Page:Popular Science Monthly Volume 90.djvu/328

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��Popular Science Monthly

��binding post F. The secondary circuit includes that part of the transformer coil cut in by the "secondary" switch, and runs from binding post H through the detector and stopping-condenser to the ground binding post F. The telephones are connected across the stopping-condenser. A crystal detector and stopping-con- denser suitable for this use were described in the Popular Science Monthly for November, 191 6, in the third article of this series. The proper sort of telephones to use was also discussed in that article. A test-buzzer arrangement for adjustment of the detector was explained in the December article, and should be combined with the outfit of Fig. 7 in order to make the adjust- ment easier and more positive.

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The circuits of Fig. 7, with the apparatus described, will give excellent results in receiving from the commercial and naval spark stations, if used with an antenna of from 150 to 250 ft.

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���These circuits with the apparatus de- scribed give excellent results in receiving

length. For shorter wavelengths than 500 meters the antenna used should be somewhat shorter, and for waves longer than 2500 meters (that on which time signals are sent) it is advantageous to use still longer wires. It is not neces- sary to erect multiple wire aerials for receiving, nor is great height essential. A 200-ft. single wire, of No. 10 gage copper, or even galvanized iron, swung horizontally between two 50-ft. masts or trees, will prove entirely satisfactory for most pur- poses. When it is desired to receive the short waves from amateur stations, which are restricted by law to wavelengths under 200 meters, a wire not much over 70 ft. in length should be used.

In attempting to "pick-up" signals with the outfit of Fig. 7, the best plan is to set the "units" switch on 5, the "coupling"

��switch on about 30^ and then to vary simultaneously the "tens" and the "second- ary" switches. As the turns on the loading coil are increased in number, those of the secondary should also be increased. When signals are heard, the best point of the secondary is selected, and the loading coil and coupling switches adjusted to give the loudest responses. One must of course be careful that his detector is adjusted proper- ly before starting to tune; for this purpose the test-buzzer is a great time-saver.

It will be noted that more turns of the loading coil and of the secondary are needed for long wavelengths than for short ones, and that when the number of "coupling" turns is reduced, the number of turns in the loading coil must be proportionally increased. This is because the loading coil and the coupling turns in series form a primary circuit, whose effective length must be adjusted for the various wavelengths in the manner described last month. It will also be noted that all stations which have the same wavelength will "come in" best with approximately the same settings, and that the wavelength of any station may be estimated roughly by considering the num- ber of turns in the loading and coupling coils which give the strongest signal from it.

A thing which is very important in the operation of this tuner, however, is not likely to be evident from the first tests made upon it. That is the relation between the sharpness of tuning and the number of turns on the coupling part of the trans- former coil. Careful observation will bring out that when the number of turns on the coupling section is reduced, and the loading coil correspondingly in- creased to the tuning point, better selectivi- ty is obtained. Usually there is a best value of the coupling turns for every station or wavelength, and its use requires the proper corresponding settings of the loading coil and secondary switches. Often when there is interference it is best to use still fewer turns of the coupling section, corre- spondingly increasing the loading coil, so as to get sharper tuning in spite of a weakened signal. The judicious selection of values for these three coil sections (primary, coupling and secondary), and the proper balancing of signal strength against sharpness of tuning, is one of the items which is most important in commer- cial radio-telegraphy. Many operators fail to get the most out of their receivers merely because they fail to pick out the best

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