1. Introduction
High-power microwave (HPM) phase shifter is a crucial component in HPM coherent synthesis systems and HPM array antennas[
1,
2,
3]. Its performance directly impacts the technical indicators of the system. Therefore, it’s significant to research phase shifters with low insertion loss, high phase shifting accuracy, and high-power capacity.
According to the implementation method, microwave phase shifters can be divided into non-mechanical and mechanical types. The most representative non-mechanical phase shifters include ferrite[
4,
5,
6], PIN diode[
7], and dielectric-loaded phase shifters[
8,
9]. Due to limitations in working mechanism and breakdown threshold of dielectric materials, the power-handling capacity of non-mechanical phase shifters is generally lower than megawatt levels, making them unsuitable for HPM applications. Mechanical phase shifters can be divided into push-pull and rotary waveguide types according to the adjustment method. Currently, push-pull waveguide phase shifters, such as the T-shaped rectangular waveguide phase shifter[
10], the waveguide broad-wall adjustable phase shifter[
11], and the narrow side slot-waveguide phase shifter[
2], are commonly used in the HPM areas to achieve microwave phase adjustment. However, push-pull waveguide phase shifters have two drawbacks: first, they require space for the short circuit slider to move in the waveguide, which makes the HPM system difficult to maintain a high vacuum level and increases the complexity of the HPM vacuum system; second, the sliding of the short circuit slider along the inner wall of the waveguide can cause wear and reduce the service life of the phase shifter.
Considering the limitations of push-pull phase shifters discussed above, researchers in the HPM field are increasingly turning to rotary phase shifters due to their more compact structure and longer lifespan. The waveguide rotating contact surface of a rotary waveguide phase shifter is less prone to wear, making it more durable. There are mainly two types of rotary waveguide phase shifters in literatures: the pressure elliptical waveguide phase shifter and the waveguide gap bridge rotary adjustable phase shifter[
12,
13]. The pressure elliptical waveguide phase shifter has a wide working frequency band, but its phase shift has a nonlinear relationship with the rotation angle, resulting in lower phase shifting accuracy. The waveguide gap bridge rotary adjustable phase shifter consists of a waveguide gap bridge, two circular polarizers, and two inverters, making its structure more complex. To achieve linear phase adjustment, this phase shifter requires the simultaneous rotation of two inverters at the same angle, making its adjustment process more complicated.
A high-power rotary waveguide phase shifter based on circular polarizers is presented. As illustrated in
Figure 1, the structure of the phase shifter comprises a linear polarized to left-handed circular polarized (LP-LHCP) mode converter, a left-handed to right-handed circular polarized (LH-RHCP) mode converter, and a linear polarized to right-handed circular polarized (LP-RHCP) mode converter. The entire phase shifter exhibits mirror symmetry. By rotating the LH-RHCP mode converter, a 360° linear phase shift can be achieved, with the phase shift being twice the rotation angle. Through the optimization and simulation analysis of the X-band rotary waveguide phase shifter, it has been demonstrated that the proposed phase shifter possesses advantages such as high phase shifting accuracy, low insertion loss, and high power-handling capacity.
The paper is organized as follows.
Section 2 introduces detailed the phase shifting principle of the proposed phase shifter.
Section 3 presents the optimization results for the three parts of the X-band rotary waveguide phase shifter.
Section 4 conducts a simulation analysis of the entire phase shifter. Finally, a conclusion is made in
Section 5.
2. Phase Shifting Principle
The LH-RHCP mode converter serves as a crucial component for phase adjustment in the presented high-power rotary waveguide phase shifter. Comprised of two mirror-symmetric circular polarizers, which can convert the input LHCP TE
11 mode to LP TE
11 mode and then to RHCP TE
11 mode. The circular polarizer is composed of a circular waveguide and a pair of rectangular grooves protruding from both sides of the waveguide, as shown in
Figure 1. To elucidate the principle underlying the conversion between LP and CP TE
11 mode by the circular polarizer, two orthogonal coordinate systems,
x-
y and
ex-
ey, are established as depicted in
Figure 2. In this context,
ex is parallel to the direction of the rectangular grooves of the circular polarizer, while
ey is perpendicular to that direction.
In order to satisfy the requirement of the CP electric fields having equal amplitudes and a phase difference of 90°, the direction of the input electric field
Ei must be oriented at an angle of 45° (resulting in a LHCP wave) or -45° (resulting in a RHCP wave) with the rectangular grooves. When the input electric field
Ei is oriented at 45° with the rectangular grooves, it can be decomposed into two orthogonal electric field components along the
ex and
ey directions, respectively, as shown in Equation (1). Due to the different cavity structures in the
ex and
ey directions, their electric field propagation constants (
βx and
βy) and transmission efficiencies (
τx and
τy) are also distinct. For the TE
11 mode in the
ey direction, the electric field is primarily concentrated in the middle of the circular waveguide, with relatively low field strength at the rectangular grooves on both sides. Consequently, the discontinuity caused by the rectangular grooves has minimal impact on the transmission efficiency of electromagnetic waves in the direction, i.e.,
τy ≈ 1. The transmission efficiency
τx of electromagnetic waves in the
ex direction is mainly related to the thickness
w and height
h of the rectangular cavity. Therefore, by selecting appropriate parameters, high transmission efficiency can be achieved, i.e.,
τx ≈ 1. Since the electric field propagation constant
βy perpendicular to the direction of the rectangular grooves is greater than the electric field propagation constant
βx along the direction of the rectangular grooves, adjusting the length
d of the rectangular grooves accordingly can result in a 90° phase difference between the electric fields in both directions, thereby forming a LHCP wave, as shown in Equation (2).
where
δ is the initial phase, and
ω is the angular frequency.
The circular polarizer can also be regarded as a four-port waveguide device, where ports 1 and 2 correspond to the degenerate TE
11x mode (TE
11 mode polarized along the
x-axis) and TE
11y (TE
11 mode polarized along the
y-axis) mode of the input port, respectively, while ports 3 and 4 correspond to the degenerate TE
11x mode (TE
11 mode polarized along the
x-axis) and TE
11y (TE
11 mode polarized along the
y-axis) mode of the output port, respectively. The generalized scattering matrix of the circular polarizer can be expressed as
To illustrate the characteristic of the circular polarizer, for Mode1:1 and Model 2:1 wave incident, respectively, at the input port with a normalized amplitude and the equal phase 0°,corresponding to input vectors
a1= [1,0,0,0]
T and
a2= [1,0,0,0]
T, the output vectors are
b1=
S'·
a1=
and
b2=
S'·
a2=
, respectively. Namely, when the input port electric field is the TE
11x mode, the circular polarizer can output a LHCP wave. Conversely, when the input port electric field is the TE
11y mode, the circular polarizer can output a RHCP wave.
When a LHCP wave with normalized amplitude and initial phase of 0° is injected into the LH-RHCP mode converter, corresponding to the input vector a = , the output vector is b=S'·a=, the conversion from LHCP to RHCP TE11 mode is achieved. Similarly, when a RHCP wave is input, the LHCP wave can be output.
The LH-RHCP mode converter rotates by an angle
θ, and the
ex-
ey coordinate system also rotates accordingly. The rotated coordinate system is defined as
e1-
e2, as illustrated in
Figure 3.
The relationship between the
ex-
ey coordinate system and the
e1-
e2 coordinate system is given by
where
T12and
Txy are the coordinate transformation matrices.
For the LH-RHCP mode converter input a LHCP wave with normalized amplitude and initial phase of 0°, the input port electric field can be expressed as
Substituting Equation (5) into (7), which can be deduced as
In the
e1-
e2 coordinate system, the total input electric field vector is further written as
By combining Equations (4) and (9), the total output electric field vector of the
e1-
e2 coordinate system can be obtained.
The expression for the output port electric field can be described as
Moreover, the output port electric field of the
ex-
ey coordinate system can be rewritten as follow by substituting Equation (6) into (11).
By comparing Equations (7) and (10), it can be observed that when the LH-RHCP mode converter rotates by an angle of
θ, the input LHCP wave can be converted to RHCP wave output, and the microwave phase undergoes a change of 2
θ. Therefore, the phase shift Δ
φ and the angle of rotation Δ
θ satisfy the following formula.
4. Simulation of the Rotary Waveguide Phase Shifter
After carefully designing the three sections of the phase shifter, the rotary waveguide phase shifter is assembled as illustrated in
Figure 1. Taking into account the practical processing conditions, the material of the converter is specified as aluminum with a surface roughness of 1.6 μm.
Figure 11 and
Figure 12 depict the reflection coefficient and transmission efficiency at varying rotation angles, respectively. Notably, the reflection of the phase shifter is less than -20 dB and the transmission efficiency exceeds 95% within the frequency range of 9.5–10.2 GHz, indicating that the working frequency band of the phase shifter spans greater than 700 MHz. Furthermore,
Figure 13 shows the output phase of the phase shifter at varying rotation angles. It can be seen that the phase shifter exhibits an approximately 360° linear phase shift within the aforementioned frequency range as it rotates from 0° to 180°.
Additional research has been conducted on deviation of the electrical phase shift and the twice mechanical angle at varying rotation angles, and the calculation formula for the phase shifting deviation is calculated as
where
φ0 is represents the initial phase of the phase shifter when the LH-RHCP mode converter is not rotated, while
φ represents the output phase of the phase shifter when the LH-RHCP mode converter is rotated by Δ
θ.
Figure 14 illustrates the phase shifting deviation of the phase shifter at different rotation angles. It can be seen that in the frequency range of 9.5–10.2 GHz, the maximum phase shifting deviation is less than 1.2° during the rotation process from 0° to 180°.
The electric field distribution of the phase shifter is shown in
Figure 15. It clearly displays the mode conversion process of the phase shifter. With an input microwave power of 0.5 W, the maximum surface field strength of the phase shifter is 3179 V/m. In HPM applications, phase shifters and other microwave transmission components typically operate under a high-level vacuum condition. Under the vacuum condition, the RF breakdown field strength is approximately 1 MV/cm[
15,
16]. For assurance, assuming the breakdown field strength is 0.7 MV/cm, the phase shifter has a power-handling capacity larger than 242 MW. Furthermore, the maximum surface field strength of the phase shifter occurs at the intersection of the rectangular groove and the circular waveguide. Hence, implementing a chamfered transition in the intersection further enhances the power-handling capacity of the phase shifter.