Wireless telephony.

From the book 'Vacuum Tubes in Wireless Communication' by Elmer E. Bucher. Published in 1919.

In general, wireless telephone conversations are transmitted by radio frequency wave motion termed the carrier wave. This carrier wave is modulated at an audio frequency by a microphone transmitter such as employed in land line telephony. Thus, any undamped wave transmitter and any type of oscillation detector giving a quantitative response may be employed provided a magnetic telephone is the current translator. The great problem heretofore in radio telephony has been the difficulty of modulating the large powers employed at the transmitter by the usual telephone microphone which at its best can handle approximately from one-half to one ampere of current. Owing to the inherent imperfections of the microphone and its limited current carrying capacity, a continuously operative high power radio telephone system was not produced until the advent of the vacuum tube. In the earliest radiophone systems, a number of microphones were connected in parallel, and then in series with some part of the antenna system or in special circuits associated inductively or conductively with the antenna system; but owing to the "packing" of the microphone and the difficulty of over-heating, only very small powers could be employed. Various types of high power microphones have been constructed, but they cannot be said to have satisfied the demands of modern engineering, i.e., they were not continuously operative. The vacuum tube seem to offer a practical solution of this problem, for they can be employed to generate radio frequency alternating currents of any desired frequency. The grid potential-plate current characteristic of a properly constructed valve indicates also that a very slight change in the grid potential will cause a relatively large variation of the plate current. Hence, if a vacuum valve be connected up for the production of continuous oscillations and a microphone and battery be connected inductively or conductively to the grid circuit, the grid potential will rise and fall in accordance with the sound modulations of the human voice and the amplitude of the radio frequency carrier wave will be modulated at vocal frequencies.

Vacuum valve connected up for the production of continuous oscillations for radio telephony.

The grid and plate circuits of the vacuum tube are magnetically coupled at L-1 and L-2, both circuits being tuned to a given frequency of oscillation by condensers C-1 and C-2. The antenna circuit A, L, may be coupled to either the plate coil L-2 or the grid coil L-1. An induction coil M-1 with the primary and secondary windings P and S respectively may be connected to the condenser C-1, the primary circuit of the coil including the microphone T and the battery B-3. By proper design of the induction coil, two or three hundred volts may be impressed upon the grid and a very great modulation of the antenna current thus secured.. Valves employed for the generation of radio frequencies at high powers have grid potentials of 150 volts negative, and the plate potentials may attain 2,000 volts or more.

Modified connection of the microphone to the valve generator for radio telephony

.

Another method of connecting the microphone to the valve generator.

August Hund’s proposed method for modulating radio frequency currents by a microphone. In this system, the antenna oscillations are modulated at vocal frequency by a three-electrode vacuum tube. The radio frequency currents for the carrier wave are generated by the vacuum tube V, the grid and plate circuits being coupled together as usual for the generating of radio frequency currents. By coupling L-2 to L-1, currents of similar frequency are induced in the antenna circuit. The antenna system further includes the coil L-3 which may have from 6 to 15 micro henries inductance. The terminals of L-3 are connected to the plates P-1 and P-2 of the three-electrode bulb V-1. The potential of the grid to filament is varied at speech frequency by the microphone T through the iron-core induction coil P-1, S-1. The potential of the grid in respect to the filament can be maintained at the most satisfactory value by battery B-3 and potentiometer P. The grid normally is held at a fairly high negative potential so that no currents leak around L-3 through the valve V-1; but when the microphone T is spoken into, the valve becomes conductive at vocal frequencies; one-half cycle of the carrier wave leaks through the conduction path from P-1 to F-1, and similarly the other half cycle through the conduction path from P-2 to F-1. Energy is thus withdrawn from successive cycles of the carrier wave in accordance with the vocal wave impressed upon the grid by the microphone T and the transformer P- S1.

Englund’s vacuum tube system for wireless telephony. In this system the radio frequency currents generated by a small three-electrode tube V-1 are amplified and modulated by a microphone coupled to the input circuit of a tube V-2. The output is coupled to L-6 through L-5. The circuit of L-6 in turn contains a bridge consisting of resistances R-1, R-2, inductance L-7, and condenser C-7. Tapped across this bridge is the inductance L-8 coupled to L-9, the terminals of which in turn being further strengthened by the amplifiers V-3. By properly balancing the bridge, no current flow through L-8 at the carrier frequency F, but currents of a frequency differing from that of the carrier frequency destroy the balance according to the frequencies of the vocal currents generated by the human voice. The output circuits of V-3 are in turn coupled to a battery of power bulbs V-4 whose output circuit is coupled to the antenna circuit A, L-13, E. In a circuit of this kind an initial radio frequency current of a few watts may set up current of similar form of many kilowatts in the antenna circuit. A feature of this system is the elimination of the carrier frequency. The antenna does not radiate except when the microphone transmitter is actuated.

.

Receiving apparatus of Englund’s system. The carrier frequency is supplied by a local generator A-1.

Another system has been disclosed by John Carson in which the antenna does not radiate except when the microphone is spoken into. It is to be noted in this diagram that the field current of a radio frequency alternator A is modulated at vocal frequencies by a microphone T, the currents of which are amplified by a vacuum tube V. The field windings F of the alternator are thus excited at speech frequency and the amplitude of the radio frequency current generated by A varied accordingly.  

Index

Next page