized without deterioration in the CP radiation. Note that there have been only simulated results in [7], since the input impedance that satisfies a VSWR of less than 2 has yet to be researched. Figure 3 shows a loop antenna located at height h above a ground plane. e antenna is composed of a circular loop of circumference C, parallel wires of distance d [6], and a vertical wire F-F′ of length h, whose bottom end F′ is excited by a coaxial line, as shown in Figure 3(c).

Loop Elements
At each value of loop height h ranging from λ 0 /10 to λ 0 /4, we appropriately choose the loop parameters (C, d) for the antenna to radiate a CP wave using the method of moments [8], where λ 0 is the free-space wavelength at a test frequency f 0 . e wire radius is the same (ρ � λ 0 /200) as that in [6]. Note that the ground plane is assumed to be of infinite extent and image theory is used in the analysis. Figure 1 shows the simulated values of (C, d) that are optimized for CP radiation versus loop height h. A loop antenna having these configuration values can radiate a CP wave with an axial ratio of less than 3 dB. It is observed that as the loop height increases from h � 0.10λ 0 , the difference between solid and dotted lines becomes small and vanishes at h � 0.18λ 0 . is means that when h < 0. e simulated CP wave bandwidths as a function of loop height h are shown in Figure 2. e bandwidth is evaluated for a 3 dB axial-ratio criterion. It is found that the bandwidths of the two CP loops for h < 0.18λ 0 increase with an increase in the loop height h. A maximum bandwidth of 7.8% is obtained at h � 0.20λ 0 , where there exists only one loop. Note that the dotted line (implying existence of the two CP loops mentioned in Figure 1) vanishes at h � 0.18λ 0 . e simulated radiation pattern of the antenna with h � 0.20λ 0 is shown in Figure 4(a). e pattern is illustrated with right-(E R ) and left-hand (E L ) CP wave components. It is seen that a CP beam is formed in a direction normal to the antenna plane, in the +z-axis direction. e half-power beam width (HPBW) is 82°and the gain is 7.3 dB. For comparison, the radiation pattern of a conventional antenna with h � 0.125λ 0 [6] is also shown in Figure 4(b), where the HPBW and gain are 70°and 9.1 dB, respectively. e HPBW of the present antenna with h � 0.20λ 0 is wider than that of the conventional one by 12°, resulting in the lower gain.

Modified Loop Elements
So far, we have found a loop antenna showing wideband CP radiation. Subsequently, we analyze modified loop antennas to know a condition for the wideband CP radiation. Figure 5 shows the modified antennas considered here. ey are created from the present antenna shown in Figure 3; one has a loop without a chord β-c shown in Figure 5(a) and the other has a loop without two arcs of α-F and β-c shown in Figure 5(b). Note that the modified antennas are still specified by two parameters, loop circumference C, and parallel-wire distance d, as shown in Figure 5. e simulated CP wave bandwidths versus loop height h are shown in Figure 6. It is found that both the modified antennas can radiate a CP wave with an axial ratio of less than 3 dB. It is also found that each modified antenna does not show wideband CP radiation in comparison with the present antenna, as shown in Figure 2. is fact means that wideband CP radiation is obtained for a composite (present) antenna consisting of the two CP-modified loops, since both the modified loops are created from the present antenna, as mentioned in Figure 5 (also recall that there exist the two CP loops when h < 0.18λ 0 , as shown in Figures 1 and 2). It can be said that a composite antenna consisting of two CP loops is a condition for the wideband CP radiation found in Section 2.

Loop-Line Antenna
In Sections 2 and 3, we have discussed wideband CP radiation which is attributed to a composite element consisting of two CP loops. Using this composite element, we design a line antenna to enhance the CP wave bandwidth. Figure 7 shows the line antenna located at height h above a ground plane. e antenna is composed of N loop elements and connecting lines of length L, and excited at the left end F using a coaxial line. A tapered feed F-F′ is adopted for input impedance matching a 50-Ω coaxial line, as shown in Figure 7(b). e tapered-feed configuration is specified by (l 1 , l 2 , and l 3 ). Note that the antenna is made of wires of radius ρ [4][5][6].
We design the line antenna to radiate a CP broadside beam (in the +z-axis direction). e beam direction and We take the following two steps to efficiently determine the parameters: (1) First, the tapered feed F-F′ is changed to a direct feed, as shown in Figure 7(c), where a straight wire F-F′ is used [7]. e line-antenna parameters (L, C, d) are determined for the CP broadside beam. e determined values of (L, C, d) are fixed in the second step. (2) e direct feed is replaced with the tapered feed F-F′. e tapered-feed parameters (l 1 , l 2 , l 3 ) are determined for a VSWR of less than 2 under the condition that the CP broadside beam in the first step does not deteriorate. Calculations in the first step show that the CP broadside beam is obtained for line-antenna parameters of (L, C, d) � (0.53λ 0 , 1.01λ 0 , 0.26λ 0 ). Further calculations in the second step reveal that a VSWR of less than 2 is obtained for taperedfeed parameters of (l 1 , l 2 , l 3 ) � (0.25λ 0 , 0.17λ 0 , 0.02λ 0 ). e abovementioned values of the configuration parameters are summarized in Table 1, together with those of a conventional line antenna [6]. ere is a difference between the values of height h, which is a key parameter for enhancing the CP wave bandwidth, as discussed in Section 2. Also, recall that the present and conventional line antennas have the same values of (ρ, N). e simulated radiation pattern is shown with solid and dotted lines in Figure 8(a). It is observed that the antenna radiates a CP beam in the +z-axis direction. e HPBW is 13°and the gain is 14.1 dB. For comparison, the simulated radiation pattern of the conventional line antenna with h � 0.125λ 0 [6] is also shown in Figure 8(b), where the HPBW and gain are 13°and 14.5 dB, respectively.
Solid lines in Figure 9(a) show the simulated axial ratio and gain in the +z-axis direction versus frequency. It is found that the axial ratio remains less than 3 dB in a frequency bandwidth of 6.1%, where the gain is more than 14 dB. For comparison, dotted lines show those of the conventional line antenna [6]. e CP wave bandwidth is 3.4%. It should be emphasized that the bandwidth of the present line antenna is wider than that of the conventional one by a factor of 1.8. e solid line in Figure 9(b) shows the simulated VSWR versus frequency, which is less than 2 in the CP wave bandwidth.
To verify the simulated results, we perform experimental work. A loop-line antenna is fabricated at f 0 � 3 GHz using the ground plane of 3λ 0 × 9λ 0 .
Photographs of the fabricated antenna are shown in Figure 10. e dots and small circles in Figure 8(a) show the experimental results of principal (E R ) and cross-polarization (E L ) components of the radiation pattern, respectively. ey agree well with the simulated results shown with solid and dotted lines. Agreement between the  experimental and simulated results is also obtained for the frequency responses of the axial ratio, gain and VSWR, as shown in Figure 9. e small circles and solid lines are experimental and simulated results, respectively. We finally compare the results with those of line antennas investigated by other researchers. Table 2 summarizes the comparison. It is emphasized that there is no need for a matched termination in the present line antenna (prototype) [6], resulting in higher radiation efficiency. e prototype has a radiation efficiency of almost 100% since there are only conductor losses that are negligible up to a 12 GHz band [4].

Conclusions
We have analyzed a loop antenna located at height h above a ground plane. It is found that when h < 0.18λ 0 there exist two circularly polarized (CP) loops having different configuration values from each other and that a maximum CP wave bandwidth of 8% is obtained at h � 0.20λ 0 . Further analysis reveals that the loop antenna has a composite configuration consisting of two CP loops, leading to wideband CP radiation. Using this composite loop as a radiation element, we design a transmission-line antenna. It is demonstrated both numerically and experimentally that the antenna radiates a broadside beam with a CP wave bandwidth of 6%, which is wider than that of a conventional antenna by a factor of 2.
Data Availability e numerical and experimental data used to support the findings of this study are included within the article.