This application relates generally to data transmission, and more particularly to data transmission over power lines. 
The use of power lines to transmit data is known.  Initially, powerline communication systems were limited to relatively low data rates, typically less than 500 kbs.  These low data rates are generally useful for applications such as remote control of various switches connected to the powerline system.  More recently, developments have been made in the area of broadband powerline communication systems, also known as powerline telecommunications (PLT) systems or broadband powerline (BPL) systems.  These systems are capable of transmitting data at significantly higher data rates than previous systems.  For example, BPL systems can transmit data at rates of 4-20 Mbps. 
While existing powerline systems are capable of transmitting data at the rates described above, they were not initially designed for data transmission.  Instead, they were designed to carry large currents at high voltages so that significant amounts of energy could be distributed at one primary low frequency (e.g., 60 Hertz). 
Powerline communication systems generally use one or more carrier frequencies in order to spread the data transmission over a wider range of frequencies.  The low data rate powerline communication systems discussed above generally utilized frequencies in the range of 9 kHz to 525 kHz.  In this frequency range the risk of emissions is low as the attenuation of the cable is low and the wavelengths used in the signaling are long with respect to the typical cable lengths in the system.  However, the high data rates of BPL systems cannot be achieved using carrier frequencies below 525 kHz.  Instead, BPL systems typically use carrier frequencies in the range of 1-30 MHz.  At these higher frequencies the powerline cables become more effective radiators of electromagnetic waves. 
One of the problems with a BPL system is the risk of interference to radio communications services caused by the generation of electromagnetic emissions from the powerlines over which the BPL system operates.  The physical attributes of the powerlines (e.g., high elevation and unshielded wiring) along with the higher carrier signal frequencies needed for high bandwidth data transmission, contribute to this interference problem.