Performance Parameters for RF coax cables

The most common performance parameters for RF coax cables are summarized in the chart below:

•  Electrical
  Frequency range
  Attenuation (max loss)
  Power handling
  RF stability
  Shielding
  Return loss / VSWR
• Mechanical
  Outside diameter
  Flexibility (min. bend radius)
  Flex life
  Weight
  Dielectric material
  Connector series
• Environmental
  Temperature range
  Humidity
  Vibration
  Resistance to solvents/UV
  Outgassing
  Altitude

• Impedance is a compromise between power handing and attenuation and changed by varying the ratio of inner & outer conductor sizes. We do not list impedance in the table above because the majority of communications systems are 50Ω, while 75Ω is standard for CATV systems. Mini-Circuits sells both 50Ω 75Ω test cables.
• Cable diameter affects many other attributes, which is why it’s usually at the top of the list in the selection process. Increasing the outer diameter of a cable can reduce its insertion loss, increase power handling and termination strength. However, wider cable diameter also results in lower maximum frequency rating, increase in minimum bend radius, less flexibility and more weight. Wider cable diameters also limit choice of connector types, which has additional consequences outlined in greater detail below.
• Bend radius, flexibility and flex life all depend on cable diameter, so customers with strict requirements for these factors will gravitate toward cables with smaller diameters. This typically means sacrificing power handling and insertion loss performance to some extent.
• Rule of thumb:
  The one-time minimum bend radius of a cable is 5x the outside diameter of the cable.
  The minimum re-bendable radius of a cable is 10x the outside diameter.
  There is some variation in this rule depending on the frequency and tolerance for RF degradation after bending.
• Power handling increases with cable diameter. Users with high power requirements may therefore need to sacrifice cable flexibility and high frequency performance. Additionally, smaller connector types and poor return loss that result from low quality connector attachment can reduce power handling. Power handling also degrades at high altitude and high temperature, so users building systems for these operating conditions should ensure the cables they choose have been properly qualified.
• Operating frequency range is typically a hard requirement defined by the bandwidth of the system, but frequency determines a number of other factors system designers need to weigh. For example, cable attenuation is a function of frequency: the higher the frequency, the leakier the signal. Better return loss is also more difficult to achieve as frequency increases. Frequency dependent losses can be mitigated by adding shielding; triple shielded cable is often used to achieve similar isolation to that of semi-rigid cables. More shielding typically requires additional outer conductor layers which adds complexity to the production process and cost to the end product.
• RF stability depends on several factors, including whether the cable will be subject to regular handling and bending (in the lab) or fixed in placed. Most cables include manufacturer ratings on minimum bend radius and total number of flex cycles before RF performance starts to degrade. Learn more about how bending affects phase stability here. RF stability is also affected by a number of environmental factors such as temperature, which becomes important when cables are used in temperature chambers for high temperature operating life (HTOL) testing, or in assembled systems where extreme temperatures are expected. Mini-Circuits offers cables specially designed for phase stability versus bend and temperature stable cables.
• Other environmental factors such as UV protection, resistance to solvents and outgassing are highly specialized requirements for niche applications demanding careful selection of all materials including the cable jacket. For outgassing concerns, the cable construction, the glue used in the heat shrink strain relief, gasket materials and epoxies are all specialized materials. Space applications will also require vented connectors with highly specialized internal designs and attachment processes to prevent voltage breakdown. These are not trivial challenges to overcome and RG cable will almost never suffice.
• Passive intermodulation (PIM) are harmonics created by non-linearity in RF connections or devices, which can result in degradation of received signal. Low PIM cable assemblies require cable and connectors built specially to reduce PIM. Historically corrugated cable (not discussed here) offers the best PIM performance but today some braided coax comes close to corrugated.