<P> The majority of modern transmitting equipment is designed to operate with a resistive load fed via coaxial cable of a particular characteristic impedance, often 50 ohms . To connect the power stage of the transmitter to this coaxial cable transmission line a matching network is required . For solid state transmitters this is typically a broadband transformer which steps up the low impedance of the output devices to 50 ohms . A tube transmitter will contain a tuned output network, most commonly a PI network, that steps the load impedance which the tube requires down to 50 ohms . In each case the power producing devices will not transfer power efficiently if the network is detuned or badly designed or if the antenna presents other than 50 ohms at the transmitter output . Commonly an SWR meter and / or directional wattmeter are used to check the extent of the match between the aerial system and the transmitter via the transmission line (feeder). A directional wattmeter indicates forward power, reflected power, and often SWR as well . Each transmitter will specify a maximum allowable mismatch based on efficiency, distortion, and possible damage to the transmitter . Many transmitters have automatic circuits to reduce power or shut down if this value is exceeded . </P> <P> Transmitters feeding a balanced transmission line will need a balun . This transforms the single ended output of the transmitter to a higher impedance balanced output . High power short wave transmission systems typically use 300 ohm balanced lines between the transmitter and antenna . Amateurs often use 300 - 450 ohm balanced antenna feeders . </P> <P> See Antenna tuner and balun for details of matching networks and baluns respectively . </P> <P> Many devices depend on the transmission and reception of radio waves for their operation . The possibility for mutual interference is great . Many devices not intended to transmit signals may do so . For instance a dielectric heater might contain a 2000 watt 27 MHz source within it . If the machine operates as intended then none of this RF power will leak out . However, if due to poor design or maintenance it allows RF to leak out, it will become a transmitter or unintentional radiator . </P>

Distortion of the modulating signal produces harmonics which cause an increase in the signal