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Page:Popular Science Monthly Volume 90.djvu/171

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Popular Science Monthly

��155

��Receiving Various Wavelengths

With the senders and receivers limited in their activities to communication be- tween a single pair of stations, it is usually not necessary to provide for variation in the tuned wavelengths of either. That is

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why the simple arrangement of Fig. 2 In the December article could be used. When it is desired to receive from a large number of outside transmitters, all using different wavelengths, it is necessary to provide apparatus whereby the effective length of the aerial at the receiver may be varied to suit the incoming wavelength. A large number of arrangements may be used for this purpose. Some of them tune the receiver very sharply, or in other words make it respond energetically to a very closely restricted range of wavelengths for each adjustment. Other sets of connec- tions are less critical in adjustment, but easier to handle.

The simplest variable tuning instrument for use at the receiver is the so-called "single-slide tuner." This is merely an inductance coil with a sliding contact whereby the number of turns in circuit may be varied at will. It may be used in place of the tuning coil shown in Fig. 3, and will allow some latitude of adjustment, though the tuning is very broad and unsatisfactory. A better mode of connection for the single- slide tuner is that of Fig. 4, in which the detector is put in a side or by-pass circuit; this gives sharper tuning and fairly strong signals.

A still better tuning arrangement fuses the "double-slide tuner," which has two variable contacts. In the catalogs of manufacturers of radio apparatus there are to be found a large number of diagrams

��showing different ways to connect the double-slide tuner; but the best possible results are to be secured from the arrange- ment of Fig. 5. One end of the coil is connected to ground, and one of the sliders to the antenna. The larger the amount of coil between the grounded end and this first slider, the longer the effective length of the aerial and the greater the wavelength for which it is tuned. The grounded end is also connected to one side of the blocking condenser described in the November article, and the other slider is connected with one terminal of the cr>-stal-detector also illustrated in November. The tele- phone has one lead connected with ground, and the other joins the open sides of the detector and blocking-condenser. The test-buzzer, which is not shown in Fig. 5, is to be arranged as explained in the December article, so that the cr>stal-detector can be adjusted to its maximum sensitiveness without waiting for signals from outside stations.

Operating the Variable Receiver

In working the apparatus set up as in Fig. 5, the first step is to make sure that the detector is adjusted to a sensitive point, and that the connections are all secure and in good condition. Then the slider connect- ed to the detector is set at a position about half-way along the coil, and the antenna- slider is moved back and forth slowly along the length of the entire tuner. When a station within range starts to send, his

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��Connecting the detector around the loading- coil produces a highly selective arrangement

signals will be heard in the telephones; it will be noted that the dots and dashes are loudest with the antenna-slider at some particular setting. Leaving the antenna- slider at this point, the detector-slider is moved back and forth until the position giving the best signals is found. This is the tuned or approximately tuned adjustment of the receiver for the specific wavelength being received.

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