management. At home, the
Communications Act of 1934 created
the Federal Communications
Commission (FCC) which administers all
telecommunications activity in the US.
No discussion of early radio
would be complete without the inventions of Major Edwin H. Armstrong.
With the possible exception of
Marconi, no other individual had such
a large impact on the development of
radio in so many areas.
Armstrong was responsible for
three major advances in the radio arts
(and many smaller ones). The first was
the regenerative receiver patented in
1914 by Armstrong while a junior in
college. The “regen” was the first electronic circuit to use feedback in order
to increase sensitivity (the ability to
receive weak signals) and selectivity
(the ability to distinguish between
signals). Figure 5 shows the schematic
of a regenerative receiver. By allowing
the receiver to operate at the edge of
oscillation, even extremely weak
Morse signals could be detected.
His patent of 1918 was even more
important. The superheterodyne
receiver converts signals from one frequency
to another by mixing them with another
frequency and then selecting from the
sum and difference frequencies (
heterodynes) that result. The superheterodyne
can receive a wide range of frequencies
by converting them to a single frequency (the intermediate frequency or IF
stage) where high-gain amplifiers and
filters can be employed. The “superhet”
design is the basis of nearly all radios in
use today, nearly 90 years later!
Third was the development of
wide-bandwidth frequency modulation
(FM) transmission and reception in
1935. Armstrong had learned that nearly all static and noise is amplitude-modulated. Attempting to remove
noise from an AM signal would also
destroy the desired signal. His solution
was to change the method of signal
modulation from AM to FM and to
increase the bandwidth of the FM signal for good fidelity. He succeeded
spectacularly as fans of FM radio know.
FM signals are the most common in the
world, with the exception of mobile
phone transmissions. Armstrong also
invented multiplexed FM, the technology used for stereo transmission.
100 Years Down the Tube
So much from something so
empty! The vacuum tube’s story is
also the story of the beginning of the
Industrial Age. Just as steam power
extended our physical abilities, radio
and electronics enabled us to extend
our communications abilities. While
the transistor and IC are the electronic “second stage,” the vacuum tube
was indubitably the rocket that lifted
us off the launching pad. NV
■ Early Broadcast Radio History —
■ 200 Meters and Down by Clinton
B. DeSoto, published by the
ARRL ( www.arrl.org) — A history of
amateur radio until the late 1920s.
Made Radio by Tom Lewis — The
lives and battles of Lee de Forest,
Edwin Armstrong, and David
■ Wikipedia — www.wikipedia.org
— Many excellent entries on radio-related topics.
■ Edwin Armstrong — users.erols.
com/oldradio — Detailed information on Edwin Armstrong’s experiences and his contributions to radio.
■ Jurassic Radio — www.oldradio.
com/archives/jurassic — Detailed
stories on early radio technology.
■ ARRL — www.arrl.org — The
National Association for Amateur
■ Empire of the Air: The Men Who
■ Radio Club of America — www.
radioclubofamerica.org — The
world’s oldest radio organization.
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