• There were some methods for communicationslike communication via smoke , mors alphabet,walkie talkie etc.• All of them has some receivers, transmitters , andmore important than all of them , has somesignals with some frequencies.
• Let’s compare the communication via smokewith walkie talkie.• In the smoke communication , telling a situation to theother people depends on the time interval betweensmokes. Different intervals for different situations. Thismeans there are some frequencies for some situations andif there are frequencies there is a signal . This signal iscalled the smoke signal.
• In the mors alphabet, walkie talkie or the othermodern communication systems , there areelectrical signals with some frequencies.• Shortly , the most important similarity is thesignals and frequencies.
• Walkie Talkies were first patentedin 1938 by Al Gross. They were firstused by the military only most likelybecause they were so expensive andvery large and heavy for the averageperson to buy and use.
• In fact, someone needed to carry part of the walkie talkie on theirback because it was so large. It is not so clear who should getcredit for the invention of the device.• Around the same time Al Gross was working on his model,Donald Hings was working on his version of the walkie talkiewhich he called a “packset”. Hings invention was used duringWar World Two starting in 1942 and was very important in thewar effort.
• After World War Two the use of the hand held radiospread throughout different public sectors. With morecompact designs police and fireman began to rely more onthe devices for communication. Later walkie talkie usemoved from public to private sectors and to everyday usefor the average person or even toys for children.
• The walkie talkie communication system consists of tworeceiver and two transmitter circuits. One receiver and onetransmitter circuits are for each walkie talkies. The generalstructure of this system :
amplifier modulatoramplifierTransducer(Microphone)Transducer(Loudspeaker)demodulatorAir medium ChannelRx antennaTx antennaBlock diagram of radio communication systemSource ofinformationUser ofinformation
• A transducer is a device that converts a signalin one form of energy to another form of energy.Microphone and the loudspeaker are thetransducers.• After the sound is converted to the electricalsignal, amplifier amplifies it in the transmittercircuit. Modulation is the carrying the messagesignal with the carrier signal . Modulator makesthis duty.• In the receiver circuit , demodulator seperates themessage signal from modulated signal , andamplifier increases the message signal to beheard easily. Loudspeaker converts the electricalsignal to the sound.• Transducer and the receiver must be in the samefrequency interval to communicate each other.
Types of CommunicationA BSimplex – A can talk to BRadio, T.V. broadcasting, CD/DVD ROMSimplest type, requires one transmitter and one receiverSimplexDuplex – A and B both can talk to each other simultaneouslyTelephone, TelegraphComplex, requires two transmitter and two receiver at both endsNeeds two different channels for simultaneous transmissionA BDuplex
• A transceiver is a small unit that combines a transmitter and areceiver• A small hand-held unit of transceiver is popularly called awalkie-talkie• The usual controls on the small transceivers unit are off-onswitch with volume control, push-to-talk button, squelch control(eliminates background noise) and jack for earphonesA BHalf-DuplexHalf-Duplex – A and B can both talk to each other but not simultaneouslyFax, CD/DVD RWNeeds one single channel for transmissionCompromise between two, don’t require separate transmitter and receiverSame antenna and circuitry may be used for both transmission and reception
WHAT IS THE MODULATION ?• Modulation is the process of superimposing asignal (message signal ) on a high frequencysignal (carrier signal ) to transmit easily.s(t)The wave youwant to transmitc(t)A wave that can betransmitted+Signal Carrier r(t)The actual wavethat is transmitted=Modulated signalt t t
DEMODULATION• Demodulation is the opposite of the modulation.Demodulation is used in receiver. When themodulated signal is come into the receiver , thedemodulator seperate the message signal from themodulated signal.r(t)The received signal atdemodulatorModulated signal s(t)Output of thedemodulatorOriginal Signalt tDemodulation
AM vs FM• In amplitude modulation (AM) , the message signal m(t)is impressed on the amplitude of the carrier signal c(t).Modulation is done with changing the amplitude of carriersignal.• In frequency modulation (FM) , the frequency of thecarrier is changed according to the variations in themessage signal.
• QUESTION IS WHAT TYPE OFMODULATION SHOULD BE USED TOTRANSMIT THE SIGNAL THROUGH THELONG DISTANCES ???• IN ORDER TO ANSWER THISQUESTION , THE ADVANTAGES FMAND AM MUST BE EXAMINED
ADVANTAGES OF FM• Resilient to the noise : AM waves do not have constantenvelopes and therefore more affected by static or noisethan FM. Unwanted electromagnetic waves do not cause thefrequency of FM carrier wave to change.• Resilient to signal strength variations• Does not require linear amplifiers in the transmitter• Enables greater efficiency than many other modes
ADVANTAGES OF AM It is simple to implement it can be demodulated using a circuit consisting of very fewcomponents AM receivers are very cheap as no specialized componentsare needed..
SHORTLY ;• Frequency modulated signal has wider coveragethan AM radio• Frequency modulated signal has better soundquality than AM radioThat’s why FM is more preferred than theAM in walkie talkies.
Carrier Frequency bandsName Freq.RangeWavelengthApplication PropagationELF 300Hzto 3kHz100 kmto 1000kmNavigation, long distancecommunication withshipsWave tube between earth surface andthe ionosphereVLF 3kHzto 30kHz10kmto 100kmNavigation, long distancecommunicationGround propagation, stableLF 30kHzto300kHz1kmto 10kmNavigation, long distancecommunication withshipsGround propagation, stableMF 300kHzto 3MHz100mto 1kmAM broadcasting, radionavigationGround-wave, sky-wave propagation.FadingHF 3MHzto30MHz10mto 100mRadio broadcasting,fixed point-to-point(around the world)Large perturbation, reflection inionosphereVHF 30MHzto300MHz1mto 10mRadio and TVbroadcasting, mobileservicesDiffractionUHF 300MHzto 3GHz10cmto 100cmCellular telephony (GSM,NMT, AMPS), digital TV,fixed point-to-point,satellite, radarShadowing by mountains and buildingsSHF 3GHzTo30GHz1cmto 10cmBroadband indoorsystems, microwavelinks, satellitecommunicationsAttenuation due to rain, snow and fogEHF 30GHzto300GHz1mmto 10mmLOS communication(short distance orsatellite)Attenuation due to rain, snow and gases
• The carrier waves frequencies for radio broadcasting are assigned byFederal Communications Commission (FCC)FREQUENCY ALLOCATION• Use of radio frequency bands of the electromagnetic spectrum is regulatedby governments in most countries, in a spectrum management processknown as frequency allocation or spectrum allocation.• Radio propagation does not stop at national boundaries. Givingtechnical and economic reasons, governments have sought to harmonisethe allocation of RF bands and their standardization.