"Donald E. Kimberlin" <0004133373@mcimail.com> (05/25/91)
In article(v11,iss.392), Jack Winslade <ivgate!Jack.Winslade@uunet. uu.net> provides a reply to a query about the technology used in Marconi's epochal transatlantic radio transmission: > In a recent message, Mike Riddle (mikee@ivgate) writes: >> Historic interlude. If I remember correctly, Marconi's original >> station on Cape Cod was a VLF installation.) Jack replied: > I believe that Marconi's original transmitter would have blanketed the > spectrum from VLF to VHF, so I guess you're correct. ;-) > From what I can remember, Marconi's rig was something like a huge > mechanical spark-gap interrupter driving a tuned circuit (LC tank) > that was supposed to be resonant at a couple of hundred kHz. Well, Jack, you drove me off to some history books, just to see how good your memory is, and it's pretty good. However, by the time of his transatlantic success, he hadn't yet used the rotary spark inter- rupter. The best short description I have at hand is from "A History of Electrical Engineering," by Percy Dunsheath, London, Faber & Faber, 1962. In the chapter, "The Electron in Engineering," Dunsheath pictures Marconi's first transmitter and receiver. The transmitter is no more than a battery interrupted by a telegraph key in the primary circuit of an induction coil that has a spark gap in its secondary with an "aerial plate" and a ground connection to the electrodes of the spark gap. (I find the notion of an "aerial plate" quite interesting, for any number of early histories running into the 1920's consider a radio antenna as one plate of a capacitor with earth forming the other plate. One book shows ever larger and larger "flat-top" antenna structures, such as the one built by Telefunken for its station LPZ near Buenos Aires. It stretched one-half kilometer wide by 2-1/2 kilometers long. That of GFEX at Hillmorton, near Rugby, England covered 900 acres of land, suspended on twelve 820-foot-high towers spaced 1320 feet apart in a figure-8 arrangement. The GFEX antenna was said to have a capacitance of .045 microfarad, with a total resistance on one ohm! Clearly, the builders of these behemoths felt that creating a huge induction field was the way to obtain a strong magnetic field. Power levels of these beasts ran from 200,000 watts to one million watts!) But Dunsheath writes of Marconi: "In 1900 Marconi made a major modification of the transmitting circuit. The spark gap was removed from the main aerial and placed in an oscillatory circuit consisting of a condenser made of several Leyden jars in parallel and the single turn rimary of a transformer, the secondary of which had many turns and was in the aerial-to-earth circuit." The text goes on to indicate this idea of placing a resonant circuit in the transmitting antenna was also applied to Marconi's receiving antenna, and with unspecified power levels at unspecified frequencies, ranges of several hundred miles were reached. Then, Dunsheath describes Marconi's transatlantic experiment thus: "The spark system of Marconi having made such strides, he was encouraged to contemplate bridging the Atlantic and in 1900 the first high-power radio station was constructed at Polhu in Cornwall. In place of the induction coil fed by a voltaic battery an alternator driven by a 25 horsepower oil engine worked through transformers to give a voltage of 20,000. The aerial system consisted of 50 wires supported on masts 200 feet high." (Another book contains a photo showing four apparently wooden towers surrounding a small building at Poldhu.) "Towards the end of 1901 Marconi left for St. John's, Newfoundland, and set up temporary aerials by means of kites and balloons, with which, on 13 December 1901, he received the agreed three-dot signals which were being transmitted from Poldhu. The wavelength used was from 2,000 to 3,000 feet." Later, Dunsheath describes that, "...different forms of interuptor were sought. Rotary contactors, in the form of discs with external spokes passing near to fixed studs, were driven by the alternator shaft and became common practice." He then goes on to describe how Poulsen in Denmark (1903) enclosed large arcs in a hydrogen (isn't that explosive?) bath and a strong magnetic field to produce undamped (continuous) oscillations up to one megahertz. The Poulsen arc converters were built into units up to 100,000 Watts each with as many as ten units in parallel used at a U.S. Navy station near Bordeaux, France producing one megawatt in this way. In the same period, Alexanderson and others were building high-frequency alternators typically of 200 kilowatts each. to produce signals a bit higher in frequency than the arc transmitters of that eraly day. So, it would appear that Marconi first used a (probably rough) form of alternator, and at the indicated wavelengths, its frequency was 328 to 492 kilohertz. (I guess frequency control wasn't too important in 1901, since there was no one else to interfere with.) But with resonant antennae and semi-smooth waves, I would take issue with Jack's surmise that the signal reached up to blue light. I'd say it was only up to about TV channel 7. ;-). But, if he had a 25 horsepower engine and high efficiency, that could produce about 18,000 watts of power. Considering it would be another decade until deForest would produce a triode vacuum tube to amplify weak received signals, getting 3,000 miles on 400 kilohertz ... which is Medium Frequency, by the way ... it took a few years for the art to settle on Low Frequency and VLF ... GFEX ran at 16 kilohertz and 540 kilowatts in its antenna in the 1920's.), Marconi did very well, indeed. Few of us in later generations ever got a medium-wave signal that far on a planned basis!