Past question and answer on tellecomunication II (EEC 218)


What is propagation of radio wave?

RADIO IS the behaviour of radio waves when they are transmitted or propagated from one point on the earth to other parts of the atmosphere.

Explain the following terms in connection with radio wave propagation.

  1. Ground wave: [or surface wave] is a surface wave that propagate close to the surface of the earth. It has a low frequency waves, especially A.M broadcast is the medium waves its employed for world-wide communication in very low frequency bands.
  2. Space wave : This may be considered to consist of two rays, one travelling in a straight line between two point and the other travelling between the same two point by means of single reflection from the earth.
  3. Sky wave: This is the propagation of electromagnetic waves bent[reflected] back to the earth’s surface by the ionosphere in the high frequency band.

List (8) eight radio frequency and their uses.

VERY LOW FREQUENCY [V.L.F]: It frequency ranges from 3-30KHz and has a wavelength of 100-10km.

  1. it is used to communicate with submarines near the surface.
  2. V.L.F is also used for radio navigation beacons(alpha) and time signals(beta)
  3. It is also used in electromagnetic geographical surveys.

LOW FREQUENCY(L.F): It ranges from 30-300KHz in frequency and 10-1Km in wavelength.

  1. it main use is for aircraft beacon
  2. navigation[loran] information
  3. it is used for weather system

MEDIUM FREQUENCY(M.F): It frequency ranges from 300-3000kHz and 1000m-100m.

  1. it is used for A.M radio broadcasting
  2. navigation radio beacons
  3. maritime ship-to-shore communication
  4. transoceanic air traffic control

HIGH FREQUENCY(H.F): It frequency ranges from 3-30 MHz and has a wavelength ranging from 100-10m. it is used in;

  1. Government time station
  2. Armateur radio and citizens band services.
  3. Aviation communication.
  4. Maritime sea-shore and ship-ship services.

VERY HIGH FREQUENCY(V.H.F): It frequency ranges from 30-300MHz and has a wavelength ranging from 10-1m. use in;

  1. F.M radio broadcast
  2. Television broadcast
  3. Land mobile station
  4. Armateur radio

ULTRA HIGH FREQUENCY(U.H.F): It frequency ranges from 300-3000MHz and has a wavelength ranging from 100-10cm.

  1. Widely used for public service agencies for two-way radio communication.
  2. Mainly used for wifi, Bluetooth and universal serial box(U.S.B) cordless phones wireless transfer.
  3. Global positioning system also uses ultra high frequency(U.H.F)
  4. One uncommon use of U.H.F is detection of partial discharges.

SUPER HIGH FREQUENCY(S.H.F): It has a frequency ranging from 3-30GHz and 10-1cm.

  1. Use for wireless LAN
  2. Use for terrestrial high -speed data links which are sometimes referred to as “backhauls”.
  3. Use for satelites uplinks/downlinks.

EXTREMELY HIGH FREQUENCY (E.H.F): It has frequency ranging from 30-300GHz and also have a wavelength of 10-1mm

  1. Use for scientific research
  2. Use in telecommunication
  3. Ues in weapon system
  4. Use in security screening

A certain broadcast transmitter transmit at 1.2MHz and uses an antenna of 150m height. The antenna current was measured to be 9A. Calculate the voltage induced in a receiving antenna, of effective height of 2m. If the receiving antenna is at a distance of 40km from the transmitter ad its impedance is 120.

V= V=
Distance = 40km, =9A, =2m,
=120, F=1.2MHz
× = C= 3×


Explain the function of antenna of a radiator.

The function of an aerial of a radiator: This is a mast or tower in which the whole structure itself function as an antenna. This is commonly used for transmitting antenna operating at low frequencies, in the VLF, LF, and MF ranges. In particular those used for AM broadcasting.
It metal mast is electrically connected to the transmitter. Its base is usually mounted on a non-conductive support to insulate it from the ground and the transmitter is connected to the mast just above it.
This type of antenna is known as an anti-fading aerial.

Define the beam width of an aerial

it can be used as a directivity of directive antenna, a beam width is calculated from a radiation pattern as an angle distance between the/ point where the signal strength drop to 0.707. The beam width of an antenna also known as the radiation pattern of an antenna. This is the area where most of the power is radiated, which is the peak power.

What is the isotropic radiator of an antenna

It’s define as a radiation which is emitted by a source in all direction with equal intensity or which reaches a location from all direction with equal intensity.

Explain the antenna efficiency

If an antenna is taken as a device which accent power from a source and radiate it into space the ratio of the power radiated to the power fed.
It’s the ratio of the power radiated to the power fed to the aerial. It’s express as a percentage.


Explain the functions of the following units in monochrome television receiver.

Focus adjustment

Focus adjustment: The focus adjustment is the electron beam. The electron beam must be focused to small port/spot light on the screen usually focus is sharp on the centre area of the tube
Function Older pictures used in magnetic focusing with a focused signal on the neck of the tube behind the deflection yoke.

for monochrome TV, the focus grid has up to 0-30v to operate.

Electrostatic focusing: is omitted from a cathode and tends to diverge because they repel each other. However the electron can be forced to converge to a point by an electric or to a magnetic field. So the voltage focused in the beam to a spot is called a crossed point beyond the control grid.

Draw the block diagram of black and white television receiver.

The principle of operation of a TV receiver: From space then, the modulated signals coming in consist of picture, sound synchronizing pulses for timing the vertical and horizontal deflections on the screen. The signal arrives at the channel selector through the R.F and selector value of transistor. The mixing is done and an I.F is common to both sound and vision emerges. This I.F is amplified by a repetition of the stages two or three times after which the sound now takes off on its own for a second conversion, detection and audio amplification to the speaker.
From the junction of where the sound took off, the video detector accepts the vision and synchronizes signals which is transferred to the video amplifier. Part of the video amplifier signal is fed to a grid of the television tube on video information, while part is fed back for automatic gain control purposes, the rest going to the sync pulse separator.
There is another locally generated vertical and horizontal bank of pulses, comparing with the in coming sync. Pulses to compensate for any differences and ensure stabilization by the time they are amplified and sent to the two deflections coil situated on the neck of the picture tube.
The end result of the synchronization stabilized pulses should give so vertical cycles per seconds; returning a moving dot on the screen which is being swept through 625 horizontal lines.


  1. Draw the block diagram of amplitude modulation (AM) radio transmitter
  2. Explain the function of each block of amplitude modulation radio transmitter in 4a.

A.M radio transmitter: Radio transmitter is an electronic device when connected to an antenna produce an electromagnetic signal (wave) such as a radio or television broadcasting station.
The block diagram consist of an oscillatory modulatory power amplifier and a filter and a matching network to connect to any antenna i.e microphone audio amplifier, oscillatory radio frequency modulation R.F modulated amplifier or power amplifier/ power supply.

  1. As microphone: is a device or tranducer that convert sound energy into electrical energy.
  2. Carrier frequency modulator oscillator: it is a device that produce constant frequency of unknown value, therefore it produce frequency used as carrier frequency in the transmitting station.
  3. Radio frequency amplifier: this stage in the transmitting station is a stage used to amplify the carrier frequency produced by the oscillator
  4. Audio modulation: stage amplifier audio signal from microphone by amplifying. It means it gives the audio signal more strength.
  5. Amplitude modulation amplifier: it is a stage where the output of the audio amplifier and R.F signal meet for amplitude modulation to take place. In short it is a stage where radio signal is used to superimpose on the carrier.
  6. R.F modulator Amp or Power amplifier: this is the final stage in the transmitting station where final amplifier station is done to give the modulated wave more length before it is put on the aerial for transmission.
  7. Power supply: it is a device that convert A.C power to supply power to all the stages in the transmitter equipment.

State 2(two) advantages of amplitude modulation (AM) transmitter

  1. Low cost of construction
  2. Used for long and short distances

It’s of low quality.
It produces high noise and distortion.
It has small bandwidth.
High power for its distribution


what is characteristics impedance of transmission line

The characteristic impedance of a transmission line is the imput impedance of an infinite length of that line. Its imput impedance is the ratio of the voltage applied across the sending end terminals to the current. flowing into the line

Phase velocity of propagation

Phase velocity of propagation: The phase velocity of a line is the velocity with which a sinusoidal wave travels along that line. There are F cycles per seconds and a sinusoidal wave travels with a phase velocity

Explain the following terms in relation to the layers of the ionosphere

  1. D-layer= is the innermost layer, 60KM to 90KM above the surface of the earth. An example of the D layer is the disappearance of distance A.M BROADCAST band which start in the day time.
  2. E-layer= is the middle layer 90KM – 120KM above the surface of the earth. At night the E layer rapidly disappear because the primary source of the ionization is no longer present. After sunset an increase in the height of the E layer maximum increases the range to which radio waves can travel by reflection from the layer.
  3. F-layer= it extends from about 200km to more than 500km above the surface of the earth. Any signal penetrate from this layer will escape into space. The lower part of the earth atmosphere is called the troposphere and it extends from the surface up to about 10km [6km at frequency above 400MHz there is signified scatter in the troposphere below the ionosphere]. This provides the means for extending the range of a communication link the signal level may be low, and link is subject to fading due to variation in the tropospheric height.
  4. Skip Distance: The skip distance is the distance from the transmitter to the point where the sky wave is first return to the earth.

What is maximum usable frequency

It is the highest frequency that can be used to establish communication using the skywave between two points. If a higher frequency is employed, the signal is received at the distance end of the link. As a wave travels through the ionosphere it is attenuated, the attenuation being inversely proportional to the frequency of the wave.

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