Radio broadcast .. (1922-30)

364 RADIO BROADCAST RADIO BROADCAST Photograph A top view of the receiver, showing the arrangement of batteries. The two adjustable loops can be seen near the front of the set stick. The coils are then stretched so the turns are spaced about \" apart. The size of the loops should be varied until the tube oscillates. When the tube begins to oscillate, the plate current will generally change. Another test for oscillation is to touch the plate terminal with a screwdriver and watch to see if the plate current changes. If a type 210 tube is being used, with, say, 200 volts on the plate, a good size spark can be seen as the screwdriver touches the plate terminal. The microphone is connected in series with the antenna. See Fig. 4. In the preliminary ex- periments, it is not necessary that a microphone be used. Perhaps the best method would be to supply the plate and filament directly from the a. c. power lines. The circuit diagrams of a transmitter using alternating current for the filament and plate supply is given in Fig. 7. It is the same as the circuit diagram shown in Fig. 4 with the exception that the energy is supplied through two transformers instead of by batteries. With a. c. on the plate, the trans- mitter output can be picked up on an ordinary crystal detector since the 6o-cycle note will be audible in the telephones. On the other hand, with battery supply for the transmitter, the signal could not be picked up unless an oscillating detector were to be used. In making the first tests it is very much easier to work with a crystal detector rather than with a tube. A fine little wavemeter can be made as illus- trated in Figs. 10 and n. This wavemeter consists of a single loop of wire, L, connected in series with a midget variable condenser, C, which should have a maximum capacity of 25 Adjustable Sliding Contact,: i0.00025 mfd. Adjustable Sliding Contact; - A - B RADIO BROADCAST Photograph FIG. 10 The wavemeter. A telephone condenser is bypass located under the panel FIG. 9 The circuit diagram of the receiver. The poten- tiometer, P, is used to control regeneration. €2 is variable and should have a maximum ca- pacity of 0.0002 mfd. Ci may be a o.oooi mfd. fixed condenser. Rg is about 5 megohms. The r.f. choke construction is described in the text micro-microfarads. The telephones in series with a good tested fixed crystal are placed across the variable condenser. A telephone bypass condenser, C2, having a capacity around o.ooi mfd., should be placed across the two terminals of the telephones. The whole set of parts may be placed on a small piece of bakelite measuring not more than 2" x 4". The details concerning the re- ceiver can be obtained from the various diagrams and photographs that are illustrated in this article. See Figs. 8 and 9. The two short- circuiting wires, LI, and L.2, on the inductances, must be adjusted so as to tune to the same wavelength as the transmitter. The potentio- meter controls oscillation. For phone work, the detector should be operated in a non-oscillating con- dition. The extension handles are plainly shown in Fig. 8. They are very FEBRUARY, 1927 essential since the hand capacity is consider- able and it will be difficult to hold the signal if the extensions are not used. For the first experimental work it is hardly necessary that the receiver be housed in a cabinet for any other reason other than the fact that it makes it somewhat more portable. A bread board layout could be put together very much simpler and would be just as good insofar as the experi- ments are concerned. In starting these experiments, the first thing to do would be to make up a transmitter and be sure that it oscillates. Two simple tests for oscillation were given at the beginning of this article. Now set up the Lecher wire and deter- mine the wavelength. The layout of apparatus used in the Laboratory is shown in Fig. I. The author is sliding the hooked wire along the Lecher wire and in this way obtaining a maxi- mum deflection on the meter. When a maximum deflection is obtained, this point is marked on the wire and then the whole apparatus is slid along further until another maximum point is obtained. The distance between these two is measured with a meter stick and this distance, multiplied by two, equals the wavelength. All the ap- paratus should then be left just as it is, and a wavemeter, such as is illustrated in Figs. 10 and n, coupled to the transmitter, and the resonance point obtained. The process of measuring the wavelength by means of Lecher wires is satisfactory but is too troublesome to bother with every time a wavelength determina- tion is to be made. 11 will be better to determine the wavelength on which one desires to work and then to adjust the oscillator to this wavelength with the aid of the Lecher wire. This point can then be determined on the wavemeter and any future calibrations can be made using it rather than the Lecher wire. It is hoped that sufficient interest will be created by this article to cause readers of RADIO BROADCAST to make up apparatus similar to that illustrated in this article. Experiments of all sorts can be made, different types of antennas can be used, and the method of coupling it to the transmitter can be varied. In the trans- mitter shown in Figs. I, 3, 4, and 5, the ante-nna is connected directly to the end of the plate in- ductance loop in which position it seems to give quite good results. Possibly better results will be obtained if a loosely coupled antenna system is used. Also the antenna used in these experi- ments was tuned to the fundamental wavelength and radiation on a harmonic frequency might give better results. RADIO BROADCAST will be pleased to receive any reports of experiments that are done on the outside, and it is expected that these reports, together with the experiments that are to be done by the magazine, will b« reported in an early issue FIG. I I A piece of wire and a midget condenser comprisi the tuned circuit of this wavemeter. Q. ma} have a capacity of o.ooi mfd. Be sure to usi a good crystal