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Part 2 Wavelength, Frequency and Modulation

Introduction 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/s or 186,000 miles/second. b. ‘Em’ waves travel through a vacuum so can be used for communication in space. ‘Em’ waves travel a long way for any given power rating.

Introduction ‘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 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 –

Wavelength & Frequency
The wavelength , is calculated by dividing the velocity of the wave V, by its frequency F. Remember Velocity = Frequency x Wavelength Velocity V is the speed of light - 3 x 108 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 300,000,000 λ V F = 1,000

Wavelength & Frequency
The length of the aerial dictates the frequency at which it will transmit and receive. 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. = λ V F

Wavelength & Frequency
We know wavelength , is calculated by dividing velocity V, by frequency F. 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) = λ V F 3 x 108 (= 3 x 100,000,000 = ) 300,000,000 λ = 200 x 103 (= 200 x 1,000 = ) 200,000 (= 1500 / 2 = 750) (= 1500 / 4 = 375)

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

Modulation

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.

Introduction MODULATION 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

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. An electronic circuit called an oscillator then produces an ‘em’ “carrier wave”. (by converting energy into a periodically varying electric output)

Modulation MODULATED 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.

Modulation Amplitude Modulation (AM), Frequency Modulation (FM).
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 This modulates the amplitude from max to zero,
and then back to maximum, producing pulses of Morse Code (dots and dashes) The simplest form of Amplitude Modulation (AM) is switching the transmitter "ON" and "OFF" to interrupt the carrier wave. ON OFF ON OFF ON OFF D A S H D O T D A S H

+ = 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 high frequency tone (the carrier wave) in step with the lower frequency audio tone. + = Carrier Wave Audio Tone Modulated Wave

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

Check of Understanding
What is the optimum length for a half-wave aerial that requires to transmit and receive 1 MHz signals? 600 metres 300 metres 150 metres 3 x 108 (= 3 x 100,000,000 = ) 300,000,000 λ = 1 x 106 (= 1 x 1,000,000 = ) 1,000,000 15 metres

Check 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 frequency that it will transmit and receive. The less efficient it is.

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? 30 m 30 km 300 m 300 km

Check of Understanding
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? 30,000 metres 3,000 metres 300 metres 3 x 108 (= 3 x 100,000,000 = ) 300,000,000 λ = 10 x 103 (= 10 x 1,000 = ) 10,000 30 metres

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

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

Check 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 and manmade noise? ANSWERS The combination of low and high-frequency currents into radio waves Audio Frequency and Radio Frequency True The need for such a large bandwidth in a limited frequency spread