Long Wave Transmitter

Alexanderson’s main task within RCA was responsibility for design and installation of the alternators which RCA sold worldwide. He also managed several important business negotiations with foreign customers. What made Alexanderson’s 200 kW transmitter such a success, that anyone dared to launch a company as big as RCA with only this product as the object for sale?

The reply is that he succeeded in constructing a complete system for transmitting and receiving based on the design of the alternator and the multiple antenna. On the receiving side his so-called “barrage receiver” prevented interference between incoming and outgoing signals at the same station, and furthermore contributed to a reduction of atmospheric and other disturbances.

Alexanderson’s transmitter was constructed of four interacting elements.

1. An AC generator producing the power needed at Grimeton radio frequencies.
2. A control system holding the speed constant within very small devition.
3. A modulation system controlling the power of the generator.
4. A multiple antenna (multiple tuned antenna) giving a signal strength 5 to 6 times higher compared with the conventional horizontal antenna in use at that time.

With this transmitting system one had, as Alexanderson expressed it, “… proved during wartime that communication over the seas has become entirely reliable every day during the year and practically each hour during the day and night.”

Here follows some data of the 200 kW alternator. It normally consisted of an asynchronous motor of 600 horse power as a driving force, linked to a gear to increase the speed of the generator.

The units were built for the wavelengths 10,500 to 24,000 meters (28.57-12.5 kHz). This was achieved by choosing various pole numbers, 1220, 976 or 772. The gears between the motor and generator were delivered in three versions 1:2.675, 1:2.973 and 1:3.324. The number of revolutions which could be delivered by the motor were 864 down to 720 r.p.m. Transmitters installed in Europe with mains frequency 50 Hz had a wavelength span from 12,500 to 28,800 meters dictated by the lower speed of the motor. The units weighed 50 tons and were always delivered in pairs, together occupying an area of 13 x 3.5 meters.

The rotor of the machine is shaped as an enormous discus-like disk of steel. In the periphery of the disk are slots filled with non-magnetic material in order to reduce the air resistance. The disk is rotated in a strong magnetic field. On both sides of the air gap there is an iron core with windings for generating the radio frequency current. When the steel disk rotates, a continually-alternating magnetic flux is created by the change of magnetic reluctance between the magnetic “poles”(the steel) and the non-magnetic “poles” (the slots) in the disk. In this way, the radio frequency voltage, increased to 2,000 V by a transformer, is fed to the antenna. The diameter of the disk is 1.6 m and at the periphery, its thickness is 7.5 cm. In order to achieve a frequency of 28.5 kHz, the rotor speed must reach 2,538 r.p.m. At this number of rotations, the peripheral speed of the heavy disk is approximately 800 km per hour (500 miles/hour).

The wavelength of the Alexanderson transmitter was entirely dependent on the alternator’s rotation speed. The antenna system was moreover so sensitive to changes in the alternator frequency, that a change of a quarter of a percent reduced the antenna current by 50 percent. Therefore, the speed regulator became an important part of the operation. A system for speed control was created, with the result that a motor with a speed of 900 rotations per minute only deviated by a single revolution between idle running and full load.

The high-frequency carrier wave from the alternator was modulated using a separate circuit, in which another design by Alexanderson was implemented, the so called “magnetic amplifier.” This can be compared with an oil-cooled transformer and is made in such a way that the permeability of the core of thin sheets of iron can be varied through magnetic saturation. A current of a few amperes in the control circuit can control hundreds of amperes in the antenna. When the transmitter was used for telegraphy, the magnetic amplifier was controlled by telegraph relays included in the ordinary wire telegraph net. During the war, when the Navy managed the New Brunswick station, the telegraph key was placed in the central operation room at the Naval Communication Department in Washington. When the station was used for radio telephony, the control current was a magnified telephone current.

The multiple antenna was developed by Alexanderson at the New Brunswick station. In the beginning, there was a horizontal antenna erected by the Marconi company, approximately 1,600 meters long and having a resistance of 3.7 ohms. Alexanderson succeeded in reducing the total resistance in the antenna by connecting the antenna at six points, with equal distance along the total length of the antenna, to ground through large inductances suitable for the actual frequency. This way Alexander-son succeeded in reducing the total ground resistance to 0.5 ohm.

In order to maintain an antenna current of 600 A at a resistance of 0.5 ohm in the multiple antenna, the power loss in the ground return circuit was in accordance with the formula (I2 x R) = 6002 x 0.5 = 180 kW, while the horizontal antenna with its resistance of 3.7 ohms required 6002 x 3.7= 1332 kW. This was an entirely new trick to reduce the antenna resistance and many were skeptical of the correctness of Alexanderson’s theories, but had to accept these experiments showing compliance between theory and practice . On the first of March 1920, RCA took over all the American Navy’s big stations on the east and west coast. It was planned to provide all the main stations with Alexanderson’s alternators and to establish a central station for wireless traffic, covering the whole world. The work was started in July of 1920 at Rocky Point on Long Island and in November of 1921, the first station was formally inaugurated, then having two complete transmitters and two antennas in operation.