Many vacuum electronic devices, such as the high-power microwave, millimeter wave, and terahertz wave devices,^[1–3] can generate the electromagnetic waves with azimuthally symmetric output modes including circular waveguide mode TM₀₁ and coaxial transverse electromagnetic (TEM) mode. These modes will be transformed into a TE₁₁ mode in a circular waveguide in order to obtain directed radiation. Coaxial plate-inserted mode converters (CPIMC) and turnstile-junction mode converters (TJMC) have received a great deal of attention because of their particular advantages of high conversion efficiency and easy-to-fabricate. However, these mode converters are only designed for a given frequency,^[4,5] whose bandwidth is very narrow. In order to satisfy the need of tunable HPMs, a tunable mode converter should be investigated. In addition, the electromagnetic wave with circular polarization may have a high probability to couple into unknown targets.^[6–8] Therefore, a tunable circular-polarized turnstile-junction mode converter (TCTMC) is proposed and investigated in this paper. Compared with TJMC, the TCTMC is designed to realize high conversion efficiency in a broad frequency range.

The rest of this paper is organized as follows. In Section 2, the structure and design approach of TCTMC are described in detail. In Section 3, the analyses and simulations are performed to verify the proposed method. Finally, a brief conclusion is drawn in Section 4.

The whole structure of the tunable high-power microwave system is illustrated in Fig. 1. It consists of tunable MILO, TCTMC, and horn antenna. The tunable MILO has been investigated in our previous work.^[4,5] Actually, the investigation of TCTMC is to meet the requirements for tunable MILO. TCTMC is also applicable to conventional HPM and other tunable HPM devices.

Fig. 1.

	Figure Option ViewDownloadNew Window
	Fig. 1. The structure of the tunable HPM system.

Figure 2 shows the three-dimensional (3D) structure diagram of TCTMC, which can convert a coaxial TEM mode into a circularly-polarized TE₁₁ mode. It is composed of a coaxial circular waveguide (CCWG), four rectangular waveguides (RWGs), eight metal rods, a transition section, and a circular waveguide (CWG). The four RWGs marked with RWG1, RWG2, RWG3, and RWG4 form an input turnstile junction and an output turnstile junction, respectively. Eight metal rods are distributed equally along the azimuthal direction, of which four metal rods as a whole are named “rod A” (close to RWG) and the others as a whole are called “rod B” (far away from RWG). The transition section includes two cone frustums and two cylinders. Both metal rods and transition section act as the matching sections to reduce or even eliminate the reflections and unwished transmissions caused by the irregular structures in the mode converter.

Fig. 2.

	Figure Option ViewDownloadNew Window
	Fig. 2. 3D structure diagram of the TCTMC.

The operating principle of TCTMC is as follows. A coaxial TEM mode microwave is injected into the CCWG and then further enters into the four RWGs. The microwave transports in the TE₁₀ mode in the four RWGs. With different lengths of the four RWGs, the microwave obtains different phases at the output turnstile junction. Consequently, when the four rectangular waveguide mode microwaves with different phases are output from the output turnstile junction, they form a coaxial TE₁₁ mode microwave and then enter into a circular TE₁₁ mode microwave in CWG.

The mode converter can achieve the output of circular-polarized TE₁₁ mode, provided that the phase shift of outputting TE₁₀ modes between adjacent RWG is π/2. The radial lengths of four RWGs are L₁, L₂, L₃, and L₄, respectively, which meet the following equation:^[9]

Assume that the broad side and narrow side of the rectangular waveguide are a and b, respectively, and the wavelength in free space is λ, then λ_g can be expressed as^[10]

First, the effective lengths of RWGs can be changed continuously in order to meet Eq. (1) in a relatively wide frequency range. Taking RWG1 for example as shown in Fig. 3, the RWG1 is welded with a sleeve, and a screw is inserted in the sleeve. The screw can be rotated and its place is fixed. When the screw is rotated, the sleeve and RWG1 are moved up or down. So the effective lengths of RWGs can be changed. Consequently, the length differences among the four RWGs can be changed to meet Eq. (1).

Fig. 3.

	Figure Option ViewDownloadNew Window
	Fig. 3. (color online) Tuning mechanism of RWG1.

Second, grooves A and B consist of four grooves arranged in the outer and inner conductors, respectively, as shown in Fig. 2. Rod B is inserted into grooves A and B. Moreover, rod B can move along the grooves while rod A is fixed. The rack and gear are introduced in order to move rod B. Based on the gear engagement principle, rod B can move along the axial direction via rolling the gear. Thus, the distance between rods A and B can be adjusted appropriately to eliminate the unwished reflections.

It is concluded that the frequency tuning can be achieved, provided that equation (1) is met and the reflection is eliminated effectively. The next part will verify the feasibility of the method.

The proposed TCTMC is investigated in a frequency range of 1.35 GHz–2.35 GHz. The optimized dimensions of TCTMC are as follows. The radius of the rod is 2 mm. The lengths of RWGs 1, 2, 3, and 4 are determined according to Eq. (1). The broad side and narrow side of the rectangular port in each of RWGs are a = 123 mm and b = 26 mm, respectively. The two radii and the length of the cone frustum in the transition section are 54 and 12, and 55 mm, respectively.

The current model is established successfully and obtains a better result via simulating and optimizing with the CST Studio Suite. The CST software has been widely used in designing the mode converter, which verifies the simulation reliability and the experimental and simulation results are in good agreement with each other.^[11–13] The objectives of this work are to improve the conversion efficiency and broaden the bandwidth. To simplify the simulation and analysis, the TCTMC without metal rods is discussed first.

Figure 4 presents the curves of conversion efficiency of TCTMC without metal rods against frequency. It is found that the conversion efficiency is higher than 90% in the whole frequency range of 1.35 GHz–2.35 GHz. Furthermore, it is even above 98% between 1.57 GHz and 2.16 GHz with a corresponding bandwidth of 31.64%. However, from Fig. 5 it can be seen that the reflections of TCTMC at both ends of the spectrum are more than −20 dB, thus lowering the conversion efficiency to some extent.

Fig. 4.

	Figure Option ViewDownloadNew Window
	Fig. 4. (color online) Curves of conversion efficiency versus frequency of TCTMC without metal rods.

Fig. 5.

	Figure Option ViewDownloadNew Window
	Fig. 5. (color online) Curves of reflection versus frequency of TCTMC without metal rods.

By contrast, the conversion efficiency of conventional TJMC of TEM to TE₁₁ is 99% at a center frequency of 1.75 GHz with an axial ratio of 0.03 dB. Over the frequency range of 1.59 GHz–1.90 GHz, the conversion efficiency exceeds 90% and the axial ratio is less than 2.5 dB with a corresponding bandwidth of 18.6%.^[9] Obviously, the TCTMC without metal rods obtains a significant improvement in comparison with the conventional TJMC, indicating that the proposed TCTMC is feasible.

For different-frequency microwaves, the reflections of modes and amplitudes are different, which needs matching structures to compensate for the reflections. The common matching structures are stepped or metal rods. However, steps cannot effectively eliminate unsymmetrical modes, such as the TE₂₁ mode. Consequently, metal rods are chosen as the matching structures in TCTMC.

When metal rods are added to the TCTMC as described in Section 2, the distance (Z) between rods A and B can be adjusted by moving rod B. Therefore, rod A and rod B play a key role in eliminating the reflections.

Figure 6 describes the plots of the conversion efficiency of the TCTMC with metal rods against frequency. The conversion efficiency exceeds 96% in the whole frequency range of 1.35 GHz–2.35 GHz. Moreover, it can be higher than 98% over the frequency range of 1.42 GHz–2.29 GHz with a bandwidth of 46.90%. It is also found that only the TE₁₁ circular waveguide mode is generated in the output of CWG at the frequency in a range of 1.35 GHz–2.35 GHz.

Fig. 6.

	Figure Option ViewDownloadNew Window
	Fig. 6. (color online) Plots of conversion efficiency versus frequency of TCTMC with metal rods.

Comparatively, the earlier mode converters have a great disadvantage. Firstly, structures of most of earlier mode converters were fixed, which can only obtain high conversion efficiency at a certain frequency point. The serpentine mode converter (SMC) by Lawson et al. can convert about 99% of the desired incident mode into a desired output mode when the operation frequency is 14.424 GHz.^[14] The coaxial waveguide mode converter by Zhang et al. can realize dual-band mode conversion but the structure cannot be changed after fabrication.^[15] The cross shaped mode converter (CSMC) by Peng et al. cannot work well except at 1.75 GHz.^[16] Secondly, for the rest of the earlier mode converters, the structures can be adjusted, like HETMC by Wang et al.^[17] However, the frequency bandwidth is relatively narrow and the whole length of HETMC is five times the length of the proposed TCTMC.

The moving metal rods can reduce the reflections of TCTMC as shown in Fig. 7. As the frequency ranges from 1.40 GHz to 2.30 GHz, the reflection coefficients of the reflected modes are almost less than −20 dB. However, the magnitude difference between TE_11X and TE_11Y becomes greater when frequency becomes lower or higher. This suggests that the output is not a standard circularly-polarized wave.

Fig. 7.

	Figure Option ViewDownloadNew Window
	Fig. 7. (color online) Plots of reflection versus frequency of TCTMC with moving rod B.

Note that in our simulations, the distance between rods A and B can be expressed as follows:

Fig. 8.

	Figure Option ViewDownloadNew Window
	Fig. 8. (color online) Nonlinearly fitted simulated distance (Z) versus frequency.

The simulated distributions (by the software CST Studio Suite) of the electric fields at input port, RWG and output port of TCTMC are shown in Figs. 9(a)–9(c), respectively. The injected TEM mode (Fig. 9(a)) in the input port transports into RWG, and is converted into TE₁₀ mode (Fig. 9(b)). Figure 9(c) shows the electric field distributions at different times in the output port, indicating that the output mode is the TE₁₁ mode. Besides, the direction of polarization changes with increasing time, which means that circularly-polarized output is obtained.

Fig. 9.

	Figure Option ViewDownloadNew Window
	Fig. 9. (color online) Electric field distributions of TCTMC.

The axial ratio (AR) determining the circular polarization, is defined as the ratio of long axis to the short axis of an elliptical polarized wave. In TCTMC, the AR of the output TE₁₁ mode can be obtained from^[18]

Figure 10 shows the comparison between ARs of the output TE₁₁ mode in TCTMC with and without metal rods. It can be seen that when the corresponding frequency is in the ranges of 1.49 GHz–2.22 GHz and 1.35 GHz–2.35 GHz, respectively, the ARs of the output TE₁₁ mode in TCTMC with and without metal rods are both kept under 1.3. These suggest that the output TE₁₁ modes in these ranges are almost circularly polarized waves.^[19] However, the AR in TCTMC with metal rods is in a range from 1.3 to 1.7 in the other frequency ranges. Although the circular polarization of the TE₁₁ mode is not so satisfactory, it is still acceptable.

Fig. 10.

	Figure Option ViewDownloadNew Window
	Fig. 10. (color online) Comparison between axial ratios of the output TE11 mode with and without matching rods.

Significantly, the AR of the TCTMC is not very good, which needs to be solved in the next investigation on TCTMC.

In this research, a tunable circularly-polarized turnstile-junction mode converter (TCTMC) is presented and investigated numerically with the CST Studio Suite. Both the length difference among the RWGs and the distance Z between rods A and B can be adjusted in order to improve the conversion efficiency and broaden the bandwidth. The conversion efficiency of TCTMC can reach above 98% with a bandwidth of 46.90% over the frequency range of 1.42 GHz–2.29 GHz.