Vhf Radio Communication Theory Essay

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.

Radio Waves

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Radio Waves


     In the modern society, radio is the most widely used medium of
broadcasting and electronic communication : it plays a major role in many areas
such as public safety, industrial manufacturing, processing, agriculture,
transportation, entertainment, national defense, space travel, overseas
communication, news reporting and weather forecasting. In radio broadcasts, they
use the radio waves which can be both microwaves and longer radio waves. These
are transmitted in two ways: amplitude modulation (AM ) and frequency modulation
( FM ). These two kinds of wave have many differences.

     Radio waves are among the many types of electromagnetic waves that
travel within the electromagnetic spectrum. Radio waves can be defined by their
frequency (in hertz, after Heinich Hertz , who first produced radio waves
electronically), which is number of times they pass through a complete cycle per
second; or by their wavelength, which is determined by the distance (by meters)
that is traveled from the crest of one wave to the crest of the next.

     Radio frequencies are measured in units called kilohertz, megahertz, and
gigahertz. (1 kilohertz = 1000 hertz : 1 megahertz = 106 hertz, 1 gigahertz =
109 hertz). All radio waves fall within a frequency range of 3 kilohertz, or
3000 cycles per second to 30 gigahertz. Within the range of frequencies, radio
waves are further divided into two groups or bands such as very low frequency (
VLF 10-30 kHz ), low frequency (LF 30-300 KHz), medium frequency ( MF 300-3000
KHz), high frequency ( HF 3-30 MHZ) and very high frequency ( VHF 30-300MHZ).

     Amplitude modulation is the oldest method of transmitting voice and
music through the airwaves is by amplitude modulation. This is accomplished by
combining a sound wave from a microphone, tape, record, or CD with a "carrier"
radio wave. The result : a wave that transmits voice or programming as its
amplitude ( intensity ) increases and decreases. Amplitude modulation is used by
station broadcasting in the AM band and by most international short wave
stations.

     Frequency modulation is another way to convey information, voice , and
music on a radio wave is to slightly change, or modulate, the frequency. The
main advantage of FM broadcasting is of it is static free. But the drawback to
FM is since the frequency is varied, station takes up more room on the band.
Frequency modulation is, of course, used on the FM band. And it is used for
"action band" and ham transmission in the VHF/UHF frequency range.

     In amplitude modulation, what is modified is the amplitude of a carrier
wave on one specific frequency.

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The antenna sends out two kinds of Am waves :
ground waves and sky waves. Ground waves spread out horizontally from the
antenna. They travel through the air along the earth's surface. Sky waves spread
up into the sky . When they reach the layer of atmosphere called the ionosphere,
they may be reflected back to earth . This reflection enables AM radio waves to
be received at great distances from the antenna.

     Frequency modulation station generally reach audiences from 15 to 65
miles ( 24-105km) away. Because of frequency of the carrier wave is modulated,
rather than amplitude, background noise is reduced. In FM transmission, the
frequency of the carrier wave varies according to the strength of the audio
signal or program. Unlike AM , where the strength of the carrier wave varies,
the strength of the carrier wave in FM remains the same , while its frequency
varies above or below a central value broadcast. FM transmission have a
broadcast waves ( 88-108 MHZ) are shorter than AM broadcast waves (540 - 1600
kHz) and do not go as far.

     In AM transmission, the amplitude of the carrier waves varies to match
changes in the electromagnetic waves coming from the radio studio. In FM
transmission, the amplitude of the carrier waves remains constant. However, the
frequency of the waves changes to match the electromagnetic waves sent from the
studio.

     Two types of radio waves are broadcast by AM transmitter : ground waves,
which spread out horizontally from the ground and travel along the earth's
surface, and air waves, which travel up into the ionosphere, allows AM
transmission to travel great distances. AM radio stations with powerful
transmitters can reach listeners as far as 1000 miles ( 1600 km ) away.

     FM radio waves also travel horizontally and skyward. However , due to
the higher frequency of the carrier waves, the waves that go skyward are not
reflected. They pass through the atmosphere and into space. Although AM waves
can be received at greater distances than FM waves , FM. waves do have
advantages. They are not affected by static as much as Am waves. Static is
caused by electricity in the atmosphere. FM waves also result in a truer
reproduction of sound than AM waves.

     Furthermore, FM has much better sound than AM because AM has different
frequency and wavelength than FM. AM stations broadcast on frequencies of
between 535 and 1605 kilohertz. The FM band extends from 88 to 108 megahertz. So
that people can compare two different bands on the radio.



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