Cable Loss

Value to Enter:

Enter either the total antenna cable loss (for each radio) or the dB loss per foot (or meter) and the length of the cable length. A default value of 3dB is provided for each radio. If you're calculating for Wi-fi access points with integrated antennas the total cable loss values should be changed to zero for both radios.

Significance of This Value:

Cable loss reduces the signal energy between the radio base station and the antenna. A low loss antenna cable (LMR 400) has a loss of 0.23 dB per foot at 2.4 GHz and 0.35 dB per foot at 5.8 GHz. Standard loss cable is often closer to 1 dB per foot. For cable runs less than roughly 10 feet the default value of 3 dB will often suffice for general calculation estimates. When running cable 100 feet up an antenna tower the manufacture's attenuation specifications must be obtained for the cable being used.

Background and Technical Perspective:

Electrical resistance is in a cable is the result of opposition to the movement of electrons. The power output of a cable can be derived from Ohm's and Watt's laws when the voltage is not alternating (DC current.) When a signal is alternating (at, for example, 2.4 GHz) the moving electrons tend to push away from the core of the conducting cable and move towards the outside of the cable. This is called the skin effect. In essence, it's as though the cable had less cross-sectional area than the area that is actually present. Skin effect causes the current to occupy a smaller cross-sectional area. Consequently, the relative resistance to current flow is greater for alternating current than for direct current. There are, as might be imagined, a wide variety of manufacturer's options as to cable types, conductor materials, and construction methods to optimize the signal carrying capabilities for a particular frequency, or range of frequencies, and for a particular use.