68 OCEAN ELECTRONIC NAVIGATIONAL AIDS 



Selectivity. — The ratio of over-all receiver and loop sensitivity at resonance, 

 to sensitivity to signals off resonant frequency should be at least the following: 



Percent frequency 

 Ratio difference 



10 1 



100 2 



1,000 4 



Image Frequency. — All image frequency response should be at least 60 db below 

 the desired signal within the frequency range of the receiver. 



>*odio Output Power. — At least 6 milliwats across a 600- or 20,000-ohm load. 



Output Impedance. — 600 or 20,000 ohms at 1,000 cycles. 



Installation. — Radio direction finder equipment should preferably be installed 

 In a position easily accessible to the ship's navigator. Inasmuch as direction 

 finder bearings are affected by objects in the path of approaching signals, 

 consideration must be given to the location of the direction finder loop 

 antenna. This antenna should not be located behind or near large metal 

 objects such as vessel stacks, superstructure, trunks, etc. In most vessels 

 the choice of loop antenna location will be a compromise, due to necessity. 

 The ideal location aboard any vessel would be one where the loop antenna 

 is well clear of all objects, permitting maximum use and accuracy of the 

 direction finder equipment. An indicating device should be installed at radio 

 operating positions to indicate when DF is in use. An interlock device to 

 prevent radio operation during DF use may be desirable. 



Power Requirements. — Inasmuch as marine direction finding equipment is used 

 in connection with distress work, it is important that the equipment be 

 available for immediate use at all times. It is therefore recommended the 

 equipment be designed principally for battery operation. Provisions should 

 be made for automatically recharging the batteries by rectifier, trickle 

 charge, floating the battery across the d-c supply source, etc. The battery 

 containers, charging equipment, etc. should be considered as part of the di- 

 rection finder equipment. In event the equipment is designed with a self- 

 contained a-c power supply, provisions for bypassing the rectifier and in- 

 troducing necessary d-c potentials should be made by providing an easily 

 accessible arrangement for emergency operation. 



