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Advanced Radio and Radar Part 2 Wavelength, Frequency and Modulation.

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1 Advanced Radio and Radar Part 2 Wavelength, Frequency and Modulation

2 Introduction Using ‘em’ energy has many advantages compared with sound energy: a.Speed of travel is extremely fast, The speed of light - 3 x 10 8 m/s, (ms -1 ) 300,000,000 meters/s or 186,000 miles/second. b. ‘Em’ waves travel through a vacuum so can be used for communication in space. so can be used for communication in space. c.‘Em’ waves travel a long way for any given power rating.

3 Introduction ‘Em’ waves travel a long way for any given power rating. Pioneer 10 was launched in Originally giving 140 Watts of power, when it sped past Saturn the radio power had decayed to 100W. The radio was turned off by NASA in 2003 when Pioneer was 8 billion miles away !! and taking 12 hours for the radio signal to reach Earth. At that time it it was transmitting a mere 40W at a frequency of 2 GHz, barely enough for a domestic light bulb. (a microwave blasts out 800w) For Example –

4 Wavelength & Frequency The wavelength, is calculated by dividing the velocity of the wave V, by its frequency F. Remember Remember Velocity = Frequency x Wavelength Velocity = Frequency x Wavelength Velocity V is the speed of light - 3 x 10 8 m/s, 300,000,000 m/s Frequency F is the number cycles per second, KHz, MHz etc. 1 KHz = 1,000 cycles Therefore a frequency of 1KHz gives a wavelength of 300km V λ F = 300,000,000 1,000

5 Wavelength & Frequency The length of the aerial dictates the frequency at which it will transmit and receive. Remember that Remember that Velocity = Frequency x Wavelength The most useful form of this expression is to calculate wavelength for aerial selection. λ /2 for horizontal polarisation, and λ /4 for vertical are particularly efficient aerial lengths. Knowing the wave velocity and frequency, we can calculate the wavelength and the best aerial lengths for that frequency. V = λ F

6 V = λ F Wavelength & Frequency We know wavelength, is calculated by dividing velocity V, by frequency F. Remember that Remember that Velocity = Frequency x Wavelength So what aerial length would suit a frequency of 200 KHz? λ = 1500 metres Therefore an aerial length of 750 or 375 metres would give the best results. ( λ /2 for horizontal, λ /4 for vertical polarisation) (= 3 x 100,000,000 = ) (= 200 x 1,000 = ) λ = 3 x x 10 3 (= 1500 / 2 = 750) (= 1500 / 4 = 375) 300,000, ,000, ,000

7 Wavelength & Frequency Remember – The shorter the length an aerial becomes, The higher the optimum frequency it will transmit and receive. The longer the length an aerial becomes, The lower the optimum frequency it will transmit and receive.

8 Advanced Radio and Radar Modulation

9 Introduction In 1901 Marconi was the first man to transmit and receive transatlantic radio signals. The radio waves were sent by switching the transmitter “OFF” and “ON” – Morse Code. Although effective, this system depended on the operators learning Morse Code.

10 Introduction For a system that everyone could use, some way of making the radio waves carry more information had to be found. ‘Em’ energy can be made to carry speech if low-frequency currents produced by speaking are combined with the high-frequency currents that produce radio waves. This combination process is called MODULATION

11 Modulation For the transmission of speech and music, the sound waves are converted by microphone into an oscillating electric current which varies at the same frequency as the sound wave. This is called an "audio-frequency" current. This is called an "audio-frequency" current. An electronic circuit called an oscillator then produces an ‘em’ “carrier wave”. (by converting energy into a periodically varying electric output)

12 Modulation This carrier wave is a continuous high radio-frequency (RF) current, having a fixed frequency from the range 100 KHz to 1 GHz. The audio-frequency (AF) current, and the radio-frequency (RF) current, are mixed in the transmitter so that the carrier wave is MODULATED so as to duplicate the sound waves fed into the microphone.

13 A carrier wave can be modulated in two ways, either by Amplitude Modulation (AM), or by Frequency Modulation (FM). The simplest form of Amplitude Modulation (AM) is switching the transmitter "ON" and "OFF" to interrupt the carrier wave. Modulation

14 The simplest form of Amplitude Modulation (AM) is switching the transmitter "ON" and "OFF" to interrupt the carrier wave. Modulation This modulates the amplitude from max to zero, and then back to maximum, producing pulses of Morse Code (dots and dashes) ON OFF ON OFF ON OFF D 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 variations to achieve an accurate reproduction. An improvement is to alter the amplitude of the An improvement is to alter the amplitude of the high frequency tone (the carrier wave) in step with the lower frequency audio tone. =+ Carrier Wave Carrier Wave Audio Tone Modulated Wave Modulated Wave

16 A long way for no power A long way for a given power Forever for a given power A short way for a given power Check of Understanding One advantage of ‘em’ waves is that they travel -

17 What is the optimum length for a half-wave aerial that requires to transmit and receive 1 MHz signals? 300 metres 300 metres 600 metres 15 metres 150 metres Check of Understanding λ = 3 x x 10 6 (= 3 x 100,000,000 = ) (= 1 x 1,000,000 = ) 300,000, ,000,000 1,000,000

18 The less efficient it is. The lower the optimum frequency that it will transmit and receive. The higher the optimum frequency that it will transmit and receive. The more efficient it is. Check of Understanding The shorter the length of an aerial becomes:

19 300 m 30 m 300 km 30 km Check of Understanding What is the wavelength of a wave of frequency 1 KHz given the speed of light is 300,000,000 m/s?

20 If the velocity of a radio wave is 300,000,000 m per sec, and the wave frequency is 10 KHz, what would be the wavelength? 3,000 metres 3,000 metres 30,000 metres 30 metres 300 metres Check of Understanding λ = 3 x x 10 3 (= 3 x 100,000,000 = ) (= 10 x 1,000 = ) 300,000, ,000,000 10,000

21 If a wavelength is 40 metres, what would the best aerial length be? 10 or 20 metres 1 or 4 metres 20 or 40 metres 5 or 10 metres Check of Understanding

22 What type of modulation does Morse Code use? Amplitude Modulation Wavelength Modulation Carrier Shift Modulation Frequency Modulation Check of Understanding

23 A few more questions. 1.What is meant by the term Modulation? 2.What do the initials AF and RF stand for? 3.True or False ? FM is a higher frequency than AM. 4.What is the great drawback with the AM system? 5.True or False ? AM is more prone to atmospheric and manmade noise? Check of Understanding

24 Advanced Radio and Radar End of Presentation


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