ADVISORY MINIMUM SPECIFICATIONS FOR MANUAL RADIO 

 DIRECTION FINDER EQUIPMENT; TYPE I 



The intent of this minimum specification is to serve as a general guide for 

 use by mariners interested in marine radio direction finding equipment for 

 navigational purposes when used on radiobeacons and on vessels where space 

 availability is no serious problem and radio operator personnel are available 

 for utilization of the direction finder for distress work. 



The instrument covered by these specifications shall, in general, consist of 

 the following: 



(a) Loop antenna, suitably supported. 



(b) Quadrantal error compensator. 



(c) Directional indicating device. 



(d) Receiver. 



(e) Sense determining device. 



toop. — The loop should be ruggedly constructed and provided with means 

 for rigid mounting. If the entire loop structure is a rotating device, it should 

 be arranged for complete and continuous rotation about its vertical axis 

 in such a manner that there is no observable lost motion between the loop 

 and the indicator. The loop drive should function with precision and 

 smoothness, and should be properly shielded and balanced electrically. 



The loop should be designed for low wind resistance and should be easily 

 rotated in a strong gale. 



Indicator. — The scale or indicating device should include an azimuth scale 

 (360°) which may or may not be used in conjunction with a gyro repeater. 

 In any event the scale or indicating device shall be such that divisions can be 

 read accurately to a portion of 1° of circular arc. 



The quadrantal error compensator shall be capable of correcting plus or 

 minus 15° of error. 



Collector ring and brush assemblies, if used, shall be low-loss, designed to 

 minimize noise and corrosion. Such assemblies are normally located in the 

 indicator housing. 



Cabinet. — Cabinet design should follow standard practice for shipboard elec- 

 tronic equipment. Provision should be made to prevent accumulation of 

 moisture due to condensation or leakage. 



Receiver circuit. — Tuned radio frequency or superheterodyne capable of CW 

 and MCW reception. 



Frequency. — 275-515 kc. This frequency range will permit the reception of 

 radiobeacon signals in the marine radiobeacon band f 285-325 kc.) and on 

 the international distress frequency (500 kc). Dial calibrations should be 

 direct reading in kilocycles. 



Over-all Sensitivity. — The complete unit including loop assembly should be 

 capable of developing an output of 6 milliwatts, with not more than 60 

 microwatts noise output when the loop is rotated for maximum signal in a 

 ground wave field strength of 50 microvolts per meter. 



Directional Sensitivity. — The directional sensitivity should be such that when a 

 vertically polarized signal of sufficient intensity is received to induce 1 micro- 

 volt into the loop, and the receiver is adjusted to deliver an output of 6 

 milliwatts at optimum loop setting, the absolute null should be not more than 

 3° in width, this measurement to be made under conditions wherein any 

 noise originating external to the receiver and picked up by the loop and 

 sense antenna shall not be of sufficient magnitude to obscure the null. 



Null Quality. — With a CW field of 50 microvolts per meter the null zones 

 shall be diametrically opposite within 2°. It shall not be possible to shift 

 the nulls by improper tuning or any normal manipulation of the controls. 

 The minima should be crisp, well defined and entirely free of residual signal 

 throughout the 360° rotation of the loop about its vertical axis. 



Directional Sense. — Provisions shall be incorporated which will definitely per- 

 mit the elimination of any ambiguity between the actual and reciprocal 

 bearing. 



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