Fundamentals of Communications Electronic Engineering
Define the term Frequency.
The number of complete oscillations an electromagnetic wave makes per second.
Define the term Oscillation.
The flow of electrical energy from a minimum to a maximum and back to a minimum.
Define the term Modulation.
Modulation is the process of imposing information on a carrier.
What are the four major modulation techniques?
Amplitude Modulation (AM)
Frequency Modulation (FM)
What are the key points to Amplitude Modulation (AM)?
The Amplitude is varried, the frequency is not.
Information is added to the carrier After the oscillator.
Low Noise Tolerance
Upper Side Band (USB) and Lower Side Band (LSB)
What are the key points to Frequency Modulation (FM)?
The Frequency is varied, the amplitude is not.
Information is added to the carried Before the oscillator.
High Noise Tolerance
Infinite diminishing sidebands
Define Simplex reguarding Block III use.
Transmit only operations.
Define Half-Duplex reguarding Block III use.
Transmit or Receive operations, but not simultaneously.
Define Duplex reguarding Block III use.
Transmit and Receive operations simultaneously.
What are two important properties of any receiver?
Selectivity - Selecting and reproducing a desired signal,(filtering unwanted signals).
Sensitivity - Reproducing a weak signal.
Mixing two frequencies to produce four outputs.
-The two original frequencies
-The sum of the two frequencies
What is the most commonly used IF frequency in the military?
What does the Limiter do?
A Limiter removes amplitude variations from an FM signal.
What does the Discriminator do?
Removes the signal from the sidebands of an FM signal.
What is Single Sideband Suppressed Carrier (SSBSC) communications?
Transmitting information using a single sideband of an AM signal, while supressing the carrier wave.
What are three Single Sideband (SSB) Advantages?
More efficient power distribution
Improved Signal to Noise Ratio (S/N R)
What are three Single Sideband (SSB) Disadvantages?
Complexity of the design
Requires extreme frequency stability
With reference to Block III, define Trunking.
Provides access to many clients, by sharing numerous frequencies. Managed by a site controller.
Define External Noise, and give three examples.
Noise generated outside of the receiver.
What is the primary factor in Internal Noise, and what are the names of the two formulas to calculate it?
- Ohm's Formula
- Receiver Noise Formula
What is used to calculate AM Receiver Sensitivity?
Noise Threshold (Tn)
What is used to calculate FM Receiver Sensitivity?
Fade Margin Threshold (Tfm)
A metallic device used for sending and receiving radio waves.
What is the definition of an efficient antenna?
Smoothly transmits all power it receives from the transmition line and radiates it in the proper direction.
Horizontal half-wave antenna.
Short-medium length Sky wave paths.
Most common field expedient wire antenna.
Straight flexible rod mounted on a ground plane.
Most common monopole antenna.
One-half wavelength or greater.
Considered long compaired to a wavelength.
An antenna with a flaring metal waveguide shaped like a horn to direct the radio waves.
Commonly used for UHF, Microwave and 300MHz and above.
An antenna that is wound into the form of a helix.
An antenna that uses a parabolic reflector (commonly called a dish) to direct radio waves.
An antenna Reflector
A device that reflects electromagnetic waves.
An antenna Array
Two or more antennas used together to make a more powerful, more controled, and/or directional antenna.
What two fields make up the electromagnetic wave of an antenna?
Electric Fields (E-lines) - Voltage in
Magnetic Fields (H-lines) - Current through
- Always Perpendicular to each other. ( + )
Define Vertical Polarization.
When the E-lines are Perpendicular to the Earth's surface.
Define Horizontal Polarization.
When the E-lines are Parallel to the Earth's surface.
Define Electromagnetic Wave Propagation
The movement of radio waves through the Earth's atmmosphere.
Define Free Space
Empty space with no free electrons, ions, mountains, buildings, vegitation, etc...
The affect of light and radio waves to change directions when passing from one medium to another.
The affect of light and radio waves to bend around a solid object.
- The lower the frequency, or longer the wavelength, the greater the bend of the wave.
Discuss Knife-Edge Diffraction
A propagation mode where radio waves are bent around sharp objects.
Define Path Loss (Path Attenuation)
The reduction in power density of an electromagnetic wave as it propagates through space.
The propagation affect that is caused by radio signals reaching a receiving antenna by two or more paths.
What are the two Free Space Loss formulas?
FSL = 37 + 20LogDM(Miles) + 20Logƒ(MHz)
FSL = 32.5 + 20LogDK(Kilometers) + 20Logƒ(MHz)
What is a Ground Wave?
Any type of radio wave that does not use the ionosphere.
What is a Sky Wave?
Any type of radio wave that uses the ionosphere.
What is a Direct Wave?
A Ground Wave that travels directly from the transmit antenna to the receive antenna.
What is a Reflective Wave?
A radio wave that reaches the receive antenna after it has been reflected from the surface of the Earth.
What is a Surface Wave?
A radio wave that follows the curvature of the Earth.
What is a Troposphere Wave?
A radio wave that is reflected off of the troposphere (the level below the stratosphere).
Discuss Anomalous Propagation
Weather phenomena that causes RF energy to propogate in an abnormal manner.
What are four precipitation related forms of Anomalous Propagation?
Atmospheric temperature inversions that cause channels, or ducts of cool air to be sandwiched between the Surface of the Earth and a layer of warm air, or between two layers of warm air. Ducting can carry Line of Sight (LOS) signals great distances.