 |
|
Show in the diagram Basic Radio Transmitter and Receiver.
Radio transmitters
A radio transmitter consists of several elements that work together to generate radio waves that contain useful information such as audio, video, or digital data.
• Power supply: Provides the necessary electrical power to operate the transmitter.
• Oscillator: Creates alternating current at the frequency on which the transmitter will transmit. The oscillator usually generates a sine wave, which is referred to as a carrier wave.
• Modulator: Adds useful information to the carrier wave.
• Amplifier: Amplifies the modulated carrier wave to increase its power, the more powerful the amplifier, the more powerful the broadcast. Antenna: Converts the amplified signal to radio waves. 
Radio receivers
A radio receiver is the opposite of a radio transmitter. It uses an antenna to capture radio waves, processes those waves to extract only those waves that are vibrating at the desired frequency, extracts the audio signals that were added to those waves, amplifies the audio signals, and finally plays them on a speaker.
Antenna: Captures the radio waves. Typically, the antenna is simply a length of wire. When this wire is exposed to radio waves, the waves induce a very small alternating current in the antenna.
RF amplifier: A sensitive amplifier that amplifies the very weak radio frequency (RF) signal from the antenna so that the signal can be processed by the tuner.
Tuner: A circuit that can extract signals of a particular frequency from a mix of signals of different frequencies. On its own, the antenna captures radio waves of all frequencies and sends them to the RF amplifier, which dutifully amplifies them all.
Detector: Responsible for separating the audio information from the carrier wave.
Audio amplifier: This component's job is to amplify the weak signal that comes from the detector so that it can be heard. This can be done using a simple transistor amplifier circuit.
2). Diffraction– When a radio wave passes a solid object, it tends to scatter causing some of the energy to pass into the area of geometric shadow. This phenomenon partly explains why radio waves in certain frequency bands are able to follow the curvature of the earth.
3). At what Frequency operate Radio Altimeter -typically, most radio altimeter units operate between 4.2 and 4.4GHz in frequency, but only use 150 megahertz within that range.
Билет №30
1). Sky wave - Sky waves are radio signals which have been bent or refracted sufficiently to return to the earth. The medium that causes such bending is the ionosphere, a region in the upper atmosphere where free ions and electrons exist in sufficient quantity to cause a marked change in the refractive index. Ultra-violet radiation from the sun is considered to be responsible for the ionization. For a given intensity of ionization, the amount of refraction becomes less as the frequency of the radio wave is increased. If the frequency is raised too much, the bending becomes too slight to bring the wave back to earth.
2). Attenuation - is simply a loss of energy and velocity. As part of the wave comes in contact with the surface it induces a current in it, thereby losing energy and slowing down. This slowing down of the bottom of the wave gives the waveform a forward and downward tilt encouraging it to follow the curvature of the earth. Waves continue until they are finally attenuated and become undetectable.
Attenuation depends on two factors:
a) The type of surface
b) Frequency in use
3). At what Frequency operate MLS - the MLS system operates at a frequency band of 5031,0 – 5090,7 MHz on two separate channels at a mutual interval of 300 kHz.
Билет № 31
1). Direct wave –in the frequency bands of VHF and above, the only propagation path that can be used is the direct wave which travels in a straight line from the transmitter to the transmitter to the receiver. Radio waves at these frequencies are not refracted in the ionosphere and their surface waves are very short due to attenuation. The curvature of the earth limits the range available by direct waves. It is said that range is limited to the "line of sight", that is the straight line joining the transmitter and receiver. In fact, there is a small amount of refraction in the lower layers of the atmosphere due to density changes which extend the radio horizon.
The formula used to calculate direct wave range is:
Range (nm) = 1.25(√h1 + √h2).
h1 is the height of the transmitter is feet AMSL.
h2 is the height of the receiver in feet AMSL.
Direct wave range depends on:
· The height of the transmitter.
· The height of the receiver.
· The height of any intervening high ground.
· The power of the transmitter.
The line of sight limitation has the advantage of preventing interference from oter transmitters which are over the horizon.
2). Doppler effect- Doppler effect is the apparent change in the frequency of radio waves due to relative motion between the transmitter (TX) and receiver (RX). If the distance between TX and RX is reducing, more radio waves arrive at the RX in unit time so the received frequency is greater than that transmitted. Similarly, if the distance is increasing the received frequency is lower.
3). Aerodrome Surveillance Radar (ASR)- airport Surveillance Radar (ASR) is an approach control radar used to detect and display an aircraft's position in the terminal area. These radar sets operate usually in E-Band(108-118 and 118—136,975 МГц), and are capable of reliably detecting and tracking aircraft at altitudes below 25,000 feet (7,620 meters) and within 40 to 60 nautical miles (75 to 110 km) of their airport.
Билет № 32
1). Surface wave- it is normally expected that radio waves will travel a straight line but in certain conditions, the waves will bend and follow the surface of the earth giving increased ranges. The two primary factors that cause this are diffraction and surface attenuation.
2). Attenuation - the term attenuation refers to the loss of power or signal strength suffered by radio waves pass they pass through matter or over a surface.
The following general rules are explained in later chapters:
· Surface attenuation increases with increase in frequency.
· Ionospheric attenuation increases with decrease in frequency.
· Radar attenuation increases with increase in frequency
Surface waves are to a greater or lesser extent absorbed by the surface over which the radio waves are passing. The ionosphere and particles in the atmosphere may to a greater or lesser extent block the passage of radio or radar energy. The processes of absorption and blocking are referred to as ATTENUATION.
3). At what Frequency operate MLS- The MLS system operates at a frequency band of 5031,0 – 5090,7 MHz on two separate channels at a mutual interval of 300 kHz. The protractor part of the MLS system provides continually information about an aircraft’s position relative to the runway both in the vertical and horizontal plane. The rangefinder part enables to measure the distance between an aircraft and the reference points in the approach process. The angular information for the approach course, descent, flare and go-around is determined by measuring the interval between two passages of an oscillating plane lobe through an onboard MLS antenna.
Билет №33