|For those interested in the development of radio and radio circuits here is a short, only mildly technical history of early radio. The first transmitters were only capable of broadcasting Morse code, not voice, so radio receivers could be very simple, and tone quality didn't matter. The radio signal had to be converted to sound and to do that a DETECTOR is needed. Early experimenters tried many types of detectors, but the crystal detector was far and away the most popular prior to WW1. The early crystal set was a very simple tuner hooked up to a crystal detector. Figure 1 is an example of an early crystal set (a 1924 Steinite).|
|Around 1920, the commercial broadcasts, using voice and music, began to become popular. At first the broadcasts were limited to 2 AM frequencies, which meant tuners could be very simple. At this time the public wanted to listen to distant stations, which required more SENSITIVITY from the sets. Vacuum tubes could amplify weak signals, and act as more sensitive detectors, so the use of battery-powered tube radios began to appear more commonly. The early tubes could not amplify very much (they could make the signal 3 to 8 times as loud, compared to 1940's tubes which can amplify 1200 times), so a circuit was developed called the ARMSTRONG REGENERATIVE circuit, which allowed the signal to pass through the same tube several times, building in strength on each pass. Figure 2 shows a one tube regenerative radio (a Montgomery Wards set from 1924). How good were these circuits? Well, they are still used today in items such as garage door opener remotes!|
|Tubes could also be added to increase the signal before it got to the regen circuit, the Radiola III being one example (Figure 3). Still, these radios could only be used with headphones, and many people wanted to use speakers (horns or "drums"), so that the whole family could listen at the same time. This required an AUDIO STAGE AMPLIFIER so that the speaker could get enough power to fill a room with sound. The Radiola III-A shows how the addition of two more tubes allowed a speaker to be driven (Figure 4).|
|In September of 1923 the government opened the AM band to as many stations as could fit, rather than the 2 frequencies allowed before. This meant that tuner circuits had to separate stations closely spaced together, and this required better SELECTIVITY, or the ability to focus in on just one station, without others blending in. One good way to do this was to use several tuners, all of which had to be set to the same frequency to pull in the station. The circuit was called the TUNED RADIO FREQUENCY or TRF. An example of the TRF can be seen in Figure 5, a model 10A Atwater Kent "breadboard" radio. Each stage is tuned by the dial in front of it. The bother and mess of battery power was a problem, and by the late 1920's more and more homes had electricity, so a way to power a radio by plugging it into the wall socket was developed, using tubes that didn't hum when powered with AC. An early example of this is the RCA model 18 (1928), shown in Figure 6.|
|People still wanted more sensitivity, and as stations became more and more crowded, more selectivity, so the modern SUPERHETERODYNE became more popular (the first commercial example was the RCA AR-812 in 1923, but because the cost of eight or more tubes was a problem then, superhets didn't really take off for years). The Atwater Kent model 80, with its more modern cathedral shape, is a superhet (Figure 7). The superhet is still in use in modern radios to this day. To me the most amazing thing is the short period of time between the development of a crystal set that takes a 100 foot outdoor antenna to get one or two stations, to a cathedral radio that can receive dozens of stations with 2 feet of wire for an antenna; quite a testament to the genius of the early radio men.|
Biographical note. The author was born in
1954 and graduated from Eastern Michigan University in 1976.
He writes for Glass Audio, Audio Electronics, and Speaker Builder magazines.
to send Richard e-mail.
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