What to consider when choosing a Coaxial Cable
Our founder has been quoted as saying “MRO services the wired part of wireless.”
Electrical and electronic cables exist to allow electrical energy, signals or data to be moved to another location with a minimum of energy loss or interference to or from other electrical sources. At DC and low-frequency AC a properly sized cable will have minimal resistive or radiation loss. However, as signals move into the radio frequency range capacitive, inductive, electromagnetic radiation and impedance mismatch become significant factors where losses are concerned. The solution that has prevailed is a Coaxial Cable. Constructed with a center conductor, a cylindrical insulator (called a dielectric) around the conductor, and an outer cylindrical conductor (called a shield or screen or, sometimes, drain) over the insulator and, often, a jacket over the outer conductor. A peculiar thing occurs with the shield, the outside surface keeps electrical interference from affecting the signal but the inside surface acts as a return path for the signal on the center conductor. The spacing between the center and the shield has to be kept to close tolerances for this to work well. At a certain point, the capacitive and inductive elements of the cable combine to present a characteristic impedance to the signal. If this impedance is matched to the source of energy and the load, cable losses can be minimized and the transmitter and receiver will operate optimally. 50 ohms has been the characteristic impedance of most signal and RF power transfer. In the CATV and video world, 75 ohms is more common. Other impedances are available.
Impedance, signal loss, power handling, and mechanical characteristics are the primary specifications to consider when selecting a coaxial cable. Cable loss, otherwise known as insertion loss has been minimized in various ways, but as the frequency transmitted or received is increased, so does the cable loss. Although cable construction and choice of materials all affect loss, the diameter of the cable is usually a good indicator of the cable loss characteristics, a low loss cable will have a bigger diameter.
Connecting a 100-watt transmitter working at 30Mhz to an antenna using 100 meters of cable.
- FLL100A: diameter 2.8mm, loss at 30MHz is 12.9 dB which means 5 watts of the transmitted power will be present at the antenna.
- FLL600: diameter 15mm, loss at 30MHZ is 0.45 dB which means 91 watts of the transmitted power will be present at the antenna.
Receiving a 2.5GHz WLAN signal with 10 uVolts at the antenna using 10 meters of cable.
- FLL100A: diameter 2.8mm, loss at 10 meters will be 13dB which means 2.24 uVolts at the receiver input.
- FLL600 diameter 15mm, loss at 10 meters will be 0.44dB which means 9.5 uVolts at the receiver input.
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Written by CJ Hodgson at MRO