the CDS computer via the encryption device and the SGS computer. Link-11  Communications  Switchboard The  communications  switchboard  provides  system flexibility   and   casualty   recovery   capabilities   by allowing manual switching of the data terminal set and   individual   HF   and   UHF   radios. A  typical switchboard will provide interconnections: The  Link-11  data  terminal HF  radio  sets  to  provide Link-11 capability the following set to one or more the  standard  HF A  Link-11  data  terminal  set  to  one  or  more UHF  radios  sets  to  provide  UHF  Link-11 capability The  same  communications  switchboard  may  also be used for connecting a Link-4A data terminal set to one  or  more  UHF  radios  to  provide  standard  UHF Link-4A (TADIL C) capability. Link-4A is covered in detail later in this book. Radios The Link-11 system can operate with either an HF radio  or  a  UHF  radio.  Long-range  communications are  achieved  by  the  use  of  the  HF  system.  UHF communications are limited to line of sight. “Line of sight” means the radio wave will not bend over the horizon;  therefore,  the  use  of  an  antenna  mounted high  on  the  mast  will  increase  the  range  of  UHF communications. Antenna  Couplers Antenna  couplers  are used to connect a specific radio set to a specific antenna. The coupler provides for the correct impedance matching of the antenna and the radio set. For many of the multi-couplers to work properly,  it  is  extremely  important  that  the  correct frequency spacing be observed. A general rule is to ensure   a   frequency   spacing   of   15   percent. Frequencies  that  are  too  close  together  can  cause interference  and  distortion,  increasing  the  signal  to noise ratio and causing bit errors in the data. Antennas In   oversimplifying   the   theory   of   antenna operation,  an  antenna  is  just  a  piece  of  wire  that radiates electromagnetic energy from the radio into the atmosphere and converts atmospheric electromagnetic   radiation   into   RF   current   to   be processed  by  the  radio.  As  electromagnetic  energy from  the  atmosphere  passes  through  this  wire,  it induces a current in the wire. This current is fed to the radio receiver. If the receiver is tuned to the same frequency  as  the  received  signal,  the  signal  can  be processed. The   same   wire   will   radiate   an electromagnetic field if current is flowing through it. The   frequency   at   which   a   radio   operates determines  what  size  antenna  is  most  suitable  for transmitting  and  receiving.  The  higher  the  frequency, the  smaller  the  antenna  will  be.  Lower  frequencies require larger antennas. For example, the full-wave length of an antenna designed to operate at 4 MHz is about 250 feet. Since this is too long for shipboard application,  antennas  are  designed  in  submultiple lengths.  These  include  half-wave  and  quarter-wave antennas. An  antenna  can  be  tuned  by  introducing  a capacitive  or  inductive  load.  This  loading  effectively changes the electrical length of the antenna and can be used to extend the frequency range of the antenna. For  more  information  on  antenna  design  and operation, refer  to  the  Navy  Electricity  and Electronics  Training  Series,  Module  10,  Introduction To  Wave  Propagation,    Transmission   Lines,   and Antennas, NAVEDTRA  B72-10-00-93. Transmission Cycle The data flow for the Link-11 transmission cycle is shown in figure 2-4. The CDS computer receives data   from   the   various   ship’s   sensors,   navigation systems, and operator entries, and stores this data in a data base. When a Link-11 transmission is required, the  computer  outputs  parallel  digital  data  through  the SGS  computer  to  the  cryptographic  device.  The cryptographic device encrypts the data and sends the encrypted  data  to  the  data  terminal  set  (DTS).  The DTS converts the digital data to analog audio tones, 2-4