Most   shipboard   communication   equipments   do not operate independently. A particular piece of elec- tronic  gear  may  be  designated  “primary”  and  still  be used in many different system operations. You need to understand all the associated equipment in a system to identify   problems   correctly   and   to   make   repairs promptly.  Thorough  knowledge  of  system  operations will enable you to say with complete confidence, this communications suite is operational. SAFETY Hazards  encountered  in  servicing  electronic equipment  and  the  precautions  to  be  taken  against them  are  covered  thoroughly  in  Electronics   Techni- cian  Volume  1,  Safety,  NAVEDTRA  12411,  and  the General  Handbook  (NAVSHIPS   0967-000-0100)   of the EIMB series. Safety is everyone’s responsibility. Observance of safety  precautions  will  keep  your  equipment  operat- ing, help your career in the Navy, and possibly deter- mine whether or not you survive.  Always follow the appropriate  safety  precautions! Note: Equipment that we cover in this and other  chapters  is  intended  to  be  merely  repre- sentative of equipment that you may encounter on board your command. We will not attempt to include all the possible equipment or equipment configurations. BASIC  SYSTEM  REQUIREMENTS Radio  equipment  can  be  divided  into  three broad   categories:   transmitting   equipment,   receiving equipment,  and  terminal  equipment.  Transmitting Figure 1-1.—Basic radio communication system. equipment   generates,   amplifies,   and   modulates   a transmitted   signal.   Receiving   equipment   receives   a radio wave, then amplifies and demodulates it to extract the  original  intelligence.  Terminal  equipment  is  used primarily to convert the audio signals of encoded or data transmission into the original intelligence. A basic radio communications system may consist of only a transmitter and a receiver, connected by the medium   through   which   the   electromagnetic   waves travel  (see  figure  1-1).  The  transmitting  equipment creates a radio-frequency (rf) carrier and modulates it with audio intelligence to produce an rf signal. This rf signal is amplified and fed to the transmitting antenna, which converts it to electromagnetic energy for propa- gation. The receiving antenna converts the portion of the electromagnetic  wave  it  receives  into  a  flow  of  alter- nating  rf  currents.  The  receiver  then  converts  these currents into the intelligence that was contained in the transmission. Terminal   equipment   is   used   primarily   where coded   transmissions   are   employed,   to   convert   the modulated  signal  into  the  original  intelligence.  Sys- tems  you  will  encounter  in  the  fleet  use  terminal  equip- ment,  such  as  AN/UCC-l,  AN/URA-17,  and  CV- 2460. THE  FREQUENCY  SPECTRUM Figure 1-2 shows the overall electromagnetic fre- quency spectrum as defined by the International Tele- communications Union. Pay particular attention to the part  used  for  communications.  Rapid  growth  in  the quantity  and  complexity  of  communications  equip- ment  and  increased  worldwide  international  require- ments for radio frequencies have placed large demands upon the rf spectrum. These demands include military and civilian applications, such as communications, lo- cation  and  ranging,  identification,  standard  time,  in- dustrial, medical, and other scientific uses. The military has modified the frequency spectrum for its use as shown in table 1-1. A few general charac- teristics are described in the following paragraphs. The   extremely-low-frequency   (elf),   very-low- frequency  (vlf),  and  low-frequency  (lf)  bands  require high  power  and  long  antennas  for  efficient  transmis- sion  (antenna  length  varies  inversely  with  the  fre- quency).   Transmission   of   these   frequencies   is normally limited to shore stations. The   commercial   broadcast   band   extends   from about 550 kHz to 1700 kHz. This limits naval use to the 1-2