Research on space temperature environmental performance and technology of aerospace RF cable assembly

The aerospace RF cable assembly is an important component for the propagation of space communication signals, which can achieve impedance matching and energy transmission of microwave signal transmission paths. In addition to the requirements for a wide frequency range, low insertion loss, and low standing wave as required for conventional RF cable components, it must also possess high reliability, spatial temperature resistance, vacuum high-power transmission, and other spatial adaptability capabilities. This article primarily discusses the damage to the insulation materials of cable components under space-limit temperatures and its impact on the performance of the cable components. The problem of deterioration and even failure of the performance of aerospace RF cable components in high and low-temperature alternating environments in space has been addressed through the adoption of corresponding measures.


Introduction
The spacecraft will be affected by frequent large-scale temperature alternation when entering and leaving the earth's shadow area.When the spacecraft faces the sun, it will bear the strong radiation of sunlight, and the surface temperature can reach 200℃.When facing away from the sun, the surface temperature can be as low as -180℃ due to the extremely cold environment in space.Especially for low earth orbit spacecraft, frequent access to the shadow area leads to rapid cyclic changes in the temperature of spacecraft components, with a temperature change rate of up to 50℃ / min [1] .The frequent alternation of large-scale ambient temperature changes will inevitably affect the performance of spacecraft.Compared with satellites or manned spacecraft flying around the earth, the space environment faced by deep space exploration spacecraft and extraterrestrial resident platforms is more complex and harsher [2]   .
With the development of aerospace technology at home and abroad, the technological level of various aerospace equipment is continuously improving, and the demand for space-borne RF cable assemblies is increasing.Many aerospace model users have increasingly high requirements for product performance.The environment where communication satellites and other spacecraft operate is extremely harsh, characterized by wide temperature ranges, alternating high and low temperatures, high vacuum, and strong radiation.Although corresponding temperature control, radiation protection, and other measures will be taken for the RF cable components for aerospace, the products still face harsh conditions: the minimum temperature tolerance is -100℃, and the maximum temperature is +150℃ [3] .This article focuses on the impact of the aerospace temperature environment on the performance of RF cable assemblies.According to the requirements of aerospace, it is necessary to ensure that the aerospace RF cable components have excellent and reliable microwave performance under a wide temperature range and repeated alternating high and low temperatures.

Problem description
In this wide temperature range of temperature alternating environment, the RF cable assembly is prone to problems such as shrinkage of insulation windings and deterioration of RF parameters [3] .During inorbit operation, the RF cable components outside the cabin are affected by multiple high-and lowtemperature changes in the space.Its VSWR (voltage standing wave ratio) will increase, the insertion loss will increase, and the performance of the transmission path will deteriorate [4] , which will affect the normal operation of the satellite transceiver link and even lead to the failure of the whole system.As shown above, taking a certain type of aerospace RF cable assembly as an example, during its development process, there was a deterioration in the microwave performance of the product after temperature cycling tests.As shown in Figure 1, the VSWR increased and the insertion loss increased, which far exceeded the technical specifications and could not meet the application requirements of the system.
Due to existing space constraints and material limitations, there are limited technical approaches to improve the temperature resistance of aerospace RF cable assemblies.The following analysis focuses on their structure and the mechanisms responsible for performance degradation.

Structure and mechanism analysis
RF cable assembly is composed of RF connectors and RF cables.The microwave performance and temperature resistance characteristics of the RF cable assembly are closely related to its structural changes.During the development of aerospace RF cable assembly, the author found that the RF connector interface changed after the temperature test (as shown in Figure 2): the insulator appeared concave or protruding.After disassembling the cable assembly, it was found that compared with the end face of the normal cable certificate, the insulation layer of the cable end face after the test was sunken or protruded (as shown in Figure 3).To analyze the changes in the temperature-alternating process of the RF cable assembly, it is necessary to understand the specific structure of the RF cable first.The structure of the RF cable is shown in Figure 4 [5] .The RF cable is composed of the cable's inner core, an insulating layer, a spiral wound inner shielding layer, a copper wire woven outer shielding layer, and an ETFE outer sheath with strong radiation resistance.The outer conductor of a flexible RF cable is composed of an inner shielding layer and an outer shielding layer.The inner shielding layer of the cable is made of silver-plated copper spirally wound.The insulating layer exerts an inward force, and the outer shielding layer exerts an outward pressure.The two forces are combined to compress the layers of the spirally wound inner shielding mesh, forming a good contact.There is a significant amount of stress inside the insulation layer surrounding the RF cable during manufacturing, which is gradually released under alternating high and low temperatures (thermal shock or thermal cycling), causing the insulation layer to shrink axially (as shown in Figure 5 and Figure 6).It destroys the impedance matching between the RF cable and the connector and worsens the VSWR and the insertion loss of the RF cable assembly.
Through several temperature tests, it is found that the thicker the cable is, the more obvious the RF cable (outer diameter of Φ8 mm ~ Φ10 mm) after 200 times of thermal shock is, and the insulation layer shrinkage can reach 6 ~ 20 mm.This physical change has a great impact on the microwave performance of the cable assembly: the maximum VSWR can be changed from the original 1.2 to 1.8, and the RF insertion loss will deteriorate to some extent.In addition, for the cable assembly with high power transmission requirements, the insulation layer shrinkage will lead to the end face of the cavity structure (as shown in Figure 6).When the release of gas accumulation in the cable cannot be discharged, the position will become a dangerous area of micro discharge and low-pressure discharge [6] .Therefore, when the RF cable assembly transmits high-power signals in the space temperature environment, the risk of failure is very high, and this problem must be solved.From the above analysis, only by solving the problem of end face shrinkage in the process of temperature change in the RF cable assembly can the occurrence of cavity structures be avoided, thereby improving the space temperature environmental adaptability of aerospace RF cable components.This article focuses on two aspects to solve this technical problem: the process optimization of aerospace RF cable assemblies and the structural optimization of aerospace RF connectors.

Process optimization of RF cable assembly
The conventional assembly process for radio frequency cable assembly has failed to enhance the temperature resistance of space radio frequency cable assembly.While existing process optimization can improve product performance consistency and qualification rate, it does little to improve the adaptability of space radio frequency cable assembly in temperature environments.This study reveals that end-face shrinkage in space radio frequency cable assemblies is closely linked to temperature environment parameters.By selecting appropriate temperature environment parameters for pretreating the cables, it is possible to effectively reduce or inhibit end-face shrinkage in finished products.Therefore, incorporating a cable pretreatment process and optimizing its parameters during the assembly of space radio frequency cables can significantly enhance their adaptability in space temperature environments.
Temperature environment parameters mainly include temperature range, temperature variation times, temperature variation rate, and hot soak time.The temperature variation curve (Figure 8) formed by these factors will affect the performance of RF cable components.By setting the temperature range, temperature variation times, temperature variation rate, and hot soak time, the temperature test of the aerospace cable assembly is carried out.The study studies the influence of temperature alternation on the stability of microwave parameters of RF cable components, studies the relationship between these four temperature environmental factors and the shrinkage of RF cable insulation and the microwave performance of RF cable, forms a systematic influence relationship curve, and analyzes the variation trend of RF cable end face and microwave parameters.It provides a reference for the subsequent optimization of the cable pretreatment process.
In this paper, the following comparative temperature tests were carried out, namely, we changed the temperature range and the number of temperature changes, the temperature cycle test was carried out on the RF cable, and the shrinkage of the insulation layer of the RF cable relative to the outer shielding layer after the test was tested.The temperature ranges are -45℃~+85℃, -65℃~+100℃, and -100℃~+150℃, respectively.The temperature changes are divided into 10 times, 20 times, 30 times, 40 times, 50 times, 100 times, and 200 times, respectively.The temperature rate is 4℃/min and the hot soak time is 20 minutes.The specific test results are shown in Table 1.4.54 9.98 3.98 0.12 0.12 0.1 0.02 0.02 0.02 -100~+150 5.96 10.02 3.86 0.14 0.12 0.1 0.04 0.02 0.02 The test results show that it is most reasonable to carry out 20 temperature tests (the temperature range is -65℃~+100℃), which can not only keep the shrinkage of the end face of the cable insulation stable quickly but also save the test time.With the increase in the number of temperature changes, the shrinkage of the end face of the insulation layer will tend to be stable.In the early stage of temperature change, the end face shrinkage of RF cable assembly changes rapidly, while in the late stage of temperature change, the end face shrinkage changes slowly.Based on the above characteristics, the end face of the cable insulation layer can be shrunk to a stable state through the above operations (pretreatment process), and then the shrunk part can be cut off and assembled into a cable assembly so that the insulation layer of the cable assembly can maintain a relatively stable state in the temperature environment.Finally, the applicability of the space temperature environment of the aerospace RF cable assembly can be effectively improved.
In the design and manufacturing process of RF cable assembly, there are three methods to improve the shrinkage of RF cable end face under changing temperature environments, as follows: (1) We optimize the insulation material manufacture of low-density PTFE winding tape to ensure a reasonable density of the PTFE winding tape, optimize its physical properties, and reduce internal stress caused by the winding tape during the winding process [7] and the shrinkage probability of the insulation layer under temperature changes.
(2) We optimize the winding tension between the insulating layer and the inner shielding layer, increase the pressing force between the shielding layer and the insulating layer of the RF cable, increase the friction between the two, hinder the movement of the insulating layer relative to the shielding layer, and reduce the shrinkage of the insulating layer under temperature changes.
(3) Compared to conventional RF cable assemblies, in the assembly process of aerospace cable assemblies, the cable pretreatment process is added, reasonable temperature environment parameters are selected, and temperature tests are conducted on aerospace RF cables to release internal physical stress in the RF cable through periodic high and low-temperature treatments.After the test, the contracted end face is cut out and then fitted with an aerospace RF connector.

Structural optimization design of RF connector
The structure of the RF connector also has a certain impact on the shrinkage of the RF cable insulation layer end surface. [8]Therefore, the temperature-alternating resistance of the RF cable components can be improved by optimizing the structure of the RF connector.The specific methods mainly include the following two:  (1) When the cable insulation layer shrinks at the end face, it will exert a force on the wire core, which will cause the wire core to move.Therefore, the double insulation support structure is selected in the connector design to limit the movement of the wire core from the front and back directions(as shown in Figure 7).The movement of the insulation layer can be restrained by fixing the wire core, to maintain the structural stability of the RF cable assembly.
(2) A taper pin structure is added at the end of the insulator of the connector (as shown in Figure 8).The cone penetrates the cable insulation layer to restrain the displacement of the insulation layer by increasing the friction force.The cone structure can be separated between the inner and outer conductors, which can effectively improve the micro-discharge and low-pressure discharge threshold and inhibit the discharge at this position.
By incorporating a pre-treatment process into the assembly process of radio frequency cables for space flight components, and optimizing this pre-treatment process, as well as designing and optimizing the structure of space flight radio frequency connectors, the shrinkage of the cable insulation can be effectively reduced or even eliminated.This ensures the structural stability and performance reliability of the radio frequency cable components when they are operating in a high-temperature space environment for an extended period.As shown in Figure 9, this article conducted a microwave performance test on the final developed aerospace RF cable assembly.The specifications, model, and length of the cable assembly are consistent with those introduced at the beginning of this article.Similarly, after repeated temperature cycling tests, the optimized design product can maintain internal structural stability and reliable performance, with a variation rate of its microwave performance parameters within 10%.

Conclusion
To improve the adaptability of aerospace RF cable assemblies towards temperature fluctuations, it is crucial to tackle concerns related to end-face contraction and performance decline caused by varying temperatures.This investigation examines the architecture of radio frequency cables while identifying key factors that contribute to end-face shrinkage in cable assemblies.Through optimization of pretreatment procedures, refinement in design aspects of space-specific RF connectors, and enhancement in manufacturing processes for these specialized cables, this research effectively mitigates performance degradation or potential failures experienced under extreme thermal conditions encountered within outer-space environments featuring oscillating high-low temperature cycles.The outcomes presented herein offer significant insights applicable to addressing analogous challenges across relevant industries.

Figure 1 .
Figure 1.Changes in microwave performance of cable assemblies before and after the test.

Figure 4 .
Figure 4. Structure diagram of RF cable.

Figure 9 .
Figure 9. Changes in microwave performance of cable assemblies before and after the test..

Table 1 .
Shrinkage of the insulation layer (mm).