C. RADIO DIRECTION FINDING (RDF) 



1. GENERAL 



Radio direction -finding systems utilizing a non-directional transmitter 

 and a direction-sensitive receiving antenna system have been in use for many 

 years as navigational systems for both aircraft and ships at sea. The simple, 

 low-cost equipment makes this system extremely useful where moderate LOP ac- 

 curacy is satisfactory. 



The RDF beacon system is an azimuthal system which measures 

 direction rather than distance. In its most general use, it is composed of a fixed 

 cw or amplitude -modulated transmitter with a non-directional antenna and a RDF 

 receiver with tracking antenna which may be automatic. The output of the re- 

 ceiver is normally used in a feedback system to orient the receiving loop antenna 

 so that its plane is at right angles to the direction of the arrival of the signal. 

 The receiving instrumentation often includes a radio magnetic indicator (RMI). 

 Controls are provided to select the following functions: 



Automatic visual bearing indication of the direction of arrival of the rf 

 signal and the simultaneous aural reception of modulated rf energy. 



b. Aural reception of rf energy, using a non-directional antenna. 



c. Aural reception of rf energy using a loop antenna. 



2. DESCRIPTION 



The system has a short-base-line, of the order of the ratio of the 

 antenna loop diameter to the rf wave length. The system is useful in the 

 allocated frequency range of 100 - 1750 kc, which includes the broadcast band. 

 However, the quality of bearings decreases at frequencies above 500 kc because 

 of polarization errors. A channel at 500 kc is reserved for international dis- 

 tress traffic. Maritime RDF traffic is assigned to a frequency band of 405 - 415 kc. 



Transmitters installed at fixed sites are generally cw and operate at 

 power levels of 0.4 - 10 kw. Antennas are typically 150 - 300 feet in height and 

 are either wire or tower type. Transmission characteristics over sea water 

 are excellent at frequencies used, and thus less power and antenna height are 

 required when the transmitters are installed on surface vessels or buoys. 



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arthur ai.''Xittle,i(nr. 



