Research and Development Work Summary of Ku-band TE21 mode Coupler

 
 
 
 
 
 
Research and Development Work
Summary of Ku-band TE21 mode Coupler

 
 
 
 
 
Alignsat Communication Technologies Co.,Ltd
  1Origin of Task
In order to ensure a high EIRP value, both satellite-satellite link communication and satellite-Earth station communication require precise alignment of transmitting antenna beam and receiving antenna beam. Once there is a deviation between the two, it will cause a decrease in gain and affect the quality of communication. In order to ensure accurate alignment of the beam, a good tracking method is to generate a beam with zero value (differential beam) in the peak direction of the communication beam (and beam) by using a specific higher order mode (differential mode) of the waveguide. A tracking error signal is generated by comparing the antenna with the sum beam and difference beam, which is used to characterize the angle of the antenna deviating from the incoming signal. The mode coupler is a key component for generating a difference beam, and its performance directly affects the antenna tracking effect.
In view of the wider and wider application of differential mode tracking in satellite communications, the Department of Satellite Communications decided to set up a project group for the development of mode couplers.

2Product Description

2.1 Mode coupler tracking principle

A typical antenna feed schematic diagram for tracking with TE21 mode and communicating with TE11 mode is shown in Figure 1.
                       Figure1 Tracking feed system
When the antenna's electric axis coincides with the incoming beam direction, there is no difference signal output; when the antenna's electric axis deviates from the incoming beam direction, the difference beam will receive the signal, and The TE21 high-order mode will be excited in the horn. After passing through the main channel, the TE21 high-order mode is coupled to the difference channel via the coupling hole, the mode is converted into the TE10 mode in the rectangular waveguide. The TE10 mode synthesizes single channel signal through the latter synthetic network. The synthetic network can be divided into linear polarization network and circular polarization network. The linear polarization network corresponds to a coupler with four coupling arms. The coupler only couples one of the two TE21 modes, but does not couple the degenerate mode. Therefore, for linear polarized incoming beam, the error information in one direction cannot be extracted; for circular polarized incoming beam, although the error information in both directions can be extracted, the polarization loss of 3dB exists because the received linear polarization does not match the circular polarization of incoming beam. The circular polarization network corresponds to couplers with eight coupling arms, which couple TE21 with its degenerate mode and form circular polarization by 90 °bridge. This network can track both linear and circular polarized incoming beam, and for circular polarized incoming beams, there is no polarization loss. It is a good solution,, but the synthetic network is quite complex, especially in the case of high frequency

2.2 Composition of Equipment

The mode coupler mainly includes the main channel (eight rows of coupling holes is distributed) and the peripheral network. It is difficult to design the distribution of coupling holes in the main channel, requiring small insertion loss. In the simulation calculation, because of the large volume, the performance of the computer is required to be very high. The peripheral network consists of bridges, power dividers and phase-stabilized cables, which are mainly used to generate the port amplitude and phase distribution required for differential mode. In the design and processing debugging, to ensure the required phase distribution, the requirements for structure and processing accuracy are very high.
The appearance diagram of the mode coupler is as follows:

Figure 2 Top view of Mode coupler

Figure3 Front view Of Mode coupler

3Key Technical Specification 

  • Operating frequency12.25GHz12.75GHz
  • Differential mode null depth:<-35dB
  • Difference between sum level & difference Level≤13dB
  • Differential isolation (without the resistance filter):>40dB

4Research Process

The development of mode coupler mainly goes through three stages: theoretical design stage, computer simulation stage, trial production stage and finalization stage

4.1 Theoretical design

The two key performances of the mode coupler are differential mode coupling and main mode rejection.
To make the TE21 mode almost completely coupled, the phase constant of the TE21 mode in the circular waveguide must be equal to the phase constant of the rectangular waveguide TE10 mode, which limits the relationship between the diameter of the main channel and the width of the difference channel. The theoretical value of the two is:
A0.5143D ……....(1)
At the same time, the null depth of the differential beam will be worsened by coupling the signal TE11 mode to the differential channel. Therefore, it is necessary to improve the isolation of the differential channel to the TE11 mode. An effective method is to make the phase constant of TE11 mode in the main channel quite different from that of TE10 mode in the difference channel, which can be achieved by selecting the size of circular waveguide in the area close to the cut-off of TE21 mode.
The design of the coupling hole size between the circular waveguide and the rectangular waveguide is accomplished by the Coupled Wave Analysis and the aperture l hole coupling. The determination of the coupling hole takes into account the following principles:
a)  Select the appropriate coupling distribution to minimize the coupling of the TE11 mode in the Tx/Rx frequency bands, and consider that the coupling between the main circular waveguide TE21 mode and the rectangular waveguide TE10 mode is maximized.
b)  The aperture selection makes the coupling performance of the mode optimal in the whole frequency band.
c)  On the condition of meeting performance requirements, try to use fewer coupling holes to shorten the longitudinal length of the coupler.
For the convenience of machining, the coupling hole should be selected as a small circular hole, and the coupling distribution is . When it is a raised cosine distribution or a constant weighted Bessel distribution, the TE11 mode and the TM11 mode which do not need to be coupled can be suppressed to 40 dB or less, and the size of the coupling body is the best. Taking the raised cosine as an example, the coupling distribution is as follows:
.........2
In the formula, N is half of the total number of coupling holes, C is a constant to be determined, and i increase from the middle to both sides.
The formulas for calculating the coupling degree of weak-coupling hybrid modes and multi-point strong-coupling are as follows:
Strong coupling    ...........3
Weak coupling ........4
 ............5
   ...........................6
Whereinare the cutoff wavelengths of each coupling modes between the circular waveguide and the rectangular waveguide, and S is the hole pitch (generally equal spacing), which is the coupling amount of the reference point of the uniform hole distribution.
can be determined by weak coupling below - 40dB.can be determined by0 dB strong coupling .
Assuming the aperture is the number of holes is 2Nthen
                              ............... 7
is reference diameterThe empirical formulas of and are:
          .....8
The unit of f is GHzThe unit of is mm
Based on the above, we designed a set of coupling hole sizes and optimized them according to the coupling amount.

4.2 Computer simulation

After the theoretical design is completed, the main body and peripheral components are simulated respectively. Because the main body structure is too complex and huge, and the calculation is too large, the model is simplified by setting symmetrical section. In the simulation process, the size, spacing and coupling wall thickness are optimized according to the size of coupling amount, and a set of optimum sizes are obtained on the condition that the isolation degree meets the requirements.
The following is a simulation model diagram of the main body of the mode coupler:

              Figure 4 Model coupler body simulation model
4.3 Trial Production and Finalization Stage
When processed components and purchased components were in place, and the first prototype product was assembled. After field testing and testing, sum & difference level of one frequency point slightly exceeded the standard, and other indicators were superior. Finally, by changing the bridge input port, re-testing, and the indicators all met the requirements
Then the second set was assembled with another power splitter, and the performance meet standard.
Considering the performance and size requirements, the mode coupler shown in Figure 3 was determined to be finalized product

5Type approval condition

5.1 Electrical performance test

The performance test was carried out on the self-developed mode coupler, and the test method was tested according to the field test. The test flow is as follows:
  1. Install the antenna, the transmitting antenna is located on the 26th floor   of  Bailong Plaza, the receiving antenna (armored antenna) is located on the top of the eighth floor of the Aerospace Building, and the mode coupler is mounted on the receiving antenna;
  2. Signal alignment, adjusting the azimuth and elevation of the transmitting antenna and the receiving antenna, so that the receiving level value is the highest value
  3. Antenna debugging, by adjusting the antenna sub-reflector, the first side lobes of the transmitting frequency point are below -14dB,first side lobes of  the receiving frequency point is below -12dB, and the left and right side lobes are substantially symmetrical;
  4. Sum mode pattern test,  conduct the test on azimuth and elevation pattern of the   three Rx frequency point ,and record the peak level;
  5. Difference mode pattern test, conduct the test on difference mode pattern of the three   Rx frequency point ,and record the peak level;
The specification of Tx/Rx isolation is tested, and the meets the requirement.

5.2 Environmental test

Environmental tests include vibration, impact and humidity-heat tests.
The test was conducted in accordance with the Ku-band TE21 Mode Coupler Test Outline.
a) Impact test
Test method: The mode coupler is fixed with Sat On-The-Move antenna, and the On-The-Move antenna is fixed on the impact test-bed through the transition section. The impact parameters of the test-bed are set as: half-sinusoidal pulse, peak acceleration 30g, pulse width 11ms. The test-bed impacts six directions of three perpendicular axes, twice in each direction, totally 12 times. After the experiment, the following items were tested:
1) Check whether the internal fixing parts of the equipment are loose or cracked.
2According to the test method in chapter 5, testing  if the difference between the sum level and the difference level meets the requirements.
Testing conclusion: After the impact test, the internal fixtures of the mode coupler are free from loosening and cracking, and the ratio of the maximum value of the sum signal to the maximum value of the difference signal meets the requirements of the index.
b) Vibration test
Test method: The mode coupler is fixed and connected with the Sat On The Move Antenna, and the sat on the move antenna is fixed on the vibration test table through the transition section. The parameter setting of the test table please refer to "W curve in Figure A1.4. of appendix "GJB150.16-86 Military Equipment Environmental Test Method - Vibration Test" W curve   Figure A1.4. Test the following items after the test:
1Check the internal fixing parts of the equipment for residual deformation, cracks, looseness and other mechanical damage.
2According to the test method in chapter 5, testing  if the difference between the sum level and the difference level meets the requirements.
Test conclusion: After the vibration test of the mode coupler, the internal fixture has no residual deformation, crack, looseness and other mechanical damage, and the ratio of the maximum value of the sum signal to the maximum value of the difference signal meets the requirements of the index.
c) Damp heat test
d) Test conclusion
According to the above test conditions, the Ku-band TE21 mode coupler has stable performance in performance test and environmental test, and the fixing parts are firm and free from damage. The appearance and performance indexes of the product before and after the test are consistent, which meets the requirements of technical conditions.

5.3 Test results and comparison with similar products

Table1 Far-field test results of self-developed mode coupler antenna
Operating FrequencyGHz 12.25 12.5 12.75
  AZ EL AZ EL AZ EL
Sum diffidence level 12.5 11 11.5 11 11.33 11.16
Null depth 36 37.5 38 36 38 38
Table 2   Comparison of Self-developed Mode Coupler with Specification Requirements
  Specification requirement Performance parameters of self-developed mode coupler
Operating frequency 12.25GHz 12.75GHz 12.25GHz12.75GHz
differential-mode null depth -35dB -36dB
Difference between sum level and difference level within13dB Within 13dB
Transmission difference isolation 40dB 42dB
Length ≤125mm 125mm
Section diameter ≤120mm 115 mm
Interface dimension One end is Φ24.8mmanother end is Φ19mm One end is Φ24.8mmanother end is Φ19mm
 
Table 3   Performance comparison between self-developed mode coupler and purchased mode coupler
  Outsourced mode coupler Self-developing mode coupler performance parameters
Operating frequency 12.2512.75GHz 12.2512.75GHz
Difference mode null depth -35dB -36dB
Difference between sum level and difference level Within 13dB Within 13 dB
Transmission difference isolation 40dB 42dB
  Outsourcing mode coupler Performance parameters of self-developed mode coupler
Length 150mm 125mm
Section diameter 130mm 115mm
Interface dimension One end is Φ24.8mmanother end is Φ19mm One end is Φ24.8mmanother end is Φ19mm
 
Conclusion: It can be seen from the comparative analysis that the performance of the newly developed mode coupler meets the requirements of the index. Compared with the purchased model coupler, the size is smaller and the structure is more robust, which can replace the original product.

6Key Technologies and Solutions

6.1 Standing Wave Coupling Model with High Coupling Degree

Standing wave coupling is to use the differential mode to form a standing wave in the main waveguide so as to be coupled and extracted multiple times in the reflection. Compared with the traveling wave coupling, there is a higher coupling degree and a smaller number of holes, which can obtain a better sum Difference level index and shorter size. Through theoretical design and simulation calculation, the 12-hole standing wave coupling mode is finally adopted, and better sum and difference level index are obtained.

6.2 Waveguide coaxial conversion of unconventional waveguide

Since the coupled waveguide is an unconventional waveguide, the mode distribution is different, and unlike the general waveguide coaxial structure, it is necessary to be coupled from the narrow side for the convenience of the peripheral network arrangement. Finally, by referring to the data, a magnetic coupling mode was found, and better electrical indexes were obtained through simulation optimization.

6.3 Peripheral Network Design of Small Contour profile

In order to meet the requirements of section diameter less than 120 mm and length less than 125 mm mentioned in the design specification, the following measures are taken:
  • Adopt standing wave coupling, the body length is limited to 125mm;
  • The waveguide coaxial output method overcomes the difficulty and complicated process of adopting the cable output, increases the reliability of the product, and reduces the assembly difficulty;
  • The use of semi-steel cable for easy bending ensures the stability of the product and reduces the assembly difficulty;
  • The division usage of  bridge and the splitter to reduce the length and difficulty of the cable connection;
  • By using PROE software to simulate the layout of the peripheral network and finds the best way of cable connection and device placement.

7 Reliability assurance measures

a) Developing Reliability Design Criteria
The reliability design criterion is to summarize the engineering experience of existing and similar products, make them organized, systematic and scientific, and become the principle and requirements that designers should follow for reliability design.
b)  Selection of components and parts
The reliability of components is the basis of the reliability of the whole machine. In order to ensure high reliability and product quality, all components of qualified suppliers are selected to ensure the inherent reliability of components.
c)  Design and Acceptance of External Parts
The product contains some external components, and we have strictly controlled and managed the technical requirements of the external components. First, the accuracy requirements are clearly marked in the drawings; at the same time, we have compiled a test method to perform a strict quality check on the processed parts.