Coax over Fiber

Radio Frequency signals are broadcast in free space and enable modern communication. In many cases the signals must be transported from a large source e.g. Transmitter to an antenna for broadcast. The connection is done via coax cables or waveguides for the higher frequencies. As the frequency goes higher and higher to accommodate the requirements of modern Radars and data Transport the attenuation of coax and waveguides are increasing to a point that is it impossible to transmit such signals over long distance.

Both RF – which stands for Radio Frequency – and light (optics) are form of radiation that differ only in the wavelength. While RF cover bands from HF (starting at 3MHz) up to mm waves (ending at 300GHz) – see here for a table that defines the different band designators), light wave propagation is usually refer to 500nm to 1650nm range (180,000 – 600,000GHz). There is a vast order of magnitude difference between the two widely utilized electromagnetic waves. Both propagate at the speed of light but they suffer different attenuation in different medias. Light can be easily focused and coupled into a fiber optic cable that typically have losses of 0.2-0.4 dB/km, while top of the line waveguide (“coax cable”) for X-band has a loss of about 0.1dB/m which is 100dB/km factor of almost 500 times higher attenuation. Hence, the ONLY long-distance solution for analog RF propagation is transport over fiber – RF over Fiber.
Due to the vast frequency difference it is possible to modulate light signal with RF signal(s) (similar to the way that audio is modulating RF waves for our radios to work). The light can be focused and coupled into single mode fiber optic link and coax over Fiber becomes synonym to RF over Fiber.
RF over Fiber is widely utilized to propagate RF signals over medium to long distanced where the cable/coax/waveguide attenuation (loss) is too high to be successfully utilized. Examples are abound:

  • Satellite communication
  • GPS propagation
  • CDL
  • Wireless Cell coverage/BaseStation extension and Transport
  • and more.

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