Optimization Design of Thermal Test Waveguide Link for Communication Satellite

. Problems in structure of water load power meter and waveguide assemblies of the communication satellite transponder thermal test. This paper analyzes water load layout and reasons of signal attenuation, and then it proposes an optimization that put water load detached from satellite’s board. To ensure the reason of waveguide signal loss, this paper discusses the internal electromagnetic field distribution and currents on waveguide wall. the result of signal loss value showed that the new optimization can decrease signal attenuation value and shorten the process of crafts implementation.


Thermal test RF link scheme design 2.1 RF transmission in thermal test
Most of the satellite transponder thermal tests use metal rectangular waveguides to design transmission links. This paper takes the design of some satellite thermal test waveguide as an example. Firstly, the water cooling load layout method and the thermal test waveguide path were designed. Secondly, a brief demonstration and analysis for the design of the internal structure and the wall current of the guided mode were carried out.

Waveguide Path Design and Analysis in thermal test
There were 32 microwave loads would be installed in a satellite thermal test and must be fixed on the satellite boards. If all the models used as the previous models (directly fixed on the floor), it will be difficult to connect the water cooling load layout. We proposed two plans to solve this problem.
a. According to the design of the repeater, the forward outputs could be divided into two groups, and then the signal power synthesized by the external output multiplexer and then connected the water-load. However, the newly-manufactured output multiplexers needed to be redesigned and debugged with a period of at least 3 months.
b. The problem of fixing the water-load was solved by increasing the adapter plate. This program required 6~8 adapter plates, but the design time of the adapter plate and waveguide could be reduced within 1 month.
After considering the degree of difficulty and progress time, Option b was more feasible than Option a.

Metal Rectangle Waveguide Wall Current Calculation Principle
Wall current size and direction were determined by the tangential magnetic field near the wall, (4) Rectangular waveguides almost working in the mode. According to(3)(4) (y=0) (y=b), , , , Therefore, when in mode , the wall current only had , in same size and direction. In the up and down wall, the current consist of and . In the same position -x, the wall current in same size but in opposite direction [3].

Pre-optimization plan and test results
This paper chosed 12 Rf links among the 32 RF links in some satellite. According to plan b in 2.2, the number of waveguide as following table 1 shows, 0.51 Although this program could meet the current thermal test requirements of transponders the design was not only too complicated, but also cost lots of time. The water-load would be fixed to the satellite's board, it can affect satellite's life. In order to simplify the tooling and shorten the test design cycle, a test scheme was put forward to mount the water-cooled load directly on the adjustable bracket. The solution could reduce the VSWR and insertion loss value of the waveguide assembly. And it also helped water-load completely depart from the satellites. The new method could effectively solve the interference problem between the transponder thermal test waveguide assembly and the on-board equipment.

Structural optimization and analysis
The new scheme minimized the waveguide design path. Water-cooled load which was fixed on the adjustable bracket could have at least 10mm adjustment margin in three dimensions. Reduced use of flexible or twisted waveguides,could greatly suppressed the attenuation of waveguide components [4]. After accounting and testing the new path, the number of waveguide components and electrical performance test results were shown in the table 3, and the VSWR data were shown in table 4.

Actual design study
Based on the research in the previous section, the water-cooled load distribution optimization design was performed for a certain type of satellite transponder thermal test waveguide assembly [5]

Conclusions
By analysing the current design of the power absorbing waveguide in the transponder sub-system tank, the factors influencing signal attenuation were described in detail. Based on the original scheme, an adjustable bracket scheme that installs the water cooling load away from the satellite panel was proposed and compared. After the signal standing wave and insertion loss in the original scheme and optimization scheme, the optimized scheme has great advantages in reducing the signal attenuation value, reducing the number of used waveguide components, reducing the use of twisted waveguides and flexible waveguides, and even not using them. At the same time, the optimized solution can reduce the weight of satellite panels and increase the service life of satellites by solving the problems of complicated waveguide design and complicated installation.