How does VSWR affect the power handling capability of a coaxial cable or connector?
Power Handling Derating with VSWR
When a transmission line has standing waves, the voltage and current vary along its length. At voltage maxima, the electric field stress in the dielectric increases, potentially leading to dielectric breakdown (arcing). At current maxima, conductor heating increases, potentially exceeding the temperature rating of the cable or connector.
The voltage maximum on a transmission line with VSWR S is V_max = V_incident × (1 + |Γ|) = V_incident × (2S/(S+1)). For VSWR 2:1, V_max is 1.33× the incident voltage. For VSWR 3:1, V_max is 1.5× the incident voltage. Since power handling is proportional to voltage squared, the derated power handling is P_rated/(VSWR) for voltage-limited components.
For connectors, the most common failure mode at high VSWR is arcing at the connector interface where the gap between inner and outer conductors is smallest. SMA connectors are typically rated for 300-500 W peak power under matched conditions, but this must be derated significantly at high VSWR. N-type and 7/16 DIN connectors handle more power due to their larger dimensions.
Frequently Asked Questions
How do I derate for VSWR?
Divide the matched power rating by the VSWR value to get the derated average power handling. For peak power (voltage breakdown), divide by VSWR². A cable rated for 100 W matched should handle only 50 W at VSWR 2:1 and 33 W at VSWR 3:1.
Does altitude affect power handling?
Yes. At high altitude (low air pressure), the breakdown voltage of air decreases, reducing the power handling of connectors and open transmission line structures. Pressurized transmission lines and sealed connectors are used in aerospace and high-altitude applications.
What about average vs peak power?
Average power is limited by heating (current maxima). Peak power is limited by voltage breakdown (voltage maxima). Both must be considered separately, and both are worsened by VSWR.