Recent Radio Inventions
A. F. Jackson
A patent issued during 1915 to C. D. Ainsworth and bearing number 1,145,735 shows an interesting arrangement of three-electrode vacuumtube detector. Fig. I indicates the construction of the device and the circuit, connections. Referring to this drawing, within an evacuated glass bulb I is sealed a support 8 which carries a tubular anode 2 and two electrodes 4 and 6, also in the form of tubes and concentric with the central conductor. The two outer cylinders are made of woven wire, 4 (which may correspond to the grid of an audion) being of somewhat finer mesh than ó. The tube is operated cold, i. e., without a filament heated by auxiliary current, and secures its conductivity through the radio-active material, such as uranium, which is placed near the electrodes at 9. The usual circuits, combining antenna and ground with inductively coupled secondary coil 10 and tuning condenser 11, are used. The central electrode, however, corresponds approximately to the plate in the usual audion arrangement, and is connected to the positive terminal of the battery j through the telephone 12. No series condenser in the circuit of electrode 4 is shown.
The patentee explains the operation of the detector by saying that the rarefied
gas within the tube is made conductive by the radiation from 9, which may be a compound of uranium, thorium, radium or actinium, and that consequently a steady small current tends to pass from to 6 and to 4. The voltage of /3 is adjusted just below that which will “break down the electrical resistance of
the ionized gas” when no signals are being received ; but when currents arc induced in the secondary system from the antenna, a re-distribution of potential takes place and the battery flows, so producing a signal in the telephones. This described operation is therefore closely analogous to that of an autocoherer, in which incoming-wave energy changes the resistance of a conductor and thus alters the amount of current flowing through it from a local battery.
In his early experiments with the audion, Lee deForest is said to have used radio-active compounds in place of the heated filament, but without success be-
cause of the difficulty in securing sufficient conductivity by ionazation. The detector of the Ainsworth patent should prove a useful instrument when developed to a practical operating point, since, as the patentee points out, constancy in operation may be expected and the nuisance and expense of filaments and heating batteries are done away with.
U. S. Patents 1,141,386 and 1,141,453, issued in 1915 to R. A. Fessenden, show not only a simple “heterodyne” receiver operating on the electrical-beats principle now so widely used, in Fig. 2, but also a method for simultaneously sending and receiving with continuous waves, as in Fig3Taking up the first of these, it is seen that the antenna 1 is connected to ground 2 through the primary of the inductive coupler 3. The secondary 4 has in series with it a variable tuning inductance 66, a condenser 5 and one winding of an electro-dynamometer-telephone, 6. The second telephone winding 14 is coupled to a small radio-frequency alternator 77 through a transformer 8, Q. The dynamometer 6, 14, consists of two coils placed end to end, one of which is stationary and may have a fine ironwire core and the other of which is mounted upon a diaphragm 7. In receiving radio signals the antenna and secondary systems are tuned exactly or approximately to the frequency of the incoming waves, so that currents of this
frequency will be induced in coil 6. The alternator, 77, is then run at a radio frequency slightly different from that being received, and its current output led to coil 77. The magnetic fields of these two coils interact one upon the other ; when the currents are relatively in one direction, the fields add and the diaphragm, 7, is attracted, and when the currents are relatively reversed, the fields oppose each other and the diaphragm is repelled. This alternate adding and opposing of fields goes on constantly because of the slight difference in frequency of the two currents, and the diaphragm is moved back and forth at a rate determined by the difference in the frequencies. If the incoming wave is of 6,000 meters length, which corresponds to 50,000 cycles per second frequency, and the local generator produces currents of 50,500 cycles frequency, the number of impulses impressed upon the diaphragm will be 500 per second. This last is called the “beat frequency” of the heterodyne receiver, and is the frequency of the signal tone heard by listening to the telephone diaphragm, 7. No beats or impulses on the diaphragm are produced unless both currents are flowing; there-
fore, although power from alternator 77 is constantly flowing, signals are heard only when waves are received on the antenna from the distant sending station. This dynamometer heterodyne gives a much louder signal than could be obtained from the magnetic effect of the incoming waves applied directly to a suitable air-core or self-excited telephone, since the magnetic force acting on the diaphragm depends upon the product of the two currents in the coils 6 and 14, and that in 14 from the local generator may be made quite large.
Figure 3 shows the duplex heterodyne system. Here the receiver just described has added to it, in series with the antenna, a radio-frequency alternator powerful enough to generate the strong waves used in sending. This transmitting alternator has its field coils, 76, supplied with power through the sending key, 78, and also has connected across its armature terminals a circuit which is
coupled to the receiver coil, 14, by way of transformer 68, 6ç. The condenser, 27, may be inserted to tune the auxiliary circuit. All the other main elements of Fig. 3 are the same as shown in Fig. 2, except that variable condenser 22 may be added to make the diaphragm-coil circuit resonant.
When the sending key, 78, is open the sending generator, 26, does not generate and the system is entirely equivalent to that shown in Fig. 2, since all the receiving portions are operative. When the key is depressed to make a Morse dot or dash, however, the generator field circuit is closed and intense radio-frequency currents are set up in the aerial system. These induce strong currents in the receiver coil, 6, which might have so great an effect on the diaphragm as to make receiving from the distant station impos-
sible. Closing the key, however, connects in the circuit 27, 68, and through the coupling transformer similar, but opposed strong currents are set up in the receiver coil 14. The intensity and phase of these is adjusted so that their magnetic field exactly neutralizes that of the transmitter currents in coil 6, and the diaphragm is therefore left undisturbed and in receiving condition even though the key is pressed down. Thus the aerial is used for sending at the same time it receives.
This duplex system makes possible the transmission of twice the normal amount of traffic between two radio stations, for messages can pass both ways simultaneously. Since the same aerial is used both for sending and receiving, there is no need for erection of separate sending and receiving stations located some distance apart and connected by wire lines, as is done at the Marconi trans-oceanic plants.
The patent specification points out a number of variations of both simple and duplex heterodyne operation ; for instance, the telephone receiver may be mechanically tuned to the beat-note frequency, or the action of the dynamometer may control a microphoniccontact relay (13, Fig. 3) operating an ordinary telephone 12 by varying the current from a local battery 11. It is also suggested that, instead of currents, the voltages set up by the received waves may be used to interact with locally generated radio-frequency voltages, upon an electro-static telephone, to produce heterodyne beats and a musical signal.
An improvement upon the dynamometer heterodyne just described is the subject of 1915 U. S. Patent number 1,141,717, issued to J. W. Lee and J. L. Flogan, Jr. In principle this receiver is identical with the older forms of heterodyne, but instead of adding the effects of the incoming and locally generated currents mechanically upon a dynamometer device, the two are combined electrically. As shown in Fig. 4, a normal receiving outfit is first set up. This may consist of the antenna A, having in series with it to ground P a loading coil B, the primary of the receiving coupling C and the secondary of another oscillation transformer E. The secondary D may have the tuning condenser Q connected across its terminals leading to the detector R and stopping condenser 6’. Across this last named are connected the ordinary telephones T and potentiometer with battery, U. In addition to these usual receiving instruments, a generator of radio frequency current is coupled to the oscillation circuits. This may be, as shown in Fig. 4, an oscillating arc ƒ having the resonant condenser H and inductance G connected serially across it and fed with direct current from O through resistance M and choke coils L, L.
The heterodyne operation of this re-
ceiver may be explained with reference to Fig. 5, which is a series of curves roughly representing the currents in the several circuits. The upper line, Ni, indicates the incoming-wave currents as they would be set up in the antenna and secondary circuits if signals were arriving but the local oscillator were not in operation. The second curve N2 shows the current of slightly different frequency which is generated by the local oscillator itself, as it would be induced in the receiving circuits if no signals were being received. The third curve, Nj, represents ^ the beatcurrent which is produced in the circuits when signals are being received and the local generator is running; this current is seen to change from zero to
maximum strength regularly, according to whether the two interacting currents aid or oppose each other. This varying radio-frequency current has a beat frequency equal to the difference of the two radio frequencies, just as in the simple magnetic heterodyne, and, when rectified by the detector R, produces in the telephone circuit a pulsating direct current corresponding to the heavy curve on axis N4. These pulses of course act on the telephone diaphragm in the well known manner and produce a musical signal tone of the beat-frequency.
This recent type of heterodyne is the forerunner of many receivers used today for continuous wave signals. In some of these the local oscillator is a suitably arranged audion bulb and the detector a second audion. Occasionally amplifiers are added, and a very sensitive receiving system thereby obtained. In some instances the same audion bulb is used as a local generator, and, simultaneously, as the detector and amplifier. The basic method of operation can be traced back, however, to the heterodyne principles explained in the above three patents and outlined herein.
A Multiple Point Switch
THE drawing shows a positive contact, smooth running multi-point switch, having V2-mch diameter switch points on %-inch centers, with the width of contact arm yg-inch. It may be seen from the drawing that all movable contacts are of the self-cleaning knife edge type. An attractive and substantial instrument is the result.
This switch may be used on the high voltage audion battery circuit by leaving each alternate contact point dead, and making connection through the central contact ring. This protects the battery against short circuits.