Morse receivers The first radio receivers used a coherer and sounding board, and were only able to receive
continuous wave (CW) transmissions, encoded with
Morse code (
wireless telegraphy). Later
transmission and reception of speech became possible, although Morse code transmission continued in use until the 1990s. All the following sections concern speech-capable radio, or wireless telephony.
Early home-made sets The idea of radio as entertainment took off in 1920, with the opening of the first stations established specifically for broadcast to the public such as
KDKA in Pittsburgh and
WWJ in Detroit. More stations opened in cities across North America in the following years and radio ownership steadily gained in popularity. Radio sets from before 1920 are rarities, and are probably military artifacts. Sets made prior to approximately 1924 were usually made on wooden
breadboards, in small cupboard style cabinets, or sometimes on an open sheet metal chassis. Homemade sets remained a strong sector of radio production until the early 1930s. Until then there were more homemade sets in use than commercial sets. Early sets used any of the following technologies: •
Crystal set •
Crystal set with carbon or mechanical amplifier • Basic
Tuned Radio Frequency (TRF) Sets •
Reaction Sets •
Super-Regenerative Receiver •
Superheterodyne Receiver Crystal sets These basic radios used no
battery, had no amplification and could operate only high-impedance
headphones. They would receive only very strong signals from a local station. They were popular among the less wealthy due to their low build cost and zero run cost. Crystal sets had minimal ability to
separate stations, and where more than one high power station was present, inability to receive one without the other was a common problem. Some crystal set users added a
carbon amplifier or a mechanical turntable amplifier to give enough output to operate a speaker. Some even used a flame amplifier.
Tuned radio frequency sets Tuned radio frequency sets (TRF sets) were the most popular class of early
radio, primarily because the
Radio Corporation of America (RCA) had a monopoly on the
superheterodyne circuit patents and it was more profitable for companies to jump into radio manufacturing TRF sets. These used several
valves (tubes) to provide RF amplification, detection, and audio
amplification. Early TRF sets only operated
headphones, but by the mid-1920s it was more common to use additional amplification to power a
loudspeaker, despite the expense. The sound quality produced from "moving-iron" speakers used on such sets is sometimes described as torturous, although by the late 1920s the Kellogg-Rice dynamic (moving-coil) speaker had begun to find favor due to its superior sound-reproduction ability. Speakers widely used on
TRF sets included: •
Moving iron speaker (horn or cone) • tin can, magnet & wire based speakers •
moving coil speaker TRF sets used no
regeneration, and were merely several stages (typically three) of tuned RF amplifiers in series feeding a detector tube which extracted the audio intelligence from the RF signal. TRF sets, depending on the number of stages they employed, could have poor-to-superb sensitivity (ability of the set to pick up faint signals) and corresponding selectivity (ability to parse adjacent stations from one another). Audio reproduction quality of TRF sets was limited by the available loudspeakers. "High Fidelity" was not to become a radio marketing concept until the mid-1930s and was not realized until the advent of
FM broadcasting.
Reaction sets, also known as regenerative receivers, rely on
positive feedback to achieve adequate
gain. This approach provided high performance with a minimum number of expensive vacuum tubes, but these receivers tended to radiate RF interference in their immediate vicinity. Consequently, there was a significant amount of hostility by neighbors of "regen" set users over maladjusted
radios transmitting squealing noises and blocking on nearby properties. Early TRF sets had typically two or three tuning knobs and tube filament voltage-control rheostats, all of which had to be set correctly to receive a station. Later (late 1920s) TRF sets had ganged tuning (one knob was used to control all stage tuning capacitors simultaneously), AC house current operation, and eliminated the filament voltage adjustments. All of these changes greatly simplified operation and made radio a household appliance that even a small child could operate, instead of the highly skilled hobbyists of the brief preceding generation.
Reaction sets also had the
filament adjustment
rheostats for each
valve, and again settings had to be right to achieve reception.
Superheterodyne receivers In the era of early radio, only
RCA and a select number of competing "prestige" radio manufacturers could afford to build a
superheterodyne receiver (superhet). RCA had exclusive rights to the superheterodyne circuit patents and extracted high licensing fees from other companies who sought to build superhet sets. RCA also vigorously prosecuted patent infringers. This situation helped propel RCA to the forefront of radio manufacturers in the 1920s due to the higher efficiency of the superhet circuit- a situation which lasted until the patents expired in the early 1930s, at which time a flood of low-cost superheterodyne receivers hit the market. Early (RCA-patent-era) superhets were often used with the relatively expensive
moving coil speakers, which offer a quality of sound unavailable from
moving iron speakers. Most post-1932 commercial radios were superhets, and this technology is still in widespread use in radio receivers today, implemented with
transistors or
integrated circuits. The advantages of superhets are: • Excellent sensitivity and selectivity • Ease of designing the set for multiple-band operation, enabling reception of foreign broadcasts ("Shortwave") • High stability • Well controlled
bandwidth • Well shaped RF
passband avoids the uncontrolled tone variations of TRF sets, and gives good
selectivity The disadvantages before about 1932 were: • High patent-licensing costs. • Need for specialized test equipment to perform conversion-stage filter alignments. In general the technical and manufacturing advantages of the superhet ensured that the TRF set became quickly obsolete once the patent restrictions on superhets were eliminated.
Farm radios Prior to the
Rural Electrification Act of 1936, the vast majority of rural farms in America did not have electricity. Many rural areas of the Midwest and South did not receive commercial power until the 1960s. Until that point, special radios were made to run on DC power. The earliest so-called "farm radios" used the "A", "B", and "C" batteries typical of 1920s radio sets; these farm radios were identical to those used in cities. Somewhat later, farm radios were made to be run on from a car or tractor battery, using an
electromechanical vibrator to create a pulsating DC current that could be stepped up through a transformer to create the high voltage needed for the plates of the tubes- exactly as contemporary car radios did. Other farm radios were designed to run on , from a bank of lead-acid storage batteries charged from a gas powered generator or a wind-charger. The 32-volt system could also power other specially made appliances as well as electric lights around the farm. Other farm radios, especially from the late 1930s to the 1950s, reverted to using a large "A-B" dry cell that provided both for the tube plates and for the tube filaments, as did most tube-based portable radios of that era.
Foxhole radios World War 2 created widespread urgent need for radio communication, and
foxhole sets were built by people without access to traditional radio parts. A
foxhole radio is a simple
crystal sets radio receiver cobbled together from whatever parts one could make (which were very few indeed) or scrounged from junked equipment. Such a set typically used salvaged domestic wiring for an antenna, a double-edged safety-razor blade and pencil lead (or bent safety-pin) for a detector, and a tin can, magnet and some wire for an earpiece. Razor blades of the era were chemically coated ("blued") and this coating could function as a diode, in the same way that a galena cat's whisker detector operates.
Wooden consoles The console radio was the center piece of household entertainment in the era of radio. They were big and expensive,beautifully made wood cabinets, often costing hundreds of dollars in the late Depression era 1930s and were often coupled with a
phonograph. Such tendeed to be a major acquisition for a middle-class family, these large console radios were usually placed in the living room. Most very early console radios were tall and narrow, but as the years went on they got shorter and wider in accordance with the Art Deco design precepts which had become popular. Consumer console radios were made by
RCA,
Philco, General Electric, Montgomery Ward (under the Airline brand name), Sears (under the Silvertone brand name),
Westinghouse, Zenith and others. Brands such as
Zenith and Stromberg-Carlson made a few high end high priced models ("Stratosphere") but mainly they produced moderately priced radios. Motorola had radios in many cars by 1940 using the previously mentioned vibrator power supply necessary for tube operation. Some premium makers such as E. H. Scott and Silver-Marshall started around $500–$800 range in the 1930s and 1940s.
Table-top wood-cased radios Table top radios came in many forms: • "Cathedral style", an upright rectangular box with a rounded top • "Tombstone style" are rectangular boxes that are tall and narrow like a tombstone • "Table top" are rectangular, with width being the larger dimension. Table top radios are usually placed in the kitchen, living room, or bedroom, and sometimes used out on the porch.
Bakelite The availability of the first mass-produced plastic
Bakelite allowed designers much more creativity in cabinet styling, and significantly reduced costs. However, Bakelite is a very brittle plastic, and dropping a radio could easily crack or break the case. Bakelite is a brown-black mouldable
thermosetting plastic, and is still used in some products today. In the 1930s some radios were manufactured using
Catalin, which is the phenolic resin component of bakelite, with no organic filler added, but nearly all historic bakelite radios are the standard black-brown bakelite color. Bakelite as used for radio cabinets was traditionally brown, and this color came from the ground walnut shell flour added to the thermosetting phenolic resin as an extender and strengthening agent.
Plastic era The affordability of more modern light coloured
thermoplastics in the 1950s made brighter designs practical. Some of these thermoplastics are slightly translucent.
Early transistor radios The invention of the
transistor made it possible to produce very small portable radios that did not need a warm-up time, and ran on much smaller
batteries. They were convenient, though the prices were initially high and the sound quality of early models was not nearly as good as tube radios. Later models equalled or surpassed tube models in audio quality. Transistors also made it possible to manufacture portable FM radios, which was impractical using tubes.
Transistor radios were available in many sizes from console to table-top to matchbox. Transistors are still used in today's radios, though the
integrated circuit containing a large number of transistors has surpassed the use of singly packed transistors for the majority of radio circuitry. Transistor radios appeared on the market in 1954, but at a high price. By the 1960s, reduced prices and an increase in desire for portability made them very popular. There was something of a marketing war over the number of transistors sets contained, with many models named after this number. Some sets even had non-functional reject transistors soldered to the
circuit board, doing absolutely nothing, so the sales pitch could advertise a higher number of transistors. Vacuum tube radios and early transistor radios were hand assembled. Today radios are designed with the assistance of computers and manufactured with much greater use of machinery. Today's radios are usually uneconomical to repair because mass production and technological improvements in numerous areas have made them so inexpensive to buy, while the cost of human labor and workshop
overheads have increased greatly in comparison. s.
Car radios The earliest car radios appeared not long after commercial radio broadcasts commenced, but were experimental only. They were expensive, required a large
aerial, reception was inconsistent, and they required adjustment in use, which was not very practical. By the early 1930s most car radios, no longer experimental, were
superheterodynes and used a
vibrator power supply to step up the low voltage to high voltage ("B+" voltage of anywhere from ) for the
vacuum tubes. Vibrators are relatively unreliable as electromechanical components of limited life, buzz audibly, and produce radio interference. A few radios used a bulkier and more expensive motor-generator or motor-alternator set called a "
dynamotor" that spun a high-voltage generator or alternator using a 6- or 12-volt DC motor. Filaments were powered using 6- and later 12-volt DC power from the vehicle's electrical system directly. With the introduction of
transistors, the earlier ones suitable for audio frequencies only, car radios were valve sets with a transistor output stage; makers promoted them as transistor sets. Some historic car radios badged as transistorised are in reality of this type. All-transistor sets eventually replaced sets with vacuum tube after transistor technology improved and prices fell significantly. Chrysler and
Philco announced an all-transistor car radio in the April 28, 1955, edition of the Wall Street Journal. This Philco car radio model was the first tubeless auto set in history to be developed and produced. It was a $150 option for 1956 Chrysler and Imperial cars and hit the showroom floor on October 21, 1955. ==Warm-up time==