advanced radio and radar part 2 wavelength, frequency and modulation

Advanced Radioand Radar

Part 2Wavelength, Frequency

and Modulation

IntroductionUsing ‘em’ energy has many advantages

compared with sound energy:

a. 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 vacuum so can be used for communication in space.

c. ‘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 2003when 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 & FrequencyThe 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 1KHzgives a wavelength of 300km

V λF=300,000,000

1,000

Wavelength & FrequencyThe 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 verticalare particularly efficient aerial lengths.

Knowing the wave velocity and frequency, we can calculate the wavelength

and the best aerial lengths for that frequency.

V = λF

V = λF

Wavelength & FrequencyWe 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)

(= 3 x 100,000,000 = )

(= 200 x 1,000 = )λ =

3 x 108

200 x 103

(= 1500 / 2 = 750)(= 1500 / 4 = 375)

300,000,000

200,000

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.

Advanced Radioand Radar

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.

IntroductionFor 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

ModulationFor 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)

ModulationThis 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.

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

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

ModulationWhist 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

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 UnderstandingOne advantage of ‘em’ waves

is that they travel -

What is the optimum lengthfor a half-wave aerial that requires

to transmit and receive 1 MHz signals?

300 metres

600 metres

15 metres

150 metres

Check of Understanding

λ =3 x 108

1 x 106

(= 3 x 100,000,000 = )

(= 1 x 1,000,000 = )

300,000,000

1,000,000

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 UnderstandingThe shorter the length of an aerial becomes:

300 m

30 m

300 km

30 km

Check of UnderstandingWhat is the wavelength of a wave of frequency 1 KHz

given the speed of light is 300,000,000 m/s?

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?

3,000 metres

30,000 metres

30 metres

300 metres

Check of Understanding

λ =3 x 108

10 x 103

(= 3 x 100,000,000 = )

(= 10 x 1,000 = )

300,000,000

10,000

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

What type of modulation does Morse Code use?

Amplitude Modulation

Wavelength Modulation

Carrier Shift Modulation

Frequency Modulation

Check of Understanding

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 andmanmade noise?

Check of Understanding

Advanced Radioand Radar

End of Presentation