In time past the most difficult of all electrical apparatus for the amateur to make, install and work was the wireless telephone. This was because it required a direct current of not less than 500 volts to set up the sustained oscillations and all ordinary direct current for lighting purposes is usually generated at a potential of 110 volts.
Now as you know it is easy to step-up a 110 volt alternating current to any voltage you wish with a power transformer but until within comparatively recent years an alternating current could not be used for the production of sustained oscillations for the very good reason that the state of the art had not advanced that far. In the new order of things these difficulties have all but vanished and while a wireless telephone transmitter still requires a high voltage direct current to operate it this is easily obtained from 110 volt source of alternating current by means of vacuum tube rectifiers.
The pulsating direct currents are then passed through a filtering reactance coil, called a reactor, and one or more condensers, and these smooth them out until they approximate a continuous direct current. The latter is then made to flow through a vacuum tube oscillator when it is converted into high frequency oscillations and these are varied, or modulated, as it is called, by a microphone transmitter such as is used for ordinary wire telephony. The energy of these sustained modulated oscillations is then radiated into space from the aerial in the form of electric waves.
The distance that can be covered with a wireless telephone transmitter is about one-fourth as great as that of a wireless telegraph transmitter having the same input of initial current, but it is long enough to satisfy the most enthusiastic amateur. For instance with a wireless telephone transmitter where an amplifier tube is used to set up the oscillations and which is made for a plate potential of 100 volts, distances up to 10 or 15 miles can be covered.
With a single 5 watt oscillator tube energized by a direct current of 350 volts from either a motor-generator or from a power transformer (after it has been rectified and smoothed out) speech and music can be transmitted to upwards of 25 miles. Where two 5 watt tubes connected in parallel are used wireless telephone messages can be transmitted to distances of 40 or 50 miles. Further, a single 50 watt oscillator tube will send to distances of 50 to 100 miles while two of these tubes in parallel will send from 100 to 200 miles. Finally, where four or five oscillator tubes are connected in parallel proportionately greater distances can be covered.
A Short Distance Wireless Telephone Transmitting Set-With 110 Volt Direct Lighting Current.--For this very simple, short distance wireless telephone transmitting set you need the same apparatus as that described and pictured in the beginning of [Chapter XVI] for a Short Distance C. W. Telegraph Transmitter, except that you use a microphone transmitter instead of a telegraph key. If you have a 110 volt direct lighting current in your home you can put up this short distance set for very little money and it will be well worth your while to do so.
The Apparatus You Need.--For this set you require: (1) one tuning coil as shown at A and B in Fig. 75; (2) one aerial ammeter as shown at C in Fig. 75; (3) one aerial condenser as shown at C in Fig. 75; (4) one grid, blocking and protective condenser as shown at D in Fig. 75; (5) one grid leak as shown at C in Fig. 77; (6) one vacuum tube amplifier which is used as an oscillator; (7) one 6 volt storage battery; (8) one rheostat as shown at I in Fig. 75; (9) one oscillation choke coil; (10) one panel cut-out as shown at K in Fig. 75 and an ordinary microphone transmitter.
The Microphone Transmitter.--The best kind of a microphone to use with this and other telephone transmitting sets is a Western Electric No. 284-W. [Footnote: Made by the Western Electric Company, Chicago, Ill.] This is known as a solid back transmitter and is the standard commercial type used on all long distance Bell telephone lines. It articulates sharply and distinctly and there are no current variations to distort the wave form of the voice and it will not buzz or sizzle. It is shown in Fig. 84 and costs $2.00. Any other good microphone transmitter can be used if desired.
Connecting Up the Apparatus.--Begin by connecting the leading-in wire with one of the terminals of the microphone transmitter, as shown in the wiring diagram Fig. 85, and the other terminal of this to one end of the tuning coil. Now connect clip 1 of the tuning coil to one of the posts of the hot-wire ammeter, the other post of this to one end of aerial condenser and, finally, the other end of the latter with the water pipe or other ground. The microphone can be connected in the ground wire and the ammeter in the aerial wire and the results will be practically the same.