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The early days of radio were exciting times. Inventors leapfrogged each other with new developments and the art progressed to a science in a very short time. By JOHN HILL Some time ago I wrote a couple of articles on the subject of vintage radio. Both stories delved into the joys and tribulations of collecting and restoring old. valve radio sets. These articles were very well received by readers and it was pleasing to know that there were others doing what I'm doing. It doesn't matter how obscure one's hobbies may be, there are always others interested in the same things. So once again I pick up the pen to write about my restorations. This time my stories will be more detailed, and each article will deal with a specific aspect of collecting and restoring vintage radios. However, before commencing on this series of useful articles, I thought it appropriate to discuss briefly some of the more interesting developments that lead up to the science of radio a§! we know it. today. Many of the things that we now take for granted in this modern world of ours began life in such a crude way that it is quite amazing that they were ever considered useful. Radio had such a beginning. In the early days, spark transmitters and coherers were used solely for Morse code transmission and reception the equipment was incapable of transmitting or receiving speech or music. Early experiments Heinrich Hertz was the originator of the spark transmitter and, in 1888, discovered that signals could be transmitted to a receiver without the use of connecting wires. The equipment that Hertz used was extremely simple and very short ranged; in fact, the 12 SILICON CHIP transmission range was within the space of a single room! How often great discoveries have such humble beginnings. Although Hertz didn't know it at the time, he had discovered radio waves. These invisible waves were soon to be referred to as "Hertzian waves". Hertz' transmitter was simply a large Leyden jar (an early type of capacitor) connected in series with a coil of wire and a spark gap. When the charged capacitor discharged across the gap, radio waves were produced by the coil which acted as a transmission aerial. The receiver consisted of a similar coil connected to a considerably smaller gap. When the crude transmitter sparked, a corresponding spark was reproduced at the receiver gap. Now those early pioneers of radio were no fools. Hertz soon calculated the speed of his Hertzian waves and determined that they travelled at exactly the same speed as light which had already been determined with reasonable accuracy by 1849. The limiting factor of Hertz' equipment was that it would only work over very short distances of a few metres. If these Hertzian waves he had discovered were to be useful at all, it would require considerable improvement to both transmitter and receiver. The first receiver refinement was the development of the "coherer" in 1892. A number of inventors can lay claim to several versions of the device but the Frenchman Edouard Branly developed the type of coherer that Marconi successfully used in his early radio experiments. Compared to modern radio receivers, the coherer is incoming signal would automatically activate the decoherer and the recorder. Marconi Collecting and restoring vintage radio sets is a fascinating hobby. This old American Silvertone is missing most of its veneer but still has its original dial (complete with tuning eye) and all its original knobs. the ultimate in crudeness. It consisted basically of a small glass tube containing two metallic plugs (usually silver) that almost met in the middle, leaving about a one millimetre gap. In this small area was placed a mixture of zinc and silver filings (although Marconi used nickel and silver because it was more sensitive). When the coherer came under the influence of Hertzian waves, the filings in the gap would cohere (stick together, just as iron filings do when under the influence of a magnetic field). Unfortunately, the filings would stay that way after the signal ceased and the device had to be tapped with a pencil or like instrument to settle the filings ready for the next signal. The big coherer breakthrough came about when a Russian by the name of Popoff invented a device for "decohering" the coherer. Popoff's invention employed an electric bell-like mechanism that kept tapping the coherer. The coherer had the unique ability of being a conductor when cohered and a non-conductor when decohered. When properly set up with a battery and relays, it would operate telegraphic recorders such as a Morse sounder or a Morse inkE;ir. The latter instrument put the dots and dashes on a strip of paper. An This early two-valve receiver is the Marconiphone V2A, made around 1923. (Photo courtesy Orpheus Radio Museum, Ballarat). Guglielmo Marconi, an Italian, was the next person of importance on the radio scene and he is often given the credit for inventing radio communications. In actual fact, Marconi invented very little but had the happy knack of improving and adapting other people's inventions for his own benefit. In 1895, Marconi broke new ground with a successful transmission over two kilometres using Branly's coherer and Popoff's decoder. For this he was granted the first English patent for Wireless Telegraphy. Marconi soon improved his equipment and set a new record when messages were transmitted and received between two British warships that were some 20km apart. At the tender age of only 23 years Marconi formed, with the backing of some wealthy English businessmen, The Wireless Telegraph and Signal Company. A vintage Martin radio. The compartment below the control panel housed the receiver's batteries. (Photo courtesy Orpheus Radio Museum). In 1899, Marconi (again using someone else's ideas, in this case, Sir Oliver Lodge) incorporated tuned circuits in .his wireless equipment and patented the idea in 1900. This was a significant step forward as it helped reduce interstation interference. By this stage, the range of Marconi's radio was around 120 kilometres and wireless equipment was mainly used for ship to shore contact. An amateur radio enthusiast during those days could spend the night staring at the coherer of his homemade receiver, hoping to pick up a signal from a passing ship. Imagine the excitement he would experience if the filings in his coherer suddenly stood to attention as a signal was received. If he knew Morse code, he could even decipher the message. American contributions The Americans also contributed well to early radio, although they were a bit slow off the mark. Two of the MARCH 1988 13 Vintage Radio more prominent names on the other side of the Atlantic were Reginald Fessenden and Lee De Forest. Fessendon thought more along the lines of radio telephony as opposed to Marconi's radio telegraphy. To this end he dabbled with high frequency alternators in order to experiment with continuous wave transmissions. He also developed an electrolytic detector which actually rectified the incoming constant wave signals. De Forest also developed an electrolytic detector which caused much conflict between he and Fessendon. The electrolytic detector consisted of a small cup of dilute acid into which a silver wire was dipped. It was similar to the crystal and cat's whisker detector which was yet to be discovered. The electrolytic detector was far more sensitive than the coherer and had the ability to rectify incoming radio-frequency signals. Meanwhile, back across the Atlantic again, Englishman Ambrose Fleming had been experimenting with his two-element valve. He found that its diode characteristic could be used to rectify or detect radio signals. It is interesting to note that Edison had observed the one way characteristic of the diode (the Edison effect) some years previously, but it was Fleming who found a use for it. This Polle Royale is an American 5-tube battery set from the mid-1920s. It employed three tuning controls and a staggered valve arrangement. While De Forest, Fleming and Edison were of the same era and all experimented ,,vith crude diodes, they should not be thought of as inventors of the device. Observation of the Edison effect goes back a long way. In 1725, Duffay discovered that if one or two closely spaced insulated metal spheres was heated, a current carrying path was formed between them. So the thermionic diode goes back two and a half centuries. Edison's experiments centred around removing the dark coating that attached itself to the inside of his incandescent lamps. He figured that a plate inside the glass envelope could prevent that from happening. Strange that a hundred years later the same problem still exists in incandescent bulbs. The Audion The inclined basket coils in this glass covered custommade receiver indicates that it employs a Neutrodyne circuit. (Photo courtesy Orpheus Radio Museum). Fleming's diode was a glass envelope containing a filament (cathode) and a plate (anode). Those· unfamiliar with vacuum tube operation may appreciate an explanation of its function. The glowing filament emits electrons which form an electron cloud that hangs around the general proximity of the filament. However, if a positive charge is connected to the plate by means of a siza]Jle "B" battery, the plate draws the negatively charged electrons to it, hence the one way flow. The negative of the filament ("A") battery and the negative of the plate ("B") battery must be connected for this effect to take place. 14 SILICON CHIP Unfortunately, Fleming's moment of glory was somewhat short lived and insignificant compared to what Lee De Forest was developing at the time. De Forest's "Audion" valve was perhaps the greatest single development in the history of radio. De Forest's brilliant idea was to put a control grid (a spiral of fine wire) between the filament and the plate, thus making a 3-element valve. Varying the 9tate of the charge on the grid controls the electron flow from cathode to anode. When the grid is negatively charged it repells the electrons back towards the filament and, when positively charged, allows most of the electrons to pass through to the plate. . Therefore, slight signal variations on the grid of De Forest's three-element valve produced larger but proportional variations in the plate current. The Audion not only rectified radio frequency signals but it amplified them as well. It was later found that the new valve could also be used in oscillator circuits. De Forest's revolutionary discovery was a great step forward in the development of radio and a whole new era was about to start. This short history of radio will continue next month and will include the period that lead up to commercial broadcasting in the early 1920s. After that, we'll start collecting and restoring vintage radios.