Coax, Twinax and Triax Cables

When looking for RF cables or connectors the first item you need to identify is the cable. You will generally find 3 types of cable. Coax, Twinax and Triax, each type will then have different variances and sizes.

Coax Cable


Used to transfer Radio Frequency (RF Signals

  • Telecommunications
  • Computer Local Access Networks
  • Video
  • Wireless Communications
  • Low level RF Signals

Two Conductors:

  1. Center conductor for signal transfer
  2. Outer Shield
    • Ground, sometimes used for signal return
    • EMI/RFI (Electromagnetic Interference/Radio Frequency Interference) protection

Twinax Cable


Referred to as twisted/shielded pair and Twin Axial

Differential data transfer

  • High/Low signal – reverse image
    • Phase Locking

Non-Differential data transfer

  • Signal and clock
  • Signal and return
  • 78-124 ohm impedance
  • 78 ohm primary

Three Conductors

  1. Center conductor (blue by convention)
  2. Intermediate
  3. Outer Shield
    • EMI/RFI protection

Triax Cable


Referred to as double shielded coax

  • Military shipboard data bus applications
  • Video
  • Low current Test and Measurement
  • 50-75 Ohm
  • 75 Ohm primary

Three Conductors

  1. Center Conductor
  2. Inner shield
  3. Outer shield
    • EMI/RFI protection

Ribbon Twinax

There also exists a product that is marketed as ribbon twinax, but application is markedly different from above.

This product is used in a variety of high speed (Upwards of 15Ghz) applications include PCI Express and other high-bandwidth multi-channel mediums. It can be found in connector terminated versions, as well as direct solder to board which can be cut to custom lengths and all of which can be folded tightly without major signal degradation.

I want to measure the impedace of a Twinax cable. With a normal coaxial cable, the impedance can be determined approximately by:
How can you determine the impedance of any Twinax cable?