Comm 01 - Propagation Paths & Basic Radio Systems Flashcards
What are the different propagation paths?
Ground wave
Tropospheric Wave Ducting
Sky wave
What wave has a portion of its radiated energy affected by the presence of the earth and its features?
Ground wave
What are the two ground wave components?
Surface and space wave
Describe the path of a surface wave.
Travels along the surface of the earth and tends to curve over the horizon due to refraction
Why are ground waves only useful in the Low and Very Low Frequency (VLF)?
Attenuation of the surface wave rapidly increases as the frequency increases.
What is the max range in the MF and low HF bands for a ground wave?
100 NM
What two components make up a space wave?
Direct wave and earth-reflected wave
What is a Space wave limited to?
LOS ranges
What is Tropospheric Wave Ducting?
Path taken by the radiation that undergoes refraction and reflection in the lower levels of the atmosphere
What causes the Tropospheric Wave Ducting to refract and reflect?
Abrupt changes in temperature and moisture
What is a Sky Wave?
Waves that travel upward then reflected back to the earth by the ionosphere.
What is the frequency range for sky propogation?
300KHz to 30 MHz
What frequency bands do Sky Waves occur in?
MF to HF
Which wave propogation gives the most reliable and distant comms?
Sky Waves
What is a communication system?
A system designed for the purpose of transmitting information rom one point to another.
A basic comm system is made up of three parts, what are they?
Transmitter, receiver and antennas
What two sections is a transmitter composed of?
A radio frequency and modulator section
What does the Oscillator Generator do?
Generates EM energy at the desired frequency, known at the carrier wave. (think a blank CD)
What does the Modulator do?
Desired intelligence is entered into the modulator by means of a microphone, keyboard etc…
Intelligence is then impressed on the carrier wave by the modulator.
What are the different types of modulation?
AM and FM
What does the Amplifier do in the transmitter?
It amplifies the modulated signal and sends it to the antenna for transmission.
What components make up the receiver?
Antenna. selective circuit, detector and audio amplifiers
What does the selective circuit do?
Provides selectivity by only allowing energy of a pre-tuned frequency to pass into the receiver.
What does the detector do?
Detects and demodulates the the intelligence from the carrier wave (splits the wave in two so we can read the intelligence portion)
What do the audio amplifiers do? Why are they necessary?
Steps up the power and reproduces the intelligence signal for use. Without amplifiers, the signal would not have enough energy to be heard from the speaker.
What are the desirable receiver functions?
Intercept some of the RF energy from the transmitter
Select the correct RF from a multitude of signals
Detect and separate the intelligence from the carrier wave
Reproduce the intelligence into some meaningful form
What are the Receiver Characteristics?
Frequency coverage
Sensitivity
Selectivity
Fidelity
Describe the frequency receiver characteristic
It refers to the range of radio frequencies the receiver can handle
Receivers cannot intercept all frequencies, they are optimized for specific frequency bands
Describe the sensitivity receiver characteristic
Refers to the ability of a receiver to intercept weak signals and extract the intelligence
A highly sensitive receiver will work for both strong and weak signals
Describe the selectivity receiver characteristic
Ability of a receiver to intercept and extract intelligence from a desired signal and exclude all others.
Describe the fidelity receiver characteristic
Ability to reproduce the transmitted intelligence
Not necessary for voice comms, it is for data link
What does an antenna do? What are its functions?
Antennas couple the radio system to free space
A single antenna can both transmit and receive, but not at the same time
An antenna contains electrons that?
Oscillate at the desired frequency and send EM waves to be formed and radiated away from it
An antenna’s oscillation contains what two type of components in it’s field?
Electric (E) and magnetic (H)
What is an antenna’s gain?
The measurement of an antenna’s ability to radiate energy in a particular direction.
True or false
Gain is a function of an antenna’s size and shape
True
Describe polarization regarding an antenna’s orientation.
It is the orientation of the E field of the wave with respect to the earth.
What determines the polarization of the the radio wave?
The orientation of the transmitting antenna.
If two antennas are not close in orientation, what is the theoretical power loss?
50 - 99%
Explain the receiving antenna behaviour.
EM waves from transmitting antenna excite or oscillate the electrons in the receiving antenna.
An electric current is formed and conducted to the receiver.
Review Questions
Describe Tropospheric Wave Ducting
Part of the radiation that undergoes refraction and reflection in the lower levels of the atmosphere.
Provides extra range to LOS comms
Unpredictable
Review Questions
What three parts make up a basic radio comm system?
Transmitter, receiver and antenna
Review Questions
What is the frequency produced by the RF section of a transmitter called?
Carrier wave
Review Questions
What does the antenna system do?
Couples the radio to free space by radiating RF energy from the transmitter and capturing the RF energy by the receiver.
Review Questions
What are the four characteristics of a receiver?
Frequency coverage
Sensitivity
Selectivity
Fidelity
