transmitter  patch  panels  are  interlocked  with  the transmitter so that no open jack connection can be energized and no energized patch cord can be removed. This provides safety for both personnel and equipment. ANTENNA  POSITIONING Raise and lower antennas - raising and lowering physically  of  antennas  is  associated  with  flight, refueling or PMS operations. Extreme care should be taken that all moving parts are in correct operating conditions   and   the   Officer   of   the   Deck   or Communications  Watch  Officers  know  prior  to  the physical movement of the antennas. USE DIRECTIONAL ANTENNAS Reception is defined as: when an electromagnetic wave passes through a receiver antenna and induces a voltage  in  that  antenna.  Further  detailed  information  on antennas,  antenna  use,  wave  propagation  and  wave generation can be found in NEETS MODULES 9, 10, and 17. Rotate For Optimum Reception This  is  accomplished  by  both  physical  and mechanical  means  of  moving  the  antenna(s)  to  properly align  and  tune  the  antenna. Align For Optimum Reception Using the correct antenna location (by rotation) and the correct equipment for the system, you will bring the antenna into alignment and be ready for the final step, which  is  tuning. Tune For Optimum Reception There are two objectives of antenna tuning: (1) to tune out the various impedances and (2) to match the length of the antenna to the frequency radiated at its characteristic impedance. Impedance:   everything   exhibits   some impedance, Even a straight piece of copper wire 3 inches long will offer some resistance to current flow, however small. The characteristic impedance of this same piece of copper wire is its overall resistance to a signal. The transmission line between an antenna and a transmitter  has  a  certain  amount  of  characteristic impedance. The antenna also has a certain amount of characteristic impedance. This basic mismatch in impedance  between  the  transmitter  and  the  antenna makes   antenna   tuning   necessary.   Naturally,   as transmitters, transmission lines, and antennas become more complex, antenna tuning becomes more critical. Antenna length adjustment: When we tune an antenna,   we   electrically   (not   physically) lengthen and shorten it. The radiation resistance varies as we vary the frequency of the transmitter and  tune  the  antenna.  The  radiation  resistance  is never  perfectly  proportional  to  antenna  length become of the effects of the antenna height above the ground and its location to nearby objects. You will find that the better the ability of the receiver  to  reject  unwanted  signals,  the  better  its selectivity, The degree of selection is determinedly the sharpness  of  resonance  to  which  the  frequency- determining circuits have been engineered and tuned. You usually measure selectivity by taking a series of sensitivity  readings.  As  you  take  the  readings,  you  step the input signal along a band of frequencies above and below  the  circuit  resonance  of  the  receiver;  for example, 100 kilohertz below to 100 kilohertz above the tuned  frequency.  As  you  approach  the  tuned  frequency, the input level required to maintain a given output level will fall. As you pass the tuned frequency, the required input level will rise. Input voltage levels are then compared with frequency. They can be plotted on paper, or you may can view them on an oscilloscope. They appear in the form of a response curve. The steepness of the response curve at the tuned frequency indicates the selectivity of the receiver, thus allowing for the optimum reception. RF SAFETY PRECAUTIONS Although    electromagnetic    radiation    from transmission  lines  and  antennas  is  usually  of insufficient strength to electrocute personnel, it can lead to other accidents and compound injuries. Voltages may be inducted in ungrounded metal objects, such as wire guys, wire cable (hawser), hand rails, or ladders, If you should come in contact with these objects, you could receive a shock or RF burn. This shock can cause you to jump or fall into nearby mechanical equipment or, when working aloft, to fall from an elevated work area. Take care to ensure that all transmission lines or antennas are deenergized before working near or on them. Guys, cables, rails and ladders should be checked for  RF  shock  dangers.  Working  aloft  “chits”  and  safety harnesses  should  be  used  for  your  safety.  Signing  a 2-29