Beyond Teflon®: RF Assemblies for Extreme Environments, is part of our Times Talks webinar series.
Teflon® coaxial cables offer excellent performance, but every material has its limits. Learn how to specify the best, hermetically sealed, custom-designed cable assemblies that fit your system. Our team has helped hundreds of customers specify and qualify the correct SiO2 cable assembly for their applications.
Watch this webinar on-demand or read the session notes below.
Teflon is a phenomenal dielectric for microwave cables. It’s lightweight, low loss, and very flexible. However, even an excellent product has limits. Extreme environmental conditions are one for Teflon. For example, applications such as hypersonic missile guidance will reach temperatures ranging from 200-250°C, or even as high as 600-1,000°C. Teflon will simply melt in those conditions.
Second would be any application that requires strict phase stability. As discussed in our recent webinars on phase performance, the ability to match multiple microwave cables to each other so the signal takes the exact same amount of time to travel through, as well as controlling how that phase relationship changes over temperature, are absolutely essential properties in microwave cables used in applications like phased array radar.
A third extreme environment is high radiation, found in applications such as particle accelerators, where two beams smashing together generate a significant amount of radioactivity.
To address the challenges, Times Microwave has developed a proprietary silicon dioxide dielectric (SiO2) that excels in these environments. Silicon dioxide is used throughout the micro-electronics industry for its excellent insulating properties. The construction of the silicon dioxide coaxial cable starts with a solid oxygen-free copper center conductor, a silicon dioxide insulating dielectric, and stainless-steel jacket that has copper cladding to act as the outer conductor.
Silicon dioxide provides excellent phase stability and low hysteresis and can perform at extreme temperatures ranging from just above absolute zero to 1,000°C. The metal and silicon dioxide dielectric construction makes the cable resist radiation naturally, up to 100 mega rads. The stainless-steel jacket is welded to the connectors with laser beam technology to create a hermetic seal.
When specifying electrical performance for these cables, the first thing to think about is attenuation or insertion loss, which will depend on the cable size selected. Next, in terms of electrical length, it should be determined if the cables in the system need to be phase matched.
There are several ways Times Microwave can phase match cables, including relative or absolute value. A third option is to establish a golden standard—Times Microwave builds and tests one of the assemblies, and the data for all future assemblies is matched to the golden standard to ensure everything produced is the same.
In terms of power handling capacity, silicon dioxide is excellent in high power applications due to the nature of its construction. However, there are a few things that will decrease the power handling capacity of the cable. The first is VSWR; the higher the VSWR, the lower the power handling capacity. Second is altitude. If the application is operating in a vacuum, it will lose the natural convection that takes heat away from the cable, which will in turn reduce the maximum power that can flow through the cable. Finally, the operating frequency will determine what the power handling capacity of the cable can be.
Following is an example of a climate monitoring satellite application that required a set of electrical performance parameters that no other cables in the world could meet. It incorporates a space-based synthetic aperture radar to find sub-millimeter sized changes in the height of an ice cap to determine how much water is present.
From an RF perspective, extreme accuracy was required—any sort of error absolutely needed to be minimized, including the transmit and receive cables. All the different antenna cables had to be phase matched so the sensors could best understand exactly what they were seeing from the radar pulses. Temperature fluctuations also had to be addressed as a space-based application will go from warm on the day side of the planet to cold at night based on its orbit.
Other mechanical elements of the silicon dioxide cable that were relevant for this application included the stainless-steel jacket with a welded connector, designed to perform very well under high vibration environments such as during launch. It is also a non-outgassing material and is compliant for radiation considerations in an orbital environment. The third feature that will impact the mechanical configuration of the cable is the connector, which is application dependent.
In this application, it was important to focus on the temperature capabilities of the silicon dioxide cable since a hypersonic missile will go through the top of the atmosphere, generating huge amounts of heat, similar to a space shuttle during re-entry.
As previously mentioned, the silicon dioxide cable itself can readily withstand those heat loads. But beyond that, one of the important requirements was phase matching at ambient temperatures. As the cable moves from cold temperatures to very hot temperatures, the phase matching between cables needs to track.
Furthermore, there are basic assumptions that likely have applied to many other specified cable assemblies that may not fit for extreme environments. For example, a shrink-down marker band that includes data such as the part number, serial number and manufacturer is a standard component in a bill of materials. However, at high temperatures, it is very likely that those marker bands will melt and create a potential foreign object debris hazard. The cable assemblies used in this application were laser marked to ensure that the important data was still included without creating any fault in the system while in operation. Custom connectors were also designed.
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