A High-Power Radar Rotary Joint

Due to the power capacity limitation of the rotary joint, mechanical scanning radars are limited to use high-power microwave sources to improve their performance furthermore. To solve this problem, an over-mode circular waveguide rotary joint with radial mutations at the rotation point is introduced in this letter. The structure of the radial mutation is optimized to suppress unwanted modes. By connecting the choke slot and the rotating part, the breakdown risks of the rotary joint can be reduced. In addition, the choke structure is connected to the inner wall of the waveguide with a gap, even the breakdown occurs in the choke, the normal modes inside the waveguide will not be affected. To verify the design, a prototype of the rotary joint is fabricated and measured with the operating band in the range of 9.5–10.5 GHz and a power capacity of 3 GW.


INTRODUCTION
Radars play an important role in modern electronic systems [1][2][3]. The using of high-power microwave sources can effectively improve the detection distance and the anti-interference performance of the radars [4]. As a key device in mechanical scanning radar systems, the rotary joint can ensure a stable transmission of RF signals during the scanning process. There are two main ways to realize the rotary joint, the first one is to use a coaxial structure, and the other is to use a waveguide structure. Usually, waveguide rotary joints based on the waveguide have a higher power capacity, which helps to further improve the power density of the radar systems [5,6]. But rotary joints in conventional forms generally cannot deal with the GW-level power generated by highpower microwave systems (HPM). To solve this problem, this letter proposes a rotary joint that can operate at 9.5-10.5 GHz with a power capacity of 3 GW level based on over-mode circular waveguide. The joint adopts a non-contact design and a new choke slot structure is designed to improve the power capacity.

DESIGN PRINCIPLE Generalized Scattering Matrix Theory
In the uniform direct lossless transmission system, the microwave mode is usually consistent, and once the structure of the waveguide is changed, such as the change of the radius of the circular waveguide or the bending of the axis, the change of the aperture of the rectangular waveguide, etc., the transmission mode in the waveguide will be changed, causing the coupling between the energies of the various modes in the waveguide, which generates new modes.
The generalized scattering matrix theory is used to analyze the abrupt structure in the circular waveguide.
The structure of the two-stage abrupt waveguide is shown in Figure 1.
In Figure 1A, B represents a uniform waveguide connecting two abrupt surfaces 1 and 2, which length is L. A and C represent a uniform waveguide connected to B, and they also represent abrupt surfaces 1 and 2. Figure 1B shows the scattering matrix of the structure of Figure 1A. S 1 and S 3 represent the scattering matrices at the abrupt surfaces 1 and 2, respectively. S represents the scattering matrix of the two-level abrupt structure, and the superscripts 2 and 3 represent the scattering parameters at the left and right ends of the uniform waveguide B, respectively. The superscripts A and C represent the scattering parameters on the left side of the mutation surface 1 and the right side of the mutation surface 2, respectively, and S L represents the transmission matrix between the mutation surfaces 1 and 2, the definition of S L is as Eq. 1 γ n represents the propagation constant of the nth mode in waveguide B. According to [7][8][9][10], the solution of each parameter in S is as follows: ⎧ ⎪ ⎪ ⎪ ⎨ ⎪ ⎪ ⎪ ⎩ S AA S 11 + S 12 S L U 2 S 33 S L S 21 S AC S 12 S L U 2 S 34 S CA S 43 S L U 1 S 21 S CC S 43 S L U 1 S 22 S L S 34 + S 44 (2) Among Eq. 3, I is the identity matrix, M is the matrix form of M mn ; Y i is the input admittance matrix seen from the ith waveguide to the sudden change; e an , e bm are the transverse mode electric fields in the A and B waveguides.

Choke Structure Design Theory
The choke structure in the rotary joint requires a high microwave transmission efficiency and a high power capacity. A successful design of the choke structure can improve the electric field distribution on the rotating surface, thus reduce the local electric field enhancement and avoid the breakdown. Meanwhile, the rotary joint can be rotated flexibly and the vacuum seal inside the waveguide can be maintained. The choke structure can be analyzed using the microwave equivalent transmission line theory and its equivalent equation is given by Eq. 4: Where R 0 、G 0 、L 0 、C 0 are the resistance, conductance, inductance and capacitance per unit length, respectively. Z、Y are the series impedance and parallel conductor per unit length of the transmission line, respectively. U(z), I(z) are the voltage and current on the transmission line. Z l is the load impedance of the choke. Z c is the characteristic impedance of the transmission line. The reflection coefficient Γ L is given by Eqs 5, 6: When βl nπ, (n 0, 1, 2, . . .), the minimum voltage and maximum current occur on the transmission line. When the equivalent length of the choke is given by Eq. 7: l n 2 λ, (n 1, 2, . . .) Minimum voltage occurs on the inner wall of the choke, so the breakdown problem can be effectively avoided.

GEOMETRY AND DESIGN
The rotary joint includes three parts: circular waveguides, a choke structure and a sealing structure. The two ends of the joint are the input and output ports respectively. Its geometry structure is shown in Figure 2. The radii of fillets 1 and 2 are 5 and 0.3 mm respectively.

Design of the Circular Waveguide
Conventional circular waveguides have a small power capacity, and if they are used in the HPW system,  breakdown will occur inside the waveguide. The over-mode waveguide is a waveguide whose size is larger than the traditional waveguide size at the operating frequencies, which can withstand a higher power due to its increased sectional area [11][12][13][14]. The field distribution in the rotary joint must have the characteristics of axisymmetric distribution to ensure stable output in the continuous rotation process. The field distribution of the TM 01 mode is axisymmetric, and the phase is stable, so TM 01 mode is used as the operating mode in this rotary joint. The designed rotary joint is excited by TM 01 mode, which also requires the use of an over-mode circular waveguide.

Design of the Choke Structure
Since it needs a certain space to rotate itself for the rotary joint, a slot is made in the waveguide wall along the radial direction as shown in Figure 2. However, a discontinuity in the radial direction of the waveguide in introduced, and can excite highorder modes in the waveguide. Figure 3 shows Simulated S 21 for each transmission mode in rotary joint when the radial slot has a radius of 39 mm, a length of 12 and 5 mm fillets for the connections. It can be seen with the introducing of the discontinuous structure exists in the rotating joint, when the TM 01 mode generated, some additional modes, such as TE 11 , TE 21 , TM 11 , TE 31 , TE 01 , and TM 21 are also excited.
As shown in the Figure 3 except for the TM 01 mode, the S 21 of TE 11 , TM 11 , and TE 31 are larger than other high-order modes, so they are selected as the analysis objects later.
As shown in Figure 4, Figure 5 and Figure 6, the optimization of fillet radius R_fillet of the waveguide radial slot, slot radius R_slot and slot length L_slot can suppress higher-order modes.
It can be seen that the suppression of the unwanted modes is achieved when the radius and the length of the radial slot are chosen to be 39 and 12 mm, respectively.
It can be derived from the choke theory in Eq. 7, when the choke slot length is 15 mm, 1/2λ, the voltage is minimized at the point where it is connected to the inner wall of the waveguide. It is helpful to avoid the breakdown problem. After optimization, the choke slot length is adjusted to 17 mm, as shown in Figure 2.c. It can be seen that the choke slot is not directly connected to the inner wall of the waveguide, but is connected to the slot along the radial direction. The field strength is relatively small and the risk of breakdown is smaller than choke slot which is connected to the inner wall. Since the choke slot is not directly connected to the inner wall of the waveguide, when the choke slot is broken down,  the influence of the electric field in the waveguide is relatively weak.
According to the measurement, the breakdown field strength of YL122 aluminum alloy material is 700 kv/cm. Figure 7 shows the field strength distribution of the inner of the waveguide when the input energy is 3 GW. It can be seen that the field strength at the location of the choke slot is about 600 kv/cm, so this rotary joint can withstand 3 GW microwave power. Figure 8 is the photograph of the rotary joint. When the rotary joint is operating in TM 01 mode, and the measured and simulated results of the transmission coefficient and the standing wave ratio are shown in Figure 9.

MEASUREMENT RESULTS
A HPM source [15][16][17] and horn antenna are used for the power capacity measurements of the rotary joint. The HPM source has a microwave pulse width of 20 ns and a output power of 3 GW. The horn antenna is a multimode conical horn with continuously variable flare angle. In the experiment, the rotary joint is connected between the HPM source and the horn antenna, and the power capacity of the rotary joint is evaluated by monitoring whether the microwave waveform of the radiation field shows tail-erosion phenomenon to determine whether the breakdown occurs during the transmission process. Figure 10 shows the radiation field waveform when 30 microwave pulses are continuously input to the rotary joint in the measurement. The vacuum value during the high-power test is 3p10 −2 Pa.
The comparison between the online waveform and the radiation field waveform in Figure 9 shows that the waveform repetition is very well and there is no tail erosion, which indicating that there is no obvious HPM breakdown in this rotary joint. As a result, the power capacity of this rotary joint meets the demand of HPM with power of 3 GW and microwave pulse width of 20 ns.

CONCLUSION
In this paper, a high-power radar rotary joint is designed based on an over-mode circular waveguide, and an innovative choke slot is used, which reduces the electric field strength at the choke slot and reduces the risks of the breakdown. The structure of the radial slot is optimized to suppress other coupling modes, and the transmission efficiency of TM 01 mode at 9.5-10.5 GHz reaches more than 98.9% while withstanding power of 3 GW.

DATA AVAILABILITY STATEMENT
The raw data supporting the conclusion of this article will be made available by the authors, without undue reservation.