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- The Radio Amateur's Hand Book - 20/44 -
Germany and even far off Russia and Japan.
A long wave set for receiving these stations must be able to tune to wave lengths up to 20,000 meters. Differing from the way in which the regenerative action of the short wave sets described in the preceding chapter is secured and which depends on a tickler coil and the coupling action of the detector in this long wave set, [Footnote: All of the short wave and intermediate wave receivers described, are connected up according to the wiring diagram used by the A. H. Grebe Company, Richmond Hill, Long Island, N. Y.] this action is obtained by the use of a tickler coil in the plate circuit which is inductively coupled to the grid circuit and this feeds back the necessary amount of current. This is a very good way to connect up the circuits for the reason that: (1) the wiring is simplified, and (2) it gives a single variable adjustment for the entire range of wave lengths the receptor is intended to cover.
The Parts and How to Connect Them Up.--The two chief features as far as the parts are concerned of this long wave length receiving set are (1) the _variable condensers_, and (2) the _tuning inductance coils_. The variable condenser used in series with the aerial wire system has 26 plates and is equal to a capacitance of _.0008 mfd._ which is the normal aerial capacitance. The condenser used in the secondary coil circuit has 14 plates and this is equal to a capacitance of _.0004 mfd_.
There are a number of inductance coils and these are arranged so that they can be connected in or cut out and combinations are thus formed which give a high efficiency and yet allow them to be compactly mounted. The inductance coils of the aerial wire system and those of the secondary coil circuit are practically alike. For wave lengths up to 2,200 meters _bank litz-wound coils_ are used and these are wound up in 2, 4 and 6 banks in order to give the proper degree of coupling and inductance values.
Where wave lengths of more than 2,200 meters are to be received _coto-coils_ are used as these are the "last word" in inductance coil design, and are especially adapted for medium as well as long wave lengths. [Footnote: Can be had of the Coto Coil Co., Providence, R. I.] These various coils are cut in and out by means of two five-point switches which are provided with auxiliary levers and contactors for _dead-ending_ the right amount of the coils. In cutting in coils for increased wave lengths, that is from 10,000 to 20,000 meters, all of the coils of the aerial are connected in series as well as all of the coils of the secondary circuit. The connections for a long wave receptor are shown in the wiring diagram in Fig. 59.
[Illustration: Fig. 59.--Wiring Diagram Showing Long Wave Receptor with Variocouplers and Bank-wound Inductance Coils]
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.
[Illustration: 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.
[Illustration: 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.]
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, shell 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
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