Presentation on theme: "Advanced Radio and Radar"— Presentation transcript:
1Advanced Radio and Radar Part 2Wavelength, Frequency and Modulation
2Introduction Using ‘em’ energy has many advantages compared with sound energy:Speed of travel is extremely fast,The speed of light - 3 x 108 m/s, (ms-1)300,000,000 meters/sor 186,000 miles/second.b. ‘Em’ waves travel through a vacuumso can be used for communication in space.‘Em’ waves travel a long way for any givenpower rating.
3Introduction ‘Em’ waves travel a long way for any given power rating. Pioneer 10 was launched in 1972.Originally giving 140 Watts of power,when it sped past Saturn the radio powerhad decayed to 100W.The radio was turned off by NASA in 2003when Pioneer was 8 billion miles away !!and taking 12 hours for the radio signalto reach Earth.At that time it it was transmitting a mere 40Wat a frequency of 2 GHz,barely enough for a domestic light bulb.(a microwave blasts out 800w)For Example –
4Wavelength & Frequency The wavelength , is calculated by dividingthe velocity of the wave V, by its frequency F.RememberVelocity = Frequency x WavelengthVelocity V is the speed of light - 3 x 108 m/s,300,000,000 m/sFrequency F is the number cycles per second,KHz, MHz etc.1 KHz = 1,000 cyclesTherefore a frequency of 1KHzgives a wavelength of 300km300,000,000λVF=1,000
5Wavelength & Frequency The length of the aerial dictates the frequencyat which it will transmit and receive.Remember thatVelocity = Frequency x WavelengthThe most useful form of this expressionis to calculate wavelength for aerial selection.λ/2 for horizontal polarisation, and λ/4 for verticalare particularly efficient aerial lengths.Knowing the wave velocity and frequency,we can calculate the wavelengthand the best aerial lengths for that frequency.=λVF
6Wavelength & Frequency We know wavelength , is calculatedby dividing velocity V, by frequency F.Remember thatVelocity = Frequency x WavelengthSo what aerial lengthwould suit a frequency of 200 KHz?λ = 1500 metresTherefore an aerial length of750 or 375 metres would give the best results.(λ/2 for horizontal, λ/4 for vertical polarisation)=λVF3 x 108(= 3 x 100,000,000 = )300,000,000λ =200 x 103(= 200 x 1,000 = )200,000(= 1500 / 2 = 750)(= 1500 / 4 = 375)
7Wavelength & Frequency Remember –The shorter the length an aerial becomes,The higher the optimum frequencyit will transmit and receive.The longer the length an aerial becomes,The lower the optimum frequency
9Introduction In 1901 Marconi was the first man to transmit and receive transatlantic radio signals.The radio waves were sentby switching the transmitter “OFF” and “ON” –Morse Code.Although effective, this system depended onthe operators learning Morse Code.
10Introduction MODULATION For a system that everyone could use, some way of making the radio wavescarry more information had to be found.‘Em’ energy can be made to carry speechif low-frequency currents produced by speakingare combined with the high-frequency currentsthat produce radio waves.This combination process is calledMODULATION
11Modulation For the transmission of speech and music, the sound waves are converted by microphoneinto an oscillating electric currentwhich varies at the same frequencyas the sound wave.This is called an "audio-frequency" current.An electronic circuit called an oscillatorthen produces an ‘em’ “carrier wave”.(by converting energyinto a periodically varying electric output)
12Modulation MODULATED This carrier wave is a continuous high radio-frequency (RF) current,having a fixed frequency from the range100 KHz to 1 GHz.The audio-frequency (AF) current,and the radio-frequency (RF) current,are mixed in the transmitterso that the carrier wave isMODULATEDso as to duplicate the sound wavesfed into the microphone.
13Modulation Amplitude Modulation (AM), Frequency Modulation (FM). A carrier wavecan be modulated in two ways,either byAmplitude Modulation (AM),or byFrequency Modulation (FM).The simplest form ofAmplitude Modulation (AM)is switching the transmitter "ON" and "OFF"to interrupt the carrier wave.
14Modulation This modulates the amplitude from max to zero, and then back to maximum,producing pulses of Morse Code(dots and dashes)The simplest form ofAmplitude Modulation (AM)is switching the transmitter "ON" and "OFF"to interrupt the carrier wave.ON OFF ON OFF ON OFFD A S H D O T D A S H
15+ = Modulation Whist this system is ideal for Morse, it is not good enough for speech or music,because sound requires more variationsto achieve an accurate reproduction.An improvement is to alter the amplitude of thehigh frequency tone (the carrier wave)in step with the lower frequency audio tone.+=Carrier WaveAudio ToneModulated Wave
16Check of Understanding One advantage of ‘em’ wavesis that they travel -A long way for a given powerA short way for a given powerForever for a given powerA long way for no power
17Check of Understanding What is the optimum lengthfor a half-wave aerial that requiresto transmit and receive 1 MHz signals?600 metres300 metres150 metres3 x 108(= 3 x 100,000,000 = )300,000,000λ =1 x 106(= 1 x 1,000,000 = )1,000,00015 metres
18Check of Understanding The shorter the length of an aerial becomes:The lower the optimum frequency that it will transmit and receive.The more efficient it is.The higher the optimum frequencythat it will transmit and receive.The less efficient it is.
19Check of Understanding What is the wavelength ofa wave of frequency 1 KHzgiven the speed of light is 300,000,000 m/s?30 m30 km300 m300 km
20Check of Understanding If the velocity of a radio wave is300,000,000 m per sec,and the wave frequency is 10 KHz,what would be the wavelength?30,000 metres3,000 metres300 metres3 x 108(= 3 x 100,000,000 = )300,000,000λ =10 x 103(= 10 x 1,000 = )10,00030 metres
21Check of Understanding If a wavelength is 40 metres,what would the best aerial length be?1 or 4 metres5 or 10 metres10 or 20 metres20 or 40 metres
22Check of Understanding What type of modulationdoes Morse Code use?Wavelength ModulationFrequency ModulationCarrier Shift ModulationAmplitude Modulation
23Check of Understanding A few more questions.What is meant by the term Modulation?What do the initials AF and RF stand for?True or False ? FM is a higher frequency than AM.What is the great drawback with the AM system?True or False ? AM is more prone to atmospheric andmanmade noise?ANSWERSThe combination of low and high-frequency currents into radio wavesAudio Frequency and Radio FrequencyTrueThe need for such a large bandwidth in a limited frequency spread