TRANSMITTERS

cations, terrestrial, and satellite relay links, radar, and special operations are some other uses. Experimental use of the extremely-high- frequency (ehf) band is ending. The Fleet Satellite (FLTSAT) Ehf Package (FEP) is attached to two modified uhf FLTSATs. The FEP is currently provid- ing ehf communications capability to Army, Navy, and Air Force ground, airborne, and oceangoing terminals. We will discuss the FEP and its purpose in chapter 3. Infrared devices and lasers use even higher fre- quency ranges. Information on equipment using these frequencies can be found in Electro-Optics, volume 9, of this training series. RADIO EMISSIONS The emission class of an rf transmitter is deter- mined by the type of modulation used. The interna- tional designation system for AM and FM emissions is shown in table 1-2. It designates the rf emission by type, mode, and supplemental characteristics. We will now discuss the basic equipment required for communications. TRANSMITTERS For rf communications to take place, a signal has to be generated. Generating the signal is the job of the transmitter. The following paragraphs will very briefly discuss basic transmitters and transmitter fundamen- tals. TRANSMITTER FUNDAMENTALS Equipment used for generating, amplifying, and transmitting an rf carrier is collectively called a radio transmitter. Transmitters may be simple, low-power units, for sending voice messages a short distance or highly sophisticated, using thousands of watts of power for sending many channels of data (voice, tele- type, telemetry, t.v., etc.,) over long distances. Basic transmitters are identified by their method of modulation: continuous wave (CW), amplitude modu- lation (AM), frequency modulation (FM), or single- sideband (ssb). We will first describe the types of modulation. We will then describe briefly the basic transmitters themselves. Table 1-2.—Types of Radio Emissions MODULATION Modulation is the process of varying some charac- teristic of a periodic wave with an external signal. The voice frequencies (about 110-3,000 Hz) are contained 1-4