Radio Propogation, Reception and Communication Flashcards
What is the speed of electromagnetic waves?
3 x 10^8 metres per second
Do EM waves require a medium to transmit through?
No
What is a wave cycle?
A single complete execution of a repeated phenomenon
What is the wavelength of an EM wave?
Distance between two points of the same phase in consecutive cycles of a wave.
What is the frequency of an EM wave?
- The number of cycles of an electromagnetic wave occurring in 1 second.
- Measured in Hz
What is the relationship between wavelength and frequency?
V = f x wavelength
What is the frequency spectrum?
The spread of possible electromagnetic frequencies.
What are the three main frequencies used in aviation?
1) High frequency (HF)
2) Very High Frequency (VHF)
3) Ultra High Frequency (UHF)
What are some positives and negatives of HF ?
- Excellent long distance qualities
- Doesn’t rely on like if aight
- Suffers from low equality transmission due to interference
- Is not used at airfields
What frequencies are HF typically? What are HF frequencies used for?
- 3 - 300 MHz
- Used for HF comms
What are some positives and negatives of VHF frequencies?
- High quality
- Low loss
- Line of sight communications between A/C and ground stations
- Short range
- Ideal for local airfield communications
What is the frequency band for VHF ?
What are they used for?
- 30 - 300 MHz
- VHF comms
- Direction Finding
- VHF Omni Ranging (VOR)
- Marker Beacons
- ILS localiser
What are some positives of Ultra High Frequency (UHF) waves?
- Clear signals with high fidelity which are easy to read
- Require line of sight
- Used for short distance communications
- Military used UHF 225 - 400 MHz
What is the frequency band for UHF? What is it used for?
300 - 3000 MHz
- Distance Measuring Equipment (DME)
- GPS
- ILS Glideslope
What is the design of a typically radio transmitter?
- Radio Frequency (RF) oscillator which generates the carrier wave
- The microphone which converts sound pressure waves into low level electrical energy
- The processed speech is amplified by an audio amplifier and transferred to the modulator which varies the carrier wave frequency
- The RF amplifier increases the power in the RF amplifier and radiates into space via the antenna.
What is the purpose of a radio receiver?
Works in reverse to a transmitter
What are the three types of radio wave propogation of interest to aviators?
1) Ground waves
2) Sky waves
3) Direct Waves
What are ground waves?
- Frequencies between 30 kHz and 3,000 kHz
- Signals tend to follow the curvature of the earth
- Affected by the nature of the ground , salt water is best, sand is worse
What are sky waves (Skip propogation)?
- Frequencies between 3 MHz and 30 MHz
- Radio waves are reflected or refracted back toward Earth from the ionosphere.
- The signal skips and creates a dead zone where a radio transmission will not be heard.
What are direct waves?
Radio waves that are considered to travel in a straight line. (Line of sight).
When may radio waves be refracted and change direction?
Boundary between coast and sea.
What band of communication operates almost exclusively with sky waves?
HF band
What is the critical angle when referencing sky waves?
The angle at which the first sky wave is returned to earth.
What is the skip zone known as?
The distance between a transmitting station and the point at which the first reflected sky wave returns to earth.
What is the dead zone?
Zone where neither sky wave or ground wave signals are present.
What is the equation to determine line of sight range for VHF/UHF range?
D = 1.25 x sqrt(H1) + 1.25 x sqrt( H2)
What is the purpose of an antenna?
Device used to radiate or receive EMR wave energy of a specific frequency.
What is the impedance of the antenna?
- The resistance to an electrical signal within an antenna.
- impedance marching is where the impedance of the antenna matches the impedance of the transmitter or receiver.
What is the radiation pattern gain?
The major direction in which EMR waves are either transmitted or received.
What is the polarisation?
- The direction in which the electrical field oscillates
- Can be Vertical, horizontal or circular
What are some factors that determine where the antennas are placed on an aircraft?
- Obstruction to signal réception
- Noise
- Aerodynamics effect
- Signal polarity
- Aircraft composition
- what antenna is used
Where is ILS antenna ideally placed?
On the front of the aircraft.
What are the different types of antenna?
- Half wave dipole
- Monopole
- Loop antenna
- Patch or plate antenna
