6722 HYDROGRAPHIC MANUAL PaGE 630 



6722. Radio Receiver 



Any one of a number of good communication receivers may be used for the radio 

 receiver shown in figure 140. Commercial receivers, such as those made by Hammarlund 

 Manufacturing Company, Inc., of New York, The Hallicrafters, Inc., of Chicago, and 

 National Company, Inc., of Maiden, Massachusetts, are used exclusively for this 

 purpose by the Coast and Geodetic Survey. 



The radio receiver should include the following features to be satisfactory for use 

 in R.A.R.: 



(a) It must be suitable for marine use. 



(6) It must be capable of withstanding excessive vibrations without damage, detuning, or modu- 

 lating the output signal. 



(c) After the warm-up period, the tuning should remain essentially constant, irrespective of 

 moderate voltage, temperature, or humidity changes. 



(d) It must be sufficiently sensitive. 



(e) It must possess high image rejection. 



(/) It should contain a crystal intermediate-frequency filter. 



(g) The frequency range of the receiver must include all the frequencies used in R.A.R. 



(h) The receiver tuning must not be critical. 



(i) The audio-frequency power output should be at least 2 watts. 



(j) It must contain an adjustable intermediate-frequency beat oscillator. 



Some of the models of radio receivers made by the above-mentioned companies 

 mee t practically all the listed requirements. To comply with most of these requirements 

 the receiver must employ a superheterodyne circuit. There should be one or two stages 

 of radio-frequency amplification to aid in amplification, image rejection, and increased 

 signal-to-noise ratio. The intermediate-frequency amplifier should contain two or more 

 stages. The width of the intermediate-frequency band should be from 5 to 8 kilo- 

 cycles. The band width of the intermediate-frequency crystal filter should not exceed 

 100 cycles, in order to eliminate as much interference as possible. 



Frequencies used by the Coast and Geodetic Survey for R.A.R. range from 2.5 

 to 4,2 megacycles (see 6442). The receiver must cover this range, in fact most of the 

 receivers used for R.A.R. cover a range from broadcast frequencies to frequencies as 

 high as 42 megacycles. 



It is desirable to have both radio-frequency and audio -frequency gain controls. 

 Most commercial communication receivers are also equipped with automatic gain 

 control but this feature is seldom used in R.A.R., the gain being adjusted manually. 

 The tuning control should be provided with band-spread features so that a critical 

 adjustment will not be required in tuning. The dial of the receiver should be cahbrated 

 to show accurately the frequency at which the receiver is tuned. 



The receiver should have terminals for both headphones and loud speaker. It has 

 sometimes been found convenient to use the output voltage at the headphone terminals 

 to operate the chronograph amplifier: therefore, the impedance at the headphone 

 terminals and of the chronograph amplifier input should be identical. The chronograph 

 amplifier input for the circuit in figure 140 is 500 ohms. If the output impedance of 

 the receiver is either one-third or triple that of the clu^onogi-aph input impedance, an 

 intermediate matching transformer should be used. An alternative method is to change 

 either the receiver's output transformer or the input transformer of the chronograph 

 amplifier so that their impedances are properly matched. 



Most of the receivers used on vessels of the Coast and Geodetic Survey are operated 

 on 110-volt 60-cycle current 



