ls in decibels relative to the main signal or carrier. For example, the harmonics may be better than -50 dB and other spurious signals better than -55 dB. Output impedance The output impedance of a transmitter is a particularly important parameter. Some transmitters will be connected to a feeder, possibly coaxial to transfer the power to an antenna at some distance from the transmitter. Others such as those used in cellular telephones feed directly into the antenna that is part of the overall telephone assembly. Whatever the system it is necessary to ensure that good match is achieved between the output of the amplifier and the load, to enable the output amplifier to operate correctly and enable the optimum power transfer to take place. The output impedance of the transmitter is measured in ohms. As might be expected it relates to the ratio of the voltage and current provided by the output. For those transmitters that are connected to a feeder, the output impedance is almost invariably 50 ohms as this is a standard impedance for coaxial feeder. Placing an incorrect load on the transmitter can result in damage. High voltages or currents can destroy the output device. Although very low output units may be able to operate into any load, those above a few watts can be damaged, and often protection circuitry is provided. Running the transmitter with a high level 194 Newnes Guide to Radio and Communications Technology of standing wave ratio in a feeder has exactly the same effect. Depending upon the distance from the load, the transmitter may see an excessive level of current or voltage. In view of this the output impedance of a transmitter is often quoted, and the maximum level of standing wave ratio may also be given. Simple Morse transmitter To give an idea of how various transmitters operate, overviews of a number of types of transmitter are given, starting with a simple example to give an idea of the basic requirements of a transmitter. An effective Morse transmitter can be made out of very few components. As Mors