CW Power or Average Power Handling is limited by the overheating of the internal material due to power dissipation. The temperature distribution in the coaxial components is not uniform in the radial direction. The highest temperature is in the region closest to the center conductor where the field distribution is highly concentrated. In this area, the dielectric component can reach the point of thermal breakdown leading to failure.
Peak Power is defined by the arc path or radial difference between the center conductor and outer conductor in the critical region.
In a simplified and conclusive manner, we can state that CW Power relates to materials where Peak Power relates to structure.
Multipaction Breakdown defines peak power in spaceflight applications where high vacuum conditions exist, typically greater than 10-5 torr. Multipaction can be calculated by a few different methods. Two of the most common methods use the R. Woo Chart through frequency-distant products and the ESA/ESTEC Multipactor Tool® using internal coax diameter dimensions. Multipaction can be neglected if the frequency is less than 5 MHz or if the signal is less than 5 V (Standard Handbook for Radio Frequency (RF) Breakdown Prevention in Spacecraft Components, page 12).
Ionization Breakdown is an event which occurs during electron resonant collisions between free electrons and gas. This phenomenon is initiated at the gaps within coaxial structures. Ionization as well as multipaction are calculated using the R. Woo Chart through frequency-distant products where one can observe the worst case corona withstanding conditions: P (torr) ≅ f (GHz). This means that the critical pressure point is at a lower altitude if the frequency is higher. Furthermore, if a system is working from ground level to high altitude, > 70,000 ft., then the critical pressure point is unavoidable.
Simple Rules of Thumb
- When altitude is between 13.3 miles (70,000 feet) and 99 miles, ionization breakdown is at play, and when altitude >100 miles, multipaction is the limiting factor.
- The smaller the airgaps inside a connector or at cable entry, the lower the chance of multipaction breakdown.
- Vented connectors minimize the risk of multipaction events in vacuum conditions.
- CW power will generally be higher for larger cables as long as the material can handle the temperature. For instance, LMR 600 handles less than half of CW Power handled by the MegaPhase GC29, even though loss is lower and the outer diameter is larger. This is due to the PE core material used in LMR products, which can only handle +85°C, max. The GC29 utilizes PTFE which handles 200 °C.
- As a general rule, the larger the connectors, the better CW power will be–similar to cables. One important factor to consider is that some connector components such as the body and nut are typically made with a variety of materials: Stainless steel, beryllium copper, brass, aluminum, bronze, etc., all with different thermal conductivity properties. There is a considerable difference in CW Power performance between a TNC made with beryllium copper compared to stainless steel.
Key Questions to Ask
- What kind of application, ground, airborne, or space (vacuum)?
- What is the operating frequency range?
- What is the max. operating temperature?
- What is the power requirement? “Average power,” “peak power,” or both? What are the values?
- What is the duty cycle in CW applications?
- Are we connecting (or mating) to a match a load?
The intent of the above content is not to become an expert in the subject, but to familiarize you with some conditions and terminology used when dealing with high power applications in ground and space applications.