THE HISTORY OF WIRELESS TELEGRAPH
The Wire Telegraph era began in the mid 19th century with the experiments of Samuel Morse and the substantial assistance of Alfred Vail.
The practical experiments of Marconi and others to transmit telegraph signals without wires were conducted during 1895-1900. This was the beginning of the "Wireless Telegraph Era". The basic Spark-Gap Transmitter at that time, consisted of a Telegraph Key, battery, electromagnetic vibrator, high voltage induction coil, spark-gap, tuning coil and Leyden jars (capacitor).
The battery voltage is connected to the primary of the high voltage induction coil through the electromagnetic vibrator contacts. The high voltage at the secondary of the induction coil, is connected to the contacts of the spark-gap and to the resonant and antenna coupling circuit, that consists of a tapped coil and capacitor (Leyden jars).
The spark-gap transmitter generates waveforms with the basic frequency of the vibrator and pulses of high frequency that is determined by the resonance frequency of the tuning coil and capacitor.
SPARK TRANSMITTER DIAGRAM
The wavelengths (or frequencies) that have been used for wireless telegraph, were in the range of 6,000 meters (50 KHz) to 200 meters (1.5 MHz), according to the following table:
High power land (Up to 100 KW): 6,000 - 1,500 m (50 KHz – 200 KHz)
Medium power land (Up to 20 KW): 1,500 – 900 m (200 KHz – 333 KHz)
Maritime ship to shore (Up to 10 KW): 800 – 450 m (375 KHz – 666 KHz)
Aviation (up to 500 W): 600 – 200 m (500 KHz – 1,500 KHz)
Wavelengths (or frequencies) shorter than 200 meters (higher than 1.5 MHz), were considered in those days as not efficient and impractical for long range communication. They were allocated for experimental stations and wireless hobbyists, who later on became the first RADIO AMATEURS.
The Danish engineer Vlademar Poulsen designed an Arc Converter in 1903 to generate Continuous Wave High Frequency for wireless telegraph transmission. The electric arc operated with carbon electrodes. A series resonant circuit was connected across the carbon arc electrodes. The Poulsen Arc Transmitters have been used for wireless telegraph at low frequencies up to tens of KiloHertz. They have been used in shore stations with power output of up to 70 kiloWatts.
There was a problem in keying big power Arc Transmitters with a morse key, due to the time required to get a stable arc, when turning on the voltage to the carbon electrodes. The problem was solved by using Frequency Shift Keying method. The arc operated continuously and the transmitting frequency that was determined by the resonance circuit, was changed by shorting some turns of the inductor coil by the Morse Key.
Poulsen Arc Transmitters replaced the Rotary Spark Gap transmitters, because they generated a pure Continuous Wave (CW), in contast to the wide spectrum waves of the Spark-Gap Transmitters.
HF ALTERNATOR TRANSMITTERS
A Swedish born engineer Ernst Alexanderson, developed High Frequency Alternating Current Generator (Alternator), during his work at GE USA. It was intended to replace the Wireless Spark and Arc Transmitters. In 1904, a contract with GE was made for building 50 KW HF alternators for operation at 100 KHz. The HF alternator transmitters of Alexanderson were in use at Wireless Telegraph shore and Trans Atlantic stations. They were too large and heavy for installation on ships.
The transmitting frequency of the HF Alternator was determined by the RPM of the motor and the number of magnetic slots on the perimeter of the ROTOR DISC. The waveform was a pure sine wave. There was a disadvantage, due to the difficulty in changiung the transmitting frequency. Alexanderson HF Alternator transmitters dominated the long range and shore wireless telegraph stations from 1910 to 1920. From 1920, Vacuum Tube transmitters with the tube oscillator have been used in all the new wireless systems.
The reception of wireless telegraph signals began with the experiments of Marconi and others, using an electromagnetic detector and a relay connected to a Telegraph Register or a Sounder. In 1894, the British Oliver Lodge developed the "COHERER" that used Iron granules between two electrodes. Both types of detectors were problematic and not sensitive enough. The experiments with GALENA CRYSTALS resulted much better performance, despite the requirement to readjust the "Cat's Whisker" contact. The Galena Crystal Detector enabled the telegraph operator to hear the telegraph signals on high impedance magnetic headphones.
The efforts to improve the reception with crystal detector receivers, were focused on the quality of the resonant circuits, coils and antenna coupling, in order to get maximum selectivity and sensitivity.
Crystal receiver (homemade 1919) with loose coupler transformer, variable capacitor and Galena detector
MARCONI Multliple Tuner Model 103 (1907)
The Wireless Telegraph caused a dramatic change in communication with ships. Until the Wireless Telegraph Era, the communication with sailing ships was limited to the line of sight range, using keyed light projectors. Navy and commercial ships equipped with Wireless Telegraph, could contact shore stations and nearby ships, in case of distress. The case of the TITANIC is well known as an example of the role that the Wireless Room played in saving so many lives.
In rural locations where Wire Telegraph was not possible, the Wireless Telegraph was an economic solution. The development of military and civil aviation, required better communication and airborne spark-gap transmitters were installed on aircraft.
THE TITANIC WIRELESS ROOM
"The Latest Signal" Film about the role of the Titanic Wireless Operatots
WW1 STERLING SPARK TRANSMITTER used by aircraft for 'spotting' the fall of artillery shells. The operator could tell the gunners if they were on target.
The Spark-Gap wireless transmitters were replaced by the new Vacuum Tube Transmitters, which include Oscillator and Radio Frequency (RF) amplifiers. The Morse Code continued to be used with the transmitted pure sine wave signals. It was necessary to add a Beat Frequency Oscillator (BFO) in the receiver to hear the Morse Code. The BFO frequency emits a carrier wave at a frequency close to the Intermediate Frequency (I.F.). The Radio Operator hears a low frequency tone which is the difference between the I.F. and the BFO frequencies. This mode of operation is called Continuous Waves (CW).
The CW mode was used during the 20th century for commercial, government, maritime and military radio communication. At the beginning of the 21th century the CW mode became almost obsolete, but is still in use by Radio Amateurs.
BFO Circuit in communication receiver
The terms RADIO or BROADCAST were not in use at the time of the Wireless Telegraph, because it was used only for telegraph communications. Transmission of wireless audio signals was achieved with the development of the Vacuum Tube and the Triode. The Radio Broadcast Era started in 1920 with the first audio wireless stations, that could transmit music and news.
In 1956, I served as a Radio Officer (Sparky) on a merchant ship, which was on the line of Tel-Aviv – Odessa. The route passed along the coasts of Turkey and the Bosforus Straits. To my surprise, I heard a Morse Code communication between a Turkish vessel and a Turkish shore station. The transmission of the Turkish vessel was from a Spark Transmitter on the 500 KHz band. To my best knowledge, the ITU ordered to stop the transmission of Spark Transmitters in 1935.
I traveled on this route many times and every time that a splatter of Morse Code was heard on the 500 KHz band, I understood that the ancient Turkish vessel was still around.
A Personal Comment Regarding Spark Transmitters in the 20th Century:
The Radio Room of the cargo ship S/S Tel-Aviv - 4XVN
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