w2-canyouhearme

8/6/2019 W2-CanYouHearMe

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E367-W2- Can you hear me?

1. Figure 1 below shows how the radio waves propagate through theatmosphere in different modes.

Figure 1: Different modes of propagation

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Mode ofPropagation

Abbreviation Frequency

(Ground) Surface wave VLF 5 kHz - 10 kHz

Sky waveMFHF

300 kHz - 3MHz3MHz - 30MHz

Space waveVHFUHF

and beyond

30 MHz- 300MHz300MHz - 3GHz


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a) Discuss within your team which mode of propagation issuitable for long distance communication.Sky waves.

b) How can we achieve long distance communication inaircrafts?

2. For example, the VHF communication system has an operating frequency

range of (30 MHz - 300 MHz). Using the equation (v = f. ), thewavelength is calculated as shown. (Assume velocity of radio wave is 3.0x108 m/s)

Description Frequency Wavelength

Very High Frequency(VHF)

30 MHz = v/f= 10 m

300 MHz = 1 m

High Frequency(HF)

3 MHz 100m

High Frequency30 MHz

10m

(a) Determine the wavelengths for High Frequency?(b) Compare the wavelengths you have calculated above. Do you

think is it practical to have such a length for your antenna design?Its not practical as antenna of such length is too big.

(c) What hanppens if such antenna lenghths are installed on theaircraft?Too bulky and heavy, causes the aircraft unable to fly.

3. The aircraft HF communication system can transmit Amplitude Modulation(AM) signal.

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Modulation is the process of impressing a low frequency information signal(m(t)) onto a higher frequency carrier signal.

Sketch the frequency spectrum of the AM signal (Vout ) based on thefollowing information.

tftv CC )2cos(10)( = carrier frequency(fc )= 5.0 MHztftv mS )2cos(5)( = modulating signal (fm ) =1.0 kHz..

4. For the modulated signals shown below, match the modulated signal withits modulation type. Explain why do we need to modulate the signalbefore transmitting? Which figure below gives the best modulation, justifyyour answer.

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Modulated signal Types of modulation

C

------

a) 100% Modulation

b) Over Modulation

c) Under Modulation

A------

B------

Carrierfrequency

5.0MHz


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5. Compare the advantages and disadvantages of AM and Single SideBand(SSB) transmissions as shown in Figure 2 (a) & (b)?

(a)

(b)

Figure 2 (a & b): AM and SSB transmission

SSB: Advantage - SSB cuts the bandwidth in half and improves the efficiency tonearly 100%.

Disadvantage - Expensive.AM : Advantage -

6. Figure 3 shows a Tuned Radio Frequency receiver (TRF).

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Transmitter

AMsignal

Receiver

TransmitterLSB

Receiver


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Figure 3 Tuned Radio Frequnecy Receiver

One of the major characteristics of any radio receiver is selectivity. It is theability of the receiver to reject unwanted signal.

a. Assume the value of an inductor (L) is 10 H , value of capacitor(C) is 8.37 nF and a Q-factor of 100, what is the bandwidth of a

tuned circuit? What are some of limitations of TRF receiver?

7. Figure 4 shows the block diagram of a typical superheterodynereceiver.

Figure 4: Block diagram of Super heterodyne receiver

a. Name the type of filter circuit used (e.g. Low-pass, High-pass,Band-stop, and Band-pass)? Explain your choice.Band-pass.

b. What are the functions of the Mixer and Local Oscillator (LO)?A mixer is a nonlinear or time-varying circuit or device that accepts as

its input two different frequencies and presents at its output a mixture

of signals at several frequencies.A local oscillator is an electronic device used to generate a signal

normally for the purpose of converting a signal of interest to a

different frequency using a mixer.

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c. Explain the significant of the Intermediate frequency (IF) signaland function of the IF amplifiers?The intermediate frequency is created by mixing the carrier signal witha local oscillator signal.

d. An envelope detector can be used to demodulate the inputsignal. What is function of the diode in the circuit ? How will theresistance value R affects the output signal?

e. What is the purpose of AGC (Automatic Gain Control)?

8. A HF communication receiver has an intermediate frequency (IF) of 455kHz. Determine the local oscillator (LO) frequency when the receiver istuned to 5.675 MHz. 6.025 MHz

9. The block diagram of a typical aircraft HF system is shown in Figure 5.Briefly explain the purpose of the following components.

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a) HF antenna

b) Antenna Tuning Unit

c) Transceiver

d) Control PanelTune frequency, adjust volume etc.


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Grumman E2C aircraft

Figure 5 (Block Diagram of HF Comms system)

10. Figure 6 shows a typical SELCAL system.The acronyom SELCAL standsfor Selective Calling

Figure 6: Typical SELCAL system

a) What is the function of the of SELCAL system.

b) Each aircraft has its own unique SELCAL code. What is thefunction of the decoder of SELCAL system?

c) How does the SELCAL system alert the flight crew?

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Long wires


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Resources:

8

Aircraft communications and navigation systems:

principles, operation and maintenance

Author: Mike Tooley and David Wyatt. TL693 TOO 2007

Practical aircraft electronic systems:Author: AlbertHelfrick TL693 HEL 1995

Principles of avionics :

Author: AlbertHelfrick TL695 HEL 2004

Aircraft digital electronic and computer systems :principles, operation and maintenance

Author: Mike Tooley TL690 TOO 2007
http://libopac.rp.edu.sg/cgi-bin/cw_cgi?doSearch+2888+3089+_+//TAircraft_communications_and_navigation_systems+-0+-1http://libopac.rp.edu.sg/cgi-bin/cw_cgi?doSearch+14472+3089+_+//TPractical_aircraft_electronic_systems+-0+-1http://libopac.rp.edu.sg/cgi-bin/cw_cgi?doSearch+14472+3089+_+//TPrinciples_of_avionics+-0+-1http://libopac.rp.edu.sg/cgi-bin/cw_cgi?doSearch+2888+3089+_+//TAircraft_digital_electronic_and_computer_systems+-0+-1http://libopac.rp.edu.sg/cgi-bin/cw_cgi?doSearch+2888+3089+_+//TAircraft_communications_and_navigation_systems+-0+-1http://libopac.rp.edu.sg/cgi-bin/cw_cgi?doSearch+2888+3089+_+//TAircraft_digital_electronic_and_computer_systems+-0+-1http://libopac.rp.edu.sg/cgi-bin/cw_cgi?doSearch+14472+3089+_+//TPractical_aircraft_electronic_systems+-0+-1http://libopac.rp.edu.sg/cgi-bin/cw_cgi?doSearch+14472+3089+_+//TPrinciples_of_avionics+-0+-1

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