Attenuation refers to any reduction in the strength of a signal. Attenuation occurs with digital or analog signals. Attenuation is the end result of signals being transmitted extended distances. Attenuation is usually expressed in units called decibels (dBs). The cable type will determine at what point along the length of the cable signal degradation occurs. Repeaters can be inserted along the length of the cable to boost the signal strength which increases the maximum attainable range of data transmitted though the cable. Attenuation Effects on Data Transmitted through Cable The majority of attenuation in Coax cable comes from resistive losses in the cable’s metallic conductors. Temperature also has effects the resistivity of the conducting surface. Resistivity is a “bulk property of material describing how well that material inhibits current flow.
This is slightly different from resistance, which is not a physical property. If one considers current flowing through a unit cube of material (say, a solid metal cube that measures 1 meter on each side), resistivity is defined as the voltage measured across the unit cube length (V/m) divided by the current flowing through the unit cube’s cross sectional area (I/m2). This results in units of Ohm m2/m or Ohm-m.” University of British Columbia Geophysical Inversion Facility Temperature affects the movement of electrons in the material. When a signal voltage is passed through a wire the electrons collides with the atoms in the material.
If more atoms are allowed collide with electrons, the greater the frictional resistance, which affects the cable ability to conduct (allow data to pass through the cable). Temperature causes the atoms with in the material to move. Increasing the temperature causes the atoms to jiggle which causes frequent collations with electrons. The opposite is true when the temperature is lowered. Attenuation is a function of the frequency of the signal, and the length and physical structure of the cable itself. Specifically, it depends on: The diameter of the inner conductor: as the diameter of the conductor increases, attenuation decreases.
The composition of the outer conductor: the more effective the screening action, the lower the attenuation. The nature of the dielectric: the lower its constant dielectric, the lower the attenuation. Therefore, an expanded dielectric supplies, at any given diameter, values of attenuation lower than a solid dielectric. Since the type of cable, the temperature, and the rate at which data is being passed through the cables are constant, the length of the cable is the only factor that affects the attenuation in the two cables. You will have less signal loss in the 500 feet cable than you would have in the 5000 feet cable. As the cable lengths get longer and longer, the rise time of the edges will exceed the time for a bit cell, and at this point the signal will be attenuated so that its peak-to-peak amplitude gets smaller and smaller. If 500 feet attenuates by 5dbm, the 5000 feet cable would have an attenuation of 15dbm, which is three times the original value.
References
- Hranac, R. (2002) Broadband: Coax Attenuation versus Temperature. Electronic Journal achieves Communications Technology. Retrieved July 9, 2005, from: http://www.ct-magazine.com/archives/ct/0402/broadband.html
- Sauerwald, S. Analog Avenue Tech Notes: Getting Data from Here to There, Quickly, While Minimizing Cost.
- Retrieved July 9, 2005, from: http://archive.chipcenter.com/analog/tn009.htm
