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I 10
RADIO BROADCAST
Natural frequency of aerial circuit.
) I ! 1 1 J I 1 . I L_
825 827 829 831 833 835 837 839 841
Frequency of Arriving Radio Wave, in thousands of cycles persecond.
FIG. 4
quencies agree, the less the current which will be produced in the tuned aerial circuit. This is clearly shown in Fig. 4, where frequencies
diflfering by 4000 cycles above and below are seen to produce only about 25 per cent, of the maximum or resonant current value. A graph of the sort reproduced as Fig. 4 is called a resonance curve, since it shows how changes in frequency above and below the resonant value result in a diminution of the current to be indicated. A little study will make clear that the steeper the sides of such a resonance curve, or the sharper its peak, the greater will be the frequency-selecting power of the circuit it represents.
Evidently the adjustable resonant circuit gives us a powerful weapon against the evil of radio interference. By setting our receiver's natural frequency Jo agree with the frequency of the arriving wave to which we desire to listen, we automatically make our instruments less receptive to interfering waves of other frequencies. It is merely essential that we use circuits whose resonance curves are sharply peaked if we are to get the highest degree of resonant selection.
What Receiving Set Shall I Buy?
A Survey of the Current Offerings and When and How They Should be Employed to Receive the Radio-Phone Programmes
By J. CONRAD FLEMMING
THE selection of a radio receiving set should be quite as simple a matter as the selection of an automobile, piano, phonograph, furniture, suit, hat, or whatnot. The only reason why it appears more difficult is because the average layman knows less about radio than he does about other things — for the time being, at least. But if one becomes even superficially acquainted with radio and the requirements of radio reception, the selection of a radio receiving set for any given circumstances becomes just as simple and as positive as the selection of a rug to match the color scheme of a given room.
Behind all receiving sets there is one fundamental factor which affects the selection of the proper type for a given bit of work, and that is the distance between the receiving station and the transmitter which is to be heard.
The radio waves which travel out in all directions from the radio-phone broadcasting station become weaker and weaker as they reach farther and farther away from the transmitter, and in due course they become so weak that they no longer affect even our most sensitive receiving sets, so that to all intents and purposes they are non-existent. But in theory the waves go on for ever, becoming weaker and weaker and still weaker, but never reaching the zero bej^ond infinity.
Now receiving sets are of varying degrees of sensitiveness. Thus an inexpensive outfit must receive a certain amount of radio energy before it will respond and give satisfactory strength of music or speech in the telephone receivers. A more expensive outfit has a better means of using the intercepted wave energy to the utmost extent, so that it will respond satisfactorily with a far weaker wave