113 Flashcards
113.1 Define each band of the electromagnetic spectrum and describe what signals are commonly used in each. EHF; SHF; UHF; VHF; HF; MF; LF; VLF; ULF; SLF; ELF
- EHF Extremely High Frequency: 30 GHZ - 300 GHZ
- SHF Super High Frequency: 3 GHZ - 30 GHZ
- UHF Ultra High Frequency: 300 MHZ - 3 GHZ
- VHF Very High Frequency: 30 MHZ - 300 MHZ
- HF High Frequency: 3 MHZ - 30 MHZ
- MF Medium Frequency: 300 KHZ - 3 MHZ
- LF Low Frequency30 KHZ: 30 KHZ - 300 KHZ
- VLF Very Low Frequency: 3 KHZ - 30 KHZ
- ULF Ultra Low Frequency: 300 HZ - 3 KHZ
- SLF Super Low Frequency: 30 HZ to 300 HZ
- ELF Extremely Low Frequency: 3 HZ – 30 HZ
113.2 Describe the communication capabilities of transmissions in each of the bands below: EHF; SHF; UHF; VHF; HF; MF; LF; VLF; ELF
a. EHF – Protected Band. Secure Voice and DATA SATCOM, IP Services
b. SHF – Wideband. High-bandwidth Data SATCOM, IP Services (DoDWB and CBSP)
c. UHF – Narrownband. Used for both LOS and SATCOM voice and data services.
d. VHF – Ideal for LOS voice comms. FM radio operates on this band.
e. HF – Around since WWI. Successful transmission of HF signals over a long distance is dependent upon refraction of radio waves by layers of the ionosphere.
f. MF - Includes the international distress frequencies. FM radio operated on this band.
g. LF - Has been used for comms since the advent of radio. Seriously hampered by atmospheric noise (at low geographical latitudes), but Low frequency waves are not so seriously affected during periods of ionospheric disturbance when comms at high frequencies are.
h. VLF - Used for submarine communications.
i. ELF - Used for submarine communications. Not used by USA since 2004.
113.3 Define and discuss the following terms: Frequency; Wave Length; Ducting; Refraction; Multiplexing; Bandwidth; Keying; Azimuth Angle; Elevation Angle
a. Frequency - The number of cycles per unit of time, usually expressed in hertz.
b. Wavelength - The distance in space occupied by one cycle of a radio wave, measured in distance from the leading edge of one cycle to the corresponding point on the next cycle.
c. Ducting - Trapping of an RF wave between two layers of the earth’s atmosphere or between an atmospheric layer and the earth.
d. Refraction - Occurs when a wave traveling through two different mediums passes through the BOUNDARY of the mediums and bends toward or away from the NORMAL.
e. Multiplexing - The simultaneous transmission of a number of intelligible signals in either or both directions using only a single RF carrier.
f. Bandwidth - The frequency range occupied by a modulated carrier wave.
g. Keying - The carrier is shifted between two differing frequencies by opening and closing a key.
h. Azimuth Angle - Angular measurement in the horizontal plane in a clockwise direction.
i. Elevation Angle - The angle between the horizontal plane and the line of sight.
113.4 Discuss the following terms and give a description of what each means in relation to the atmosphere: Troposphere; Stratosphere; Ionosphere;
a. Troposphere High concentrations of moisture at this level cause two common effects. Either a signal is attenuated and passed on to the higher atmospheres and into space, or during stable weather when the troposphere is warmer than normal, it will result in ‘ducting’ which carries some VHF/UHF signals significantly around the horizon of the world, all the way to the end of the warm front. This has been shown to be possible to carry a signal from hundreds to even thousands (3000) of miles in perfect conditions.
b. Stratosphere The stratosphere temperature is nearly constant and there is little water vapor present. This results in little to no effect on all radio waves.
c. Ionosphere This layer is even better than the troposphere at allowing long distance communications because of the interaction of highly charged particles on MF and HF signals. These signals can reflect RF of these frequencies off the boundary enabling trans-continental communication via ‘skywave’ or ‘skip-waves’. The ideal conditions of the ionosphere required for this are intermittent (best at night and in the winter). ELF through LF are always guided by the ionosphere.
113.5 Discuss the following terms and give a description of what each means in relation to the atmosphere. Frequency; Radio waves; Propagation Paths
a. Frequency Cycles per unit time. Specific frequencies are known to reflect/refract off sections of the atmosphere allowing indirect, over the horizon comms without the use of satellites.
b. Radio Waves Are keyed electromagnetic patterns that enable communication at great distances via line of sight, satellite relay, and by bouncing the signal off of the atmosphere.
c. Propagation Paths This is the path a signal has taken from a transmitter to a receiver. It includes all the bends, shifts, and bounces the signal may have endured in passing through, across, and around different densities of atmosphere and mediums in the atmosphere
113.6 Describe the following modulation types and their common usage: USB; LSB; CW; AM; FM
a) USB - Upper Sideband. All of the sum frequencies above the carrier wave.
b) LSB - Lower Sideband. All the difference frequencies below the carrier wave.
c) CW - Continuous Wave. An electromagnetic wave of constant amplitude and Frequency.
d) AM - Amplitude Modulation. Any method of modulating an electromagnetic carrier frequency by varying its amplitude in accordance with the message intelligence that is to be transmitted.
e) FM - Frequency modulation. Conveys information over a carrier wave by varying its frequency.
113.7 Define the following terms as they apply to RADAR: PRI; PRF; PRT; RF; BW; CW; PW; Scan
a. PRI - Pulse Repetition Interval. The duration between the start of one pulse and the start of the next pulse (the reciprocal of PRF).
b. PRF - Pulse-Repetition Frequency. The number of pulses a radar transmits each second, measures in pulses per second.
c. PRT Pulse Repetition Time. The same as the PRI, used interchangeably
d. RF - Radio Frequency. Any frequency or EM energy capable of propagation into space.
e. BW - Band Width. The section of the frequency spectrum that specific signals occupy.
f. CW - Continuous Wave. An electromagnetic wave of constant amplitude and frequency.
g. PW - Pulse Width. Duration of time between the leading and trailing edges of a pulse.
h. 8. Scan - The time it takes an RF beam to complete one search cycle of the scan pattern.
113.8 Define the following: Half Duplex; Full Duplex; Semi-duplex; Simplex; Broadcast
a. Half Duplex - This mode has one-way flow of information between terminals. Technical arrangements often permit transmission in either direction, but not simultaneously. This term must be qualified to show s/o (send only), r/o (receive only), or s/r (send or receive).
b. Full Duplex - This is a method of operation in which telecommunications between stations takes place simultaneously in both directions using two separate frequencies. In other words, a ship may send and receive different messages at the same time. The term “full duplex” is synonymous with “duplex.”
c. Semi-duplex - communication is dual-simplex one end and full duplex the other.
d. Simplex - This mode uses a single channel or frequency to exchange information between two or more terminals. Communications is in one direction only.
e. Broadcast - within a communications system, allowing the transmission of signals in all directions with all other units receiving simultaneously.
113.9 Describe modulation and demodulation for signals analysis.
- Modulation is the process of varying one or more properties of a periodic waveform, called the carrier signal, by overlaying a data signal, usually in the form of FM or AM signal to make a data signal. Examples or which are AM and FM radio.
- Demodulation is the process of reversing this. The goal is to extract the data from the analog signal.
