CHAPTER XIII
HETERODYNE OR BEAT LONG WAVE TELEGRAPH RECEIVING SET
Any of the receiving sets described in the foregoing chapters will respond to either: (1) a wireless telegraph transmitter that uses a spark gap and which sends out periodic electric waves, or to (2) a wireless telephone transmitter that uses an arc or a vacuum tube oscillator and which sends out continuous electric waves. To receive wireless _telegraph_ signals, however, from a transmitter that uses an arc or a vacuum tube oscillator and which sends out continuous waves, either the transmitter or the receptor must be so constructed that the continuous waves will be broken up into groups of audio frequency and this is done in several different ways.
There are four different ways employed at the present time to break up the continuous waves of a wireless telegraph transmitter into groups and these are: (_a_) the _heterodyne_, or _beat_, method, in which waves of different lengths are impressed on the received waves and so produces beats; (_b_) the _tikker_, or _chopper_ method, in which the high frequency currents are rapidly broken up; (_c_) the variable condenser method, in which the movable plates are made to rapidly rotate; (_d_) the _tone wheel_, or _frequency transformer_, as it is often called, and which is really a modified form of and an improvement on the tikker. The heterodyne method will be described in this chapter.
What the Heterodyne or Beat Method Is.--The word _heterodyne_ was coined from the Greek words _heteros_ which means _other_, or _different_, and _dyne_ which means _power_; in other words it means when used in connection with a wireless receptor that another and different high frequency current is used besides the one that is received from the sending station. In music a _beat_ means a regularly recurrent swelling caused by the reinforcement of a sound and this is set up by the interference of sound waves which have slightly different periods of vibration as, for instance, when two tones take place that are not quite in tune with each other. This, then, is the principle of the heterodyne, or beat, receptor.
In the heterodyne, or beat method, separate sustained oscillations, that are just about as strong as those of the incoming waves, are set up in the receiving circuits and their frequency is just a little higher or a little lower than those that are set up by the waves received from the distant transmitter. The result is that these oscillations of different frequencies interfere and reinforce each other when _beats_ are produced, the period of which is slow enough to be heard in the headphones, hence the incoming signals can be heard only when waves from the sending station are being received. A fuller explanation of how this is done will be found in Chapter XV.
The Autodyne or Self-Heterodyne Long-Wave Receiving Set.--This is the simplest type of heterodyne receptor and it will receive periodic waves from spark telegraph transmitters or continuous waves from an arc or vacuum tube telegraph transmitter. In this type of receptor the detector tube itself is made to set up the _heterodyne oscillations_ which interfere with those that are produced by the incoming waves that are a little out of tune with it.
With a long wave _autodyne_, or _self-heterodyne_ receptor, as this type is called, and a two-step audio-frequency amplifier you can clearly hear many of the cableless stations of Europe and others that send out long waves. For receiving long wave stations, however, you must have a long aerial--a single wire 200 or more feet in length will do--and the higher it is the louder will be the signals. Where it is not possible to put the aerial up a hundred feet or more above the ground, you can use a lower one and still get messages in _International Morse_ fairly strong.
The Parts and Connections of an Autodyne, or Self-Heterodyne, Receiving Set.--For this long wave receiving set you will need: (1) one _variocoupler_ with the primary coil wound on the stator and the secondary coil and tickler coil wound on the rotor, or you can use three honeycomb or other good compact coils of the longest wave you want to receive, a table of which is given in Chapter XII; (2) two _.001 mfd. variable condensers_; (3) one _.0005 mfd. variable condenser_; (4) one _.5 to 2 megohm grid leak resistance_; (5) one _vacuum tube detector_; (6) one _A battery_; (7) one _rheostat_; (8) one _B battery_; (9) one _potentiometer_; (10) one _.001 mfd. fixed condenser_ and (11) one pair of _headphones_. For the two-step amplifier you must, of course, have besides the above parts the amplifier tubes, variable condensers, batteries rheostats, potentiometers and fixed condensers as explained in Chapter IX. The connections for the autodyne, or self-heterodyne, receiving set are shown in Fig. 60.
[Ill.u.s.tration: Fig. 60.--Wiring Diagram of Long Wave Antodyne, or Self-Heterodyne Receptor.]
The Separate Heterodyne Long Wave Receiving Set.--This is a better long wave receptor than the self heterodyne set described above for receiving wireless telegraph signals sent out by a continuous long wave transmitter. The great advantage of using a separate vacuum tube to generate the heterodyne oscillations is that you can make the frequency of the oscillations just what you want it to be and hence you can make it a little higher or a little lower than the oscillations set up by the received waves.
The Parts and Connections of a Separate Heterodyne Long Wave Receiving Set.--The parts required for this long wave receiving set are: (1) four honeycomb or other good _compact inductance_ coils of the longest wave length that you want to receive; (2) three _.001 mfd. variable condensers_; (3) one _.0005 mfd. variable condenser_; (4) one _1 megohm grid leak resistance_; (5) one _vacuum tube detector_; (6) one _A battery_; (7) two rheostats; (8) two _B batteries_, one of which is supplied with taps; (9) one _potentiometer_; (10) one _vacuum tube amplifier_, for setting up the heterodyne oscillations; (11) a pair of _headphones_ and (12) all of the parts for a _two-step amplifier_ as detailed in Chapter IX, that is if you are going to use amplifiers.
The connections are shown in Fig. 61.
[Ill.u.s.tration: Fig. 61.--Wiring Diagram of Long Wave Separate Heterodyne Receiving Set.]
In using either of these heterodyne receivers be sure to carefully adjust the B battery by means of the potentiometer.
[Footnote: The amplifier tube in this case is used as a generator of oscillations.]
CHAPTER XIV
HEADPHONES AND LOUD SPEAKERS
Wireless Headphones.--A telephone receiver for a wireless receiving set is made exactly on the same principle as an ordinary Bell telephone receiver. The only difference between them is that the former is made flat and compact so that a pair of them can be fastened together with a band and worn on the head (when it is called a _headset_), while the latter is long and cylindrical so that it can be held to the ear. A further difference between them is that the wireless headphone is made as sensitive as possible so that it will respond to very feeble currents, while the ordinary telephone receiver is far from being sensitive and will respond only to comparatively large currents.
How a Bell Telephone Receiver Is Made.--An ordinary telephone receiver consists of three chief parts and these are: (1) a hard-rubber, or composition, sh.e.l.l and cap, (2) a permanent steel bar magnet on one end of which is wound a coil of fine insulated copper wire, and (3) a soft iron disk, or _diaphragm_, all of which are shown in the cross-section in Fig. 62. The bar magnet is securely fixed inside of the handle so that the outside end comes to within about 1/32 of an inch of the diaphragm when this is laid on top of the sh.e.l.l and the cap is screwed on.
[Ill.u.s.tration: Fig. 62.--Cross-section of Bell telephone Receiver.]
[Ill.u.s.tration: original Underwood and Underwood. Alexander Graham Bell, Inventor of the Telephone, now an ardent Radio Enthusiast.]
The ends of the coil of wire are connected with two binding posts which are in the end of the sh.e.l.l, but are shown in the picture at the sides for the sake of clearness. This coil usually has a resistance of about 75 ohms and the meaning of the _ohmic resistance_ of a receiver and its bearing on the sensitiveness of it will be explained a little farther along. After the disk, or diaphragm, which is generally made of thin, soft sheet iron that has been tinned or j.a.panned, [Footnote: A disk of photographic tin-type plate is generally used.] is placed over the end of the magnet, the cap, which has a small opening in it, is screwed on and the receiver is ready to use.
How a Wireless Headphone Is Made.--For wireless work a receiver of the watch-case type is used and nearly always two such receivers are connected with a headband. It consists of a permanent bar magnet bent so that it will fit into the sh.e.l.l of the receiver as shown at A in Fig. 63.
[Ill.u.s.tration: Fig. 63.--Wireless Headphone.]
The ends of this magnet, which are called _poles_, are bent up, and hence this type is called a _bipolar_ receiver. The magnets are wound with fine insulated wire as before and the diaphragm is held securely in place over them by s.c.r.e.w.i.n.g on the cap.
About Resistance, Turns of Wire and Sensitivity of Headphones.--If you are a beginner in wireless you will hear those who are experienced speak of a telephone receiver as having a resistance of 75 ohms, 1,000 ohms, 2,000 or 3,000 ohms, as the case may be; from this you will gather that the higher the resistance of the wire on the magnets the more sensitive the receiver is. In a sense this is true, but it is not the resistance of the magnet coils that makes it sensitive, in fact, it cuts down the current, but it is the _number of turns_ of wire on them that determines its sensitiveness; it is easy to see that this is so, for the larger the number of turns the more often will the same current flow round the cores of the magnet and so magnetize them to a greater extent.
But to wind a large number of turns of wire close enough to the cores to be effective the wire must be very small and so, of course, the higher the resistance will be. Now the wire used for winding good receivers is usually No. 40, and this has a diameter of .0031 inch; consequently, when you know the ohmic resistance you get an idea of the number of turns of wire and from this you gather in a general way what the sensitivity of the receiver is.
A receiver that is sensitive enough for wireless work should be wound to not less than 1,000 ohms (this means each ear phone), while those of a better grade are wound to as high as 3,000 ohms for each one. A high-grade headset is shown in Fig. 64. Each phone of a headset should be wound to the same resistance, and these are connected in series as shown. Where two or more headsets are used with one wireless receiving set they must all be of the same resistance and connected in series, that is, the coils of one head set are connected with the coils of the next head set and so on to form a continuous circuit.
[Ill.u.s.tration: Fig. 64.--Wireless Headphone.]
The Impedance of Headphones.--When a current is flowing through a circuit the material of which the wire is made not only opposes its pa.s.sage--this is called its _ohmic resistance_--but a _counter-electromotive force_ to the current is set up due to the inductive effects of the current on itself and this is called _impedance_. Where a wire is wound in a coil the impedance of the circuit is increased and where an alternating current is used the impedance grows greater as the frequency gets higher. The impedance of the magnet coils of a receiver is so great for high frequency oscillations that the latter cannot pa.s.s through them; in other words, they are choked off.
How the Headphones Work.--As you will see from the cross-sections in Figs. 62 and 63 there is no connection, electrical or mechanical, between the diaphragm and the other parts of the receiver. Now when either feeble oscillations, which have been rectified by a detector, or small currents from a B battery, flow through the magnet coils the permanent steel magnet is energized to a greater extent than when no current is flowing through it. This added magnetic energy makes the magnet attract the diaphragm more than it would do by its own force.
If, on the other hand, the current is cut off the pull of the magnet is lessened and as its attraction for the diaphragm is decreased the latter springs back to its original position. When varying currents flow through the coils the diaphragm vibrates accordingly and sends out sound waves.
About Loud Speakers.--The simplest acoustic instrument ever invented is the _megaphone_, which latter is a Greek word meaning _great sound_. It is a very primitive device and our Indians made it out of birch-bark before Columbus discovered America. In its simplest form it consists of a cone-shaped horn and as the speaker talks into the small end the concentrated sound waves pa.s.s out of the large end in whatever direction it is held.
Now a loud speaker of whatever kind consists of two chief parts and these are: (1) a _telephone receiver_, and (2) a _megaphone_, or _horn_ as it is called. A loud speaker when connected with a wireless receiving set makes it possible for a room, or an auditorium, full of people, or an outdoor crowd, to hear what is being sent out by a distant station instead of being limited to a few persons listening-in with headphones. To use a loud speaker you should have a vacuum tube detector receiving set and this must be provided with a one-step amplifier at least.
To get really good results you need a two-step amplifier and then energize the plate of the second vacuum tube amplifier with a 100 volt B battery; or if you have a three-step amplifier then use the high voltage on the plate of the third amplifier tube. Amplifying tubes are made to stand a plate potential of 100 volts and this is the kind you must use. Now it may seem curious, but when the current flows through the coils of the telephone receiver in one direction it gives better results than when it flows through in the other direction; to find out the way the current gives the best results try it out both ways and this you can do by simply reversing the connections.
The Simplest Type of Loud Speaker.--This loud speaker, which is called, the Arkay, [Footnote: Made by the Riley-Klotz Mfg. Co., Newark, N. J.] will work on a one- or two-step amplifier. It consists of a bra.s.s horn with a curve in it and in the bottom there is an adapter, or frame, with a set screw in it so that you can fit in one of your headphones and this is all there is to it. The construction is rigid enough to prevent overtones, or distortion of speech or music.
It is shown in Fig. 65.
[Ill.u.s.tration: Fig. 65.--Arkay Loud Speaker.]
Another Simple Kind of Loud Speaker.--Another loud speaker, see Fig.
66, is known as the _Amplitone_ [Footnote: Made by the American Pattern, Foundry and Machine Co., 82 Church Street, N. Y. C.] and it likewise makes use of the headphones as the sound producer. This device has a cast metal horn which improves the quality of the sound, and all you have to do is to slip the headphones on the inlet tubes of the horn and it is ready for use. The two headphones not only give a longer volume of sound than where a single one is used but there is a certain blended quality which results from one phone smoothing out the imperfections of the other.
[Ill.u.s.tration: Fig. 66.--Amplitone Loud Speaker.]
A Third Kind of Simple Loud Speaker.--The operation of the _Amplitron_, [Footnote: Made by the Radio Service Co., 110 W. 40th Street, N. Y.] as this loud speaker is called, is slightly different from others used for the same purpose. The sounds set up by the headphone are conveyed to the apex of an inverted copper cone which is 7 inches long and 10 inches in diameter. Here it is reflected by a parabolic mirror which greatly amplifies the sounds. The amplification takes place without distortion, the sounds remaining as clear and crisp as when projected by the transmitting station. By removing the cap from the receiver the sh.e.l.l is screwed into a receptacle on the end of the loud speaker and the instrument is ready for use. It is pictured in Fig. 67.
[Ill.u.s.tration: Fig. 67.--Amplitron Loud Speaker.]
A Super Loud Speaker.--This loud speaker, which is known as the _Magnavox Telemegafone_, was the instrument used by Lt. Herbert E.
Metcalf, 3,000 feet in the air, and which startled the City of Washington on April 2, 1919, by repeating President Wilson"s _Victory Loan Message_ from an airplane in flight so that it was distinctly heard by 20,000 people below.
This wonderful achievement was accomplished through the installation of the _Magnavox_ and amplifiers in front of the Treasury Building.
Every word Lt. Metcalf spoke into his wireless telephone transmitter was caught and swelled in volume by the _Telemegafones_ below and persons blocks away could hear the message plainly. Two kinds of these loud speakers are made and these are: (1) a small loud speaker for the use of operators so that headphones need not be worn, and (2) a large loud speaker for auditorium and out-door audiences.
[Ill.u.s.tration: original Underwood and Underwood. World"s Largest Loud Speaker ever made. Installed in Lytle Park, Cincinnati, Ohio, to permit President Harding"s Address at Point Pleasant, Ohio, during the Grant Centenary Celebration to be heard within a radius of one square.]
Either kind may be used with a one- or two-step amplifier or with a cascade of half a dozen amplifiers, according to the degree of loudness desired. The _Telemegafone_ itself is not an amplifier in the true sense inasmuch as it contains no elements which will locally increase the incoming current. It does, however, transform the variable electric currents of the wireless receiving set into sound vibrations in a most wonderful manner.