Attenuation and bandwidth
Unlike twisted pair, coaxial cables can use the lower side of the spectrum for transmitting, right down to the frequency band around 0 Hz (the DC). In practice using the lower bands imposes difficulties on Transmitter/Receiver design and so frequencies under 60Hz are rarely used. Following the frequency axis in the opposite direction, the usable part of the spectrum extends up to about 500Mhz. As can be seen in the figure below, the cable attenuation is proportional in the square root of the frequency. The cable's outer conductor minimizes the loss due to radiation from the inner conductor, which results in lower attenuation in comparison to twisted pair. The 500Mhz bandwidth is obviously superior to the few MHz bandwidth available on twisted pair. These cable characteristics allow data rates of hundreds of Mbps with repeaters required every few Kilometers (compared to few Mbps rates on twisted pair for the same repeater spacing). A coax cable serving a single high bit rate transmission is said to be operating in baseband mode. The alternative way to utilize the bandwidth is to divide it into smaller frequency bands and assign them to multiple independent transmissions – this is known as broadband mode (or more generally as frequency division multiplexing – FDM).
Distortion and noise
The way the cable's outer conductor encloses the inner conductor makes it less vulnerable to external electromagnetic interference and cross talk. This leaves performance limitations imposed by thermal noise and when in broadband mode intermodulation noise (explained next). Say signal A and signal B are assigned frequency bands #1 and #2 respectively. The output that a nonlinear devices in the modulation/demodulation circuit produces for signal A, might have energy in frequencies outside frequency band #1, say in band #2. Since A and B are independent (uncorrelated) signals, as far as signal B's concerned, this addition to its assigned frequency space is noise, and this noise is known as intermodulation noise.
Applications
Television – coax is widely used in carrying TV signals. Traditional TV networks employed coax cables throughout the entire distribution network whereas today the backbones of these networks have been replaced with fiber optic cables. With a single TV channel taking up only a 6-8MHz band (depending on the whether the NTSC or the PAL broadcasting standard is being used) a single coax cable can carry dozens of channels over a few tens of kilometers.
Telephone – In the past, coax cables were heavily used in the telephone network, with a single coax cable being capable of carrying over 10,000 voice channels at once. Today, the role of coax in telephony has greatly diminished with fiber optic cables, terrestrial microwave and satellite communication taking its place.
WAN – Modern use of the cable TV infrastructure extends beyond the transmission of TV signals and provides additional services such as internet services. TV signals occupy only the 54MHz-500MHz frequency band. This leaves plenty of bandwidth to assign additional frequency bands for data uplink and downlink channels with rates of few Mbps. As traditional cable TV infrastructure was designed only for unidirectional communication, the introduction of internet services required the replacement of old repeaters with new ones capable of forwarding different frequency bands in different directions (consumer to provider and vice versa).
LAN – Coax cables were employed in LANs dating back to the earliest Ethernet networks where is was used as a shared media. The early 10base5 standard made use of the thick 10mm coax cables in baseband mode , allowing rates of up to 10Mpbs over 500 meter segments. The later popular 10base2 standard made a switch to the more easily manageable 5mm coax cables, retaining 10Mpbs rates over shorter 185 meter segments. Today both of these standards are obsolete and newer twisted pair media standards have taken their place.