3. EQUIPMENT AND INSTRUMENTS 



installation would be for a metal ship to 

 have two masts and support a "T" type an- 

 tenna halfway between them, the downlead 

 coming to an insulated support then directly 

 into the antenna coupling unit. The length 

 of the downlead plus the length of the long- 

 est leg of the flat top should not be longer 

 than 95 feet. For ships with only one mast 

 a support should be found as high as pos- 

 sible some distance away from the mast to 

 attach a guy so a modified inverted "L" an- 

 tenna can be erected. The total length of the 

 wire should not be more than 95 feet. Mutual 

 coupling to adjacent antennas can be a se- 

 rious problem. If the EPI antenna is close 

 to another antenna that is being operated 

 by a transmitter in the vicinity of 2 mega- 

 cycles it might be impossible to get the EPI 

 antenna to load properly. In such a case the 

 interfering antenna should be disconnected 

 at its transmitter and left floating while 

 EPI is in use. When operating and cali- 

 brating EPI all antennas which are at all 

 near the EPI antenna should be ungrounded. 



3-28 Locating the ground station anten- 

 na. — The ground station EPI antenna is usu- 

 ally a 100-foot aluminum mast with a 100- 

 foot radial ground plane consisting of 32 

 copper wires. The most important require- 

 ments are listed below : 



(a) It is desirable that the ground be 

 fairly level and clear for a distance of about 

 100 feet from the center of the system. 



(b) While the location may be on high 

 ground, there is no advantage in having the 

 station elevated except when Shoran is used 

 for calibration. In fact, other considerations 

 make it more desirable to have the station 

 on low ground near the water's edge. 



(c) Have the station so located that the 

 signals will not have to pass over more than 

 a quarter of a mile of land from any direc- 

 tion to the service area. 



(d) Where the ground wire is close to the 

 water's edge, the wire should never be under 

 water at any stage of the tide. 



(e) The antenna should not be located 

 near high tension wires. In damp weather 

 such lines are capable of emitting strong 

 interference. 



55 



(f) If possible, keep away from existing 

 high-power radio transmitters such as a 

 broadcast station and long-range communi- 

 cation stations. If there is doubt about an 

 area in this respect, it is wise to set up a 

 sensitive communications receiver in the 

 area and monitor the frequencies to be used 

 for EPI keeping in mind that the bandwidth 

 of an EPI receiver is 85 kilocycles. Strong 

 signals as much as 200 kilocycles away from 

 the EPI center frequency can seriously harm 

 distance signals from the ship unit. 



3-29 EPI calibrations.— The EPI sys- 

 tem is not self-calibrating, and the correc- 

 tion to observed distances must be obtained 

 by direct comparison with known distances. 

 Three methods of calibrating EPI equipment 

 are described in Chapter 2 of Special Pub- 

 lication No. 265-A, EPI Manual. Note that 

 when the Shoran calibration method is used, 

 the Shoran must also be calibrated and ap- 

 propriate corrections applied to observed dis- 

 tances before a comparison is made. 



EPI equipment should be calibrated : 



(a) At the beginning of each field trip. 



(b) After each shutdown of equipment. 



(c) Whenever a change is made in the 

 equipment which might affect the calibration. 



(d) At the end of each trip. 



(e) At other times when convenient. 

 Calibrations should not be made until both 



ship and shore equipments have been in op- 

 eration a minimum of 45 minutes. A record 

 should be kept on the ship and at each shore 

 station of all repairs, changes in equipment, 

 or accidents which might affect the calibra- 

 tion (see 5-91). 



It is advisable to record all EPI calibra- 

 tions in a separate sounding record for each 

 project, or each season when working on 

 continuing projects. At the end of a season 

 or a project, a special report on EPI cor- 

 rections should be submitted and reference 

 made to it in appropriate descriptive reports 

 (see 7-27). 



3-30 Effect of ships' heading. — As men- 

 tioned in 3-27, there is a possibility of di- 

 rectional effects through mutual coupling of 

 adjacent antennas or other interference on 



