Coaxial Cables and Connectors
Coaxial cables have an inner conductor insulated from surrounding “screen” or “shroud” conductor – the screen is “grounded” in operation to reduce external interference coupling into the inner conductor. The inner conductor carries the radio signal.
Industrial wireless devices are designed to operate with a 50 ohm load – that is, the coaxial cable and antennas are designed to have a 50 ohm impedance to the radio, at RF frequencies.
At the high frequencies used in wireless, all insulation appears capacitive, and there is loss of RF signal between the inner conductor and the screen. The quality of the insulation, the frequency of the RF signal and the length of the cable dictates the amount of loss. Generally, the smaller the outer diameter of the cable, the higher the loss; and loss increases as frequency increases. Cable loss is normally measured in dB per distance – for example, 3 dB per 10 metres, or 10db per 100 feet.
The following table shows the losses of typical types of coaxial cables:
Cables need special coaxial connectors fitted. Generally connectors have a loss of 0.1 to 0.2 dB per connector.