Radio and Radar: Radio

1. Advanced Radio And Radar
Radio

2. • There are various methods of
communications. The voice being a main one.
• A transmitter and receiver are required.
• Voice – Transmitter
• Ears – Receiver

3. • Speed of travel is quite slow in air: 340 m/s at
20ºC or 760 mph (the speed of propagation of
sound).
• Sound will not travel through a vacuum.
• It needs a substance or “medium” to transmit
the energy.
• The medium can also be liquid (water or
mercury) or a solid (bar of steel or a brick wall)

4. • The energy in the molecules vibrate off each
other in the same direction as the
propagation.
• This is why sound travels further in solids as
oppose to gases – there are more molecules.
• The direction of travel is a longitudinal wave

5. • Sound does not travel very far in air. The air
acts like a shock absorber and the sound
energy is converted into extremely small
amounts of heat.
• So sound travels faster and much more
efficiently in liquid and therefore, much
further. Solid mediums even more so.

7. • Depending on a number of complex factors,
radio waves can propagate through the
atmosphere in various ways.
These include:
• Ground waves
• Ionospheric waves
• Space waves
• Tropospheric waves.

8. • As their name suggests, ground waves (or
surface waves) travel close to the surface of
the earth and propagate for relatively short
distances at HF and VHF.
• Ground waves have two basic components; a
direct wave and a ground reflected wave.
• The direct path is called line of sight.

10. • A radio communications system consists of a
transmitter (Tx), to send the message and a receiver
(Rx) to receive the reply.
• The link between the Tx and Rx is this time not sound
energy, but electromagnetic (em) energy, - radio
waves.
• Just like light from the sun, radio waves can travel not
only through air, but also through a vacuum.
• They travel at the same extremely high speed.
• Both the transmitter and receiver use antennas to
radiate and capture the radio signal.

12. What is electromagnetic energy?
• When an alternating electric current flows in a
wire, both magnetic and electric fields are
produced outside the wire.
• Some can be used for radio communications –
radio waves.
• The frequency of the alternating current will
determine the frequency of the ‘electromagnetic’
waves produced, and its power rating will govern
the range of radiation.
• There is no theoretical limit to the frequency of
‘electromagnetic’ waves, and the expression
"electromagnetic spectrum" has been coined to
embrace all radiations of this type, which include
heat and light.

13. • Electromagnetic radiation travels in waves in a
similar fashion to sound waves travelling
through air.
• The waves travel in all directions from their
source rather like the pattern produced when
a stone is dropped into the water in a still
pond.

15. • Frequency - The number of complete vibrations
or fluctuations each second (i.e. cycles per sec).
• Amplitude - The distance between O on the
Amplitude axis and a crest.
• Wavelength - The distance between any two
identical points in a wave (literally the length of
one wave).
• Velocity - The speed with which the waves moves
is given by the formula: v = f λ

16. • Microwaves are radio waves with frequencies
higher than television signals.
• The wavelengths of microwaves are of the
order of a few millimeters.
• We know that sound waves spread and bend
around the corner of an obstacle.
• This is because the wavelength of sound wave
is generally comparable to the size of the
obstacle.
• Unlike a sound wave, a light wave keeps itself
along a straight path.

17. • The light waves bend by only a small amount
at the corners of the obstacles.
• This is because the wavelength of light waves
is smaller as compared to the wavelength of
sound waves.
• Therefore, lesser the wavelength of a wave,
smaller is its bending at the corners of
ordinary obstacles and greater the ability of
the wave to follow a straight path.

18. • The wavelength of microwaves is very small as
compared to the wavelength of radio waves.
So, microwaves are better suited to beam
signals in a particular direction

19. Modulation
• For the transmission of sounds such as speech
and music, the sound waves are converted by
a microphone into an oscillating electric
current which varies at the same frequency as
the sound wave.
• This is called an "audio-frequency" current.

20. • There are two ways to carry analog
information with radio waves.
• These are Amplitude Modulation and
Frequency Modulation (AM and FM)

22. • Amplitude modulation (AM) is a technique
used in electronic communication, most
commonly for transmitting information via a
radio carrier wave.
• With AM, the amplitude of the wave is made
to vary in accordance with the audio wave.
• It is modulated in amplitude by the signal that
is to be transmitted.

23. • With Frequency Modulation, the frequency of
the carrier wave is made to vary in accordance
with the audio wave.
• It is modulated in frequency by the signal that
is to be transmitted.
• AM radio ranges from 535 to 1705 kilohertz
and FM radio ranges in a higher spectrum
from 88 to 108 megahertz.
• A transmitter and receiver and required for
both modulations.

27. The above scheme suffers from the following
drawbacks:
• EM waves in the frequency range of 20 Hz - 20
kHz (audio-frequency range) cannot be
efficiently radiated and do not propagate well
in space.
• Simultaneous transmission of different signals
by different transmitters would lead to
confusion at the receiver.

28. • We need to devise methods to convert or
translate the audio signals to the radio-
frequency range before transmission and
recover the audio-frequency signals back at
the receiver.
• Different transmitting stations can then be
allotted slots in the radio-frequency range and
a single receiver can then tune into these
transmitters without confusion.

29. • The frequency range 500 kHz to 20 MHz is
reserved for amplitude-modulated broadcast,
which is the range covered by most three
band transistor radios.
• The process of frequency translation at the
transmitter is called modulation.
• The process of recovering the audio-signal at
the receiver is called demodulation

31. • A radio transmitter produces radio waves.
• These waves are sent out through an antenna
to a receiver.
• The transmitter is a source of electrical energy,
producing alternating current of a desired
frequency of oscillation.
• A radio receiver receives these radio waves
and converts them back into audio or visual
information.
• The waves a received by an antenna once
again.

32. • The process for a transmitter is the audio
stage which increases the weak signal coming
from the microphone.
• The modulator then modulates the audio
signal onto the radio frequency carrier.
• Then at the frequency generator stage, the
frequency is defined on which the transmitter
will operate.
• finally the RF power amplifier stage, the
power amplification of the radio signal is
carried out.
• It makes the signal stronger so that it can be
transmitted into the aerial.

33. • The process for the receiver is the tuning and RF
amplifier stage which selects the signal we want
to hear as there are many radio signals being
transmitted on different radio frequencies.
• The RF amplifier increases the signal received
from the air by the antenna.
• Antennas are connected to receivers by special
wires known as feeders.
• The detection stage is next which the process is
where the original modulating signal is recovered.
It is the opposite of the modulation stage in
transmitting.
• Then it’s the audio amplifier stage which is where
the detected audio signal is increased to a level
which can be played and heard.

36. • Master Oscillator - This generates a sinusoidal
voltage (the carrier) at the required RF
frequency (FO). Oscillators are often crystal-
controlled to ensure good frequency stability.
• Buffer Amplifier - This isolates the oscillator
from the power amplifying stage, and
prevents instability occurring.
• Power Amplifier - This is used to increase the
power of the signal to the required level
before radiation from the aerial (AF).
• Amplifier - This amplifies the microphone
signal to the desired level for output.

37. The modulation takes place in the power
amplifier stage.
If the input frequencies to the modulator are FO
from the oscillator and AF from the
microphone, we find that the output of the
power amplifier will consist of 3 frequencies:
• The carrier (FO ).
• The carrier minus the tone frequency (speech)
(FO – AF).
• The carrier plus the tone frequency (FO + AF).

39. • For example, if the audio frequency ranged
from 300 to 3000 Hz and the carrier was 1
MHz, then the frequencies in the output
would look like the diagram above.
• In the diagram you can see two sidebands to
the carrier frequency, an upper sideband and
a lower sideband.
• Some modes of operation use only one, and
this is called single sideband (SSB)
transmission.

40. • Transmitting only one sideband reduces the
size and weight of the transmitter – important
factors when talking about aircraft systems.
• The great drawback with the AM system is the
need for such a large bandwidth (i.e. all
frequencies including both sidebands,
approximately 6KHz) in a limited frequency
spread (30 KHz to 3 MHz i.e. Medium band).
• This means in reality that the AM system
could only have 148 stations at any one time.

41. • Obviously, when many transmitters are
crammed into a small band and overlap each
other there is a big problem with signals from
other transmissions breaking into the one you
are using – this is known as "interference".
• To overcome this, the use of short-range
frequency modulated systems has become
popular.

42. • In those early models of receiver the problems
encountered were noise (too much
interference), poor amplification, limited
selectivity, poor sensitivity (ability to remain
on a station) and lack of fidelity (quality of
sound).
• To overcome some of these problems, the
superheterodyne (superhet) receiver was
developed.
• Heterodyne is the term used to describe the
mixing of one frequency with a slightly
different frequency to produce something
called "beats".

43. • If two notes of nearly equal frequency are
sounded together, a periodic rise and fall in
intensity (i.e. a beat) can be heard.
• For example, if an audio note of 48 Hz is
sounded together with one of 56 Hz then the
rhythmic beat of 8 Hz (56 - 48) would be
heard.

44. • The basic block diagram of a basic superhet
receiver is shown below.
• This details the most basic form of the
receiver and serves to illustrate the basic
blocks and their function.
• The way in which the receiver works can be
seen by following the signal as is passes
through the receiver.

46. • Front end amplifier and tuning block: Signals enter the
front end circuitry from the antenna. This circuit block
performs two main functions:
• Tuning: Broadband tuning is applied to the RF stage. The
purpose of this is to reject the signals on the image
frequency and accept those on the wanted frequency.
• It must also be able to track the local oscillator so that as
the receiver is tuned, so the RF tuning remains on the
required frequency.
• Amplification: In terms of amplification, the level is
carefully chosen so that it does not overload the mixer
when strong signals are present, but enables the signals to
be amplified sufficiently to ensure a good signal to noise
ratio is achieved.
• The amplifier must also be a low noise design. Any noise
introduced in this block will be amplified later in the
receiver.

47. • Mixer / frequency translator block: The
tuned and amplified signal then enters one
port of the mixer.
• The local oscillator signal enters the other
port.
• The performance of the mixer is crucial to
many elements of the overall receiver
performance.
• It should be as linear as possible. If not, then
spurious signals will be generated and these
may appear as 'phantom' received signals.

48. • Local oscillator: The local oscillator may
consist of a variable frequency oscillator that
can be tuned by altering the setting on a
variable capacitor.
• Alternatively it may be a frequency synthesizer
that will enable greater levels of stability and
setting accuracy.

49. • Intermediate frequency amplifier, IF
block: Once the signals leave the mixer they
enter the IF stages.
• These stages contain most of the amplification
in the receiver as well as the filtering that
enables signals on one frequency to be
separated from those on the next.
• Filters may consist simply of LC tuned
transformers providing inter-stage coupling, or
they may be much higher performance
ceramic or even crystal filters, dependent
upon what is required.

50. • Detector / demodulator stage: Once the
signals have passed through the IF stages of
the superheterodyne receiver, they need to be
demodulated.
• Different demodulators are required for
different types of transmission, and as a result
some receivers may have a variety of
demodulators that can be switched in to
accommodate the different types of
transmission that are to be encountered.

51. • Audio amplifier: The output from the
demodulator is the recovered audio.
• This is passed into the audio stages where
they are amplified and presented to the
headphones or loudspeaker

52. • Reception on the AM bands is limited in both
quality of reproduction and bandwidth
availability.
• FM systems are less likely to be affected by
"noise" and give increased signal performance.
• The FM receiver circuitry is similar to the AM
system but uses a discriminator (also called a
ratio detector) in place of a demodulator.
• The discriminator is a circuit which has been
designed to detect small differences in
frequencies.
• These differences are converted to a voltage
output that represents the AF component input.

54. • The first element in the process of receiving a
radio message is the aerial.
• An aerial can vary from a length of wire
supported off the ground to a complex array
designed to select only certain frequencies,
but whatever its shape, its purpose is to
detect the tiny amounts of ‘em’ energy
radiated from the transmitter.

55. • If an aerial in the form of a length of wire is
placed into an electromagnetic field, tiny
voltages are induced in it.
• These voltages alternate with the frequency of
the ‘em’ radiation and are passed to the
receiver circuitry for processing.
• The signal strength that the aerial inputs to
the receiver is very tiny the order of 5 ì(micro)
volts (0.000005 volts).
• Therefore the receiver circuits have to be
extremely sensitive.

56. • The circuits must also isolate the wanted
signal from all the unwanted ones being
received, and this is achieved by using tuned
circuits.
• A tuned circuit simply allows a single
frequency to pass, thus filtering out all the
unwanted signals.
• The best known version of a tuned circuit is
the "crystal set" or "cat’s whisker" as it was
called in the 1920’s and 30’s.

58. What is the speed of light ?
• 3 x 108 ms-1
• 3 x 106 ms-1
• 30 x 109 ms-1
• 30 x 101 ms-1
• 3 x 108 ms-1

59. The relationship between frequency (f),
wavelength (l) and velocity of light (v) is given
in the formula:
• velocity = frequency x wavelength (v = f x l)
• velocity = frequency + wavelength (v = f + l)
• velocity = frequency - wavelength (v = f - l)
• Frequency = velocity - wavelength (f = v - l)
• velocity = frequency x wavelength (v = f x l)

60. If the velocity of radio waves is 3 x 108, what
would be the value of l for frequency of 3 x
106 ?
• 1000m
• 10m
• 100m
• 1m
• 100m

61. What does the abbreviation SSB stand for ?
• Single Side Band
• Single Silicone Band
• Ship to Shore Broadcast
• Solo Side Band
• Single Side Band

62. What is the purpose of an aerial on a receiver?
• To convert the electromagnetic waves (‘em’) into
tiny voltages
• To convert the electromagnetic waves (‘em’) into
large voltages
• To convert the electromagnetic waves (‘em’) into
very large voltages
• To convert the electromagnetic waves (‘em’) into
a constant voltage
• To convert the electromagnetic waves (‘em’) into
tiny voltages

63. What does superheterodyne receivers make use
of?
• Bleats
• Boats
• Beats
• Bullets
• Beats

64. What do FM receivers use to demodulate
signals?
• Distractor
• Modulator
• Discriminator
• Disputer
• Discriminator