30 OCEAN ELECTRONIC NAVIGATIONAL AIDS 



ute on — 2 minutes off is the usual program of transmission) , engine generator 

 starting, timing of lights, sound fog signals, warning signals, and radiotele- 

 phone schedules. 



WARNING DEVICE. — In order to insure reliable service from the radiobeacon, 

 an automatic warning device is used in the station which rings a bell when- 

 ever the transmitted signal is interrupted, out of time, or seriously impaired 

 in strength or modulation. Aural reception of the radiobeacon signal is also 

 provided for monitoring purposes. The unit, known as the Radiobeacon 

 Supervisor and Alarm, consists of a radio receiver fed from a short antenna 

 of suflBcient length to pick up the radiobeacon signal, a clock which drives 

 cam-operated contacts, and various copper-oxide rectifiers, relays, resistors, 

 etc., which serve to operate a spring- wound warning bell under contingencies 

 noted above. A loud speaker and an output meter provide the operator with 

 data on the radiobeacon signal. 



ANTENNA. — The transmitting antenna is selected to suit the physical factors 

 of the site, the transmitter power and the desired range. For high power 

 radiobeacons, insulated steel towers are used. Antenna coupling houses fed 

 through concentric cable are used with tower-type antennas. For low power 

 or limited range, an insulated vertical mast or whip-type antenna, or a 

 vertical wire with T fiat-top, is satisfactory. A good ground is essential for 

 an efficient antenna system, and in some locations where the soil is rocky 

 or sandy a counterpoise system of radials is employed. 



ACCESSORIES. — In addition to the major items described, various accessory 

 and power supply items are required at a radiobeacon station. Accessories 

 include racks and special panels carrying switches, terminals, cable adaptors, 

 storage lockers, and shelves. These items are necessary for the proper 

 installation, inter-connection and switching of the radiobeacon equipment. 

 For 115-volt d-c radiobeacon installation, a 54-cell storage battery, two d-c 

 engine generators, and two rotary converters, are required. If 115-volt a-c 

 power is available, the only power equipment needed is an auxiliary 115-volt 

 a-c engine generator, and special rectifiers to provide d-c power for the radio- 

 beacon timers. Spare parts, including vacuum tubes, are provided for 

 maintenance purposes. 



CLASS D MARKER RADIOBEACONS ON LIGHTED BUOYS 



The United States Coast Guard has developed a low power, class D radio 

 marker beacon for installation on lighted buoys. Such installations are 

 required where harbor entrances and channels must be marked by a radio- 

 beacon but where it is not practicable to affect a shore installation. Figure 

 2-10 shows the type of buoy used for the installation and the method of 

 mounting the antenna. 



RADIO DIRECTION FINDERS FOR SHIP USE, AS DEVELOPED IN THE 



UNITED STATES 



The radio direction finder is an instrument for observing, by means of radio, 

 the direction of a station sending radio signals. Briefly, in navigation, it is 

 an instrument for taking radio bearings. As generally used in marine navi- 

 gation in the United States, it consists of a loop antenna mounted above the 

 ship's pilot house, with its axis extending downward into the pilot house, and 

 carrying a handwheel and reference indicator over a magnetic compass, 

 dumb compass, or gyro repeater in the pilot house. (See figs. 2-5 and 2-11.) 

 This loop can be rotated by the navigator or observer. The loop is connected 

 to a radio receiver in the pilot house. Using this receiver, the navigator 

 picks up the desired station, then revolves the loop and notes the varying 

 strength of the signal until a point is reached where the signal is lost entirely 

 or nearly lost. This is called observing the minimiun. At this point the 

 plane of the loop is perpendicular to a line connecting the ship and the sta- 

 tion heard, and the reference indicator is so placed with respect to the loop 

 that it then points directly to the station. Such radio bearings may then 

 be used in navigation on the same general principles as sight bearings are used. 



In a well-designed and adjusted radio direction finder, the point of mini- 

 mum, or no signal heard, is sharp, and bearings may be taken with an 

 accuracy of 1° or 2°. Even when the minimum is not well defined, a fairly 

 accurate bearing may be obtained by swinging the loop to each side, until 

 the signal becomes just audible, and taking the mean of the readings in these 

 two positions. 



