RF Component & Device Test Series: Coaxial Cable Assembly Performance Testing Part 2

In part 1 of this series we delved into several of the commonly specified electrical performance characteristics for coaxial cable assemblies. This post wraps up the most common parameters:

•  Capacitance
•  Group Delay
•  Velocity of Propagation
•  RF Shielding/Leakage
•  Maximum Input Power (Power Handling)
•  Maximum Voltage/Breakdown Voltage
•  Phase Stability (during flexure)

Capacitance

The capacitance of a coaxial cable is typically measured as pico-Farads per length (pF/L) of cable and is a function of the spacing of the inner and outer conductor as well as the relative permittivity of the dielectric. The capacitance of a coaxial cable matters as it directly contributes to the cutoff frequency. Hence, using a dielectric with a higher capacitance will exhibit a lower cutoff frequency than a dielectric with a lower capacitance.

Group Delay

The group delay of a coaxial cable assembly is derived from calculating the phase variation over frequency, which is an estimation of the time it takes for a pulse to travel the length of a coaxial depending on frequency. The flatness of the group delay is an important factor in many applications, where an inconsistent group delay may result in performance degradation, such as with phase sensitive radar. Group Delay is measured by comparing the phase variation of a VNA signal sent through the coaxial assembly and plotted over frequency.

Velocity of Propagation (Velocity Factor)

The velocity of propagation, on the other hand, is a measure of the time it takes for a signal to travel through the transmission line compared to the speed of light. The velocity of propagation is readily determined from the effective relative permittivity of the dielectric within the coaxial assembly. Hence, coaxial cable assemblies with dielectrics with higher relative permittivity will have a lower velocity factor than dielectrics with a lower relative permittivity.

RF Shielding or Leakage

The shielding capability of a coaxial cable assembly is the ability of the assembly to prevent signal energy from escaping the assembly or interesting the assembly from an external source. As coaxial cable assemblies are often used for precision applications, the RF shielding effectiveness is often specified as a minimum requirement for many critical applications. In many applications a perfect RF shield isn’t feasible to use as the outer conductor of the coaxial cable assembly, and compromises for flexibility, weight, size, and cost are made that result in the use of corrugation, braids, tape, foil, and other conductive material arrangements. RF shielding or leakage can be measured by how much signal energy escapes the coaxial cable compared to the signal energy contained within the coaxial assembly.

Maximum Input Power (Maximum Power Handling) & Maximum Voltage/Breakdown Voltage

The maximum amount of input power a coaxial cable assembly can handle before performance is degraded depends on the geometry of the coax, construction methods, and material properties of the dielectrics and conductors used in the cable. Essentially, the maximum power handling is the lowest amount of power where a coaxial cable assembly begins to degrade or fail from thermal breakdown. Using higher conductivity conductors and low loss dielectrics can increase the power handling of a coaxial cable assembly of a given size, but ultimately the power handling is limited by the size and heat dissipation capability of the coaxial cable assembly and the temperature at which the dielectrics degrade.

Voltage breakdown, or maximum voltage, is generally determined by the lowest voltage where dielectric breakdown occurs and conduction through the dielectric happens. Both maximum power handling and voltage breakdown can be determined by destructive testing of sample coaxial assemblies.

Phase Stability (During Flexure)

Phase stability of a coaxial cable assembly is how constant the phase of a signal passing through the coax is maintained while a coaxial cable assembly is undergoing mechanical flexure. Phase stability is very important for test and measurement application, where the calibration and measurement repeatability is dependent on the test coaxial cables performing consistently in over many use cycles. This is readily measured with a VNA and a coaxial cable assembly attached to a fixture that includes a mechanical structure that consistently flexes the cable over time.

Learn more about Pasternack’s extensive line of Coaxial Cable Assemblies here https://www.pasternack.com/nsearch.aspx?Category=Cable+Assemblies&sort=y&view_type=grid