CHAPTER 1
Introduction
It is hard to imagine what the modern world
would look like without the constant exchange of a huge quantity of
information. It is currently disseminated by various means such as
newspapers, telephone and the Internet. However the fastest way, and
sometimes the only way, is by radio. This is where the transfer is by
electromagnetic waves, traveling at the speed of light. In radio
communication, a radio transmitter comprises one side of the link and a
radio receiver on the other. No conductor of any kind is needed between
them, and that's how the expression Wireless Link came into being.
In the early days of radio engineering the terms Wireless Telegraph
and Wireless Telephone were also used, but were quickly replaced with
Radio Communication, or just Radio.

Radio communication is created by means of
electromagnetic waves, of which the existence and features were
theoretically described and predicted by James Maxwell, in 1864.
First experimental proof of this theory was given by Heinrich Hertz in 1888,
ten years after Maxwell's death.
It was already known at that time that electric current exists in
oscillatory circuits made of a capacitor of capacity C and coil of
inductance L. It was Thomson, back in 1853 that determined the
frequency of this arrangement to be:

Hertz
used an oscillatory circuit with a capacitor made of two bowls, K1 and K2
(Pic. 1.1), and the "coil" was made of two straight conductors. The bowls
could be moved along the conductors. In this way the capacitance of the
circuit could be altered, and also its resonance frequency. With every
interruption from the battery, a high voltage was produced at the output of
the inductor, creating a spark between the narrow placed balls k1 and k2.
According to Maxwell's theory, as long as there was a spark, i.e.
alternating current in the circuitry, there was an electromagnetic field
surrounding the conductors, spreading itself through the surrounding space.
A few metres away from this device Hertz placed a bent conductor with metal
balls k3, k4 placed on the ends, positioned very close to each other.
This also was an oscillatory circuit, called the resonator.
According to Maxwell's theory, voltage induced by the electromagnetic waves
should be created in the resonator. Voltage existence would be shown by a
spark between the balls k3 and k4.
And that's the way it was: Whenever there was a spark in the oscillator
between the balls k1 and k2, a spark would also be produced by the
resonator, between balls k3 and k4.
With various forms of the arrangement in Pic. 1.1, Hertz proved that
electromagnetic waves behave as light since they could also be reflected and
refracted.
It was also shown that light is of electromagnetic nature, as stated by
Maxwell.
Hertz, however, did not believe in the practical value of his
electromagnetic waves experiments. The range of the link was no further than
a few meters. The transmitted signal was very weak, therefore the signal in
the receiver had a very small amplitude and it wasn't possible to detect it
at a greater distance. The possibility of amplifying the signal in the
receiver did not exist at the time.
Besides the short range, another shortcoming of the link was noted: If
another similar transmitter was working nearby, a receiver detected all the
signals at the same time. It did not have the ability of isolation.
However crude and simple these experiments were at the time, they
represented the birth of a new scientific branch - Radio Engineering.
The pioneers of radio were Popov and Marconi, but the place of honor belongs
to Nikola Tesla, who demonstrated wireless broadcasting in 1893, at the
Franklin Institute.
Pic.1.2 shows the arrangement of this broadcast system.
Tesla's idea was to produce electromagnetic waves by means of oscillatory
circuits and transmit them over an antenna. A receiver would then receive
the waves with another antenna and oscillatory circuit being in resonance
with the oscillatory circuit of the transmitter. This represented the
groundwork of today's radio communications.
In 1904 John Flemming created the diode, and in 1907 Lee De Forest invented
the triode. That year can be considered the birth of electronics, with the
triode being the first electronic component used in a circuit for signal
amplification.
Rapid development of radio engineering over
the ensuing years produced many innovations and after the First World War a
huge number of radio stations emerged.
At that time TRF (Tuned Radio Frequency) receivers were used. Compared to
modern receivers they had both poor selectivity and sensitivity, but back
then they fulfilled the demands. The number of radio stations was much less
than today and their transmitting power was much smaller. The majority of
listeners were satisfied with the reception of only local stations. However
as the number of stations increased, as well as their transmitting power,
the problem of selecting one station out of the jumble of stations, was
becoming increasingly more difficult.
It was partially solved with an increase in the number of oscillatory
circuits in the receiver and the introduction of positive feedback, but the
true solution was the invention of the superheterodyne receiver. This
was accomplished by Lewy (1917), and improved by E.H. Armstrong (1918).
An enormous impact on the world of radio was the invention of the transistor
by Bardeen, Bretten & Schockley, in 1948. This reduced the size of the radio
receiver and made truly portable sets a reality.
This was followed by the introduction of the integrated circuit, enabling
the construction of devices that not only proved better in every way than
those using values, but also new designs.
Radio amateurs' contribution to radio engineering should also be
emphasized.
In the beginning, radio communication was being conducted in the LW and MW
bands. But achieving long-distance reception required very powerful
transmitters. The SW band was considered to be useless for radio broadcast
on long distances and was given to radio amateurs.
The were banned from using LW and MW bands by commercial radio stations.
However, something unexpected happened: Amateurs were able to accomplish
extremely long distance transmissions (thousands of kilometres), by using
very low-power transmitters. This was later explained by the influence of
the ionosphere layer, the existence of which was also predicted by Tesla.
Modern radio receivers differ greatly from the "classical" types, however
the working principles are the same.
The only significant difference is in the way
the receiver is tuned to a station. Classical devices used a variable
capacitor, coil or varicap diode, with the frequency read from a scale with
movable pointer. In modern devices, the adjustment is done with a frequency
synthesizer controlled by a microprocessor and the reading is displayed on
an optical readout.
The inclusion of a microprocessor enables any
one of a large number of pre-tuned stations to be selected and displayed and
the use of a remote control makes the receiver even more user friendly.
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