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BCOC - Annex B > EMS > Flashcards

EMS Flashcards

1
Q

What is propagation?

A

Movement through a medium

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2
Q

What are 3 atmospheric propagation effects?

A
  1. Reflection
  2. Refraction
  3. Diffraction
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3
Q

What is Reflection?

A

Waves that are neither transmitted or absorbed, but are reflected from the surface of the medium they encounter

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4
Q

What is Refraction?

A

When a wave passes from one medium into another medium that has a different propagation velocity, a change in the direction of the wave will occur.

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5
Q

What is diffraction?

A

The bending of a wave’s path when it meets an obstruction.

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6
Q

What ways that electromagnetic energy in the form of radio waves travel between antennas?

A
  • Ground Waves

- Sky Waves

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7
Q

What are the types of Ground Waves?

A
  • Surface Waves
  • Space Waves
  • These are waves that travel very near the surface of the earth.
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8
Q

What are sky waves?

A

These are waves that are reflected and refracted back to the Earth from the ionosphere.

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9
Q

What are surface waves?

A

Travels along surface of the Earth.

Follows curvature of Earth by diffraction; based on wavelength (larger wave lengths diffract better.)

Earth attenuates signal strength

3 Factors to consider to mitigate attenuation:
Polarization: Vertical better for surface wave.
Terrain conductivity: Fresh water best, Jungle worst.
Frequency: Lower freq = better surface wave. Generally 1.6-30 MHz

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10
Q

What are space waves?

A

Antennas need to “see” each other:
Radio Line-Of-Sight (LOS) = 4/3 of natural horizon due to refraction and diffraction.

Suffers little attenuation from ground, but susceptible to fading because multiple versions of the same wave are incident on the antenna from different paths.
Direct Path and Reflected Path

Vertically polarized antennas are vastly superior to horizontally polarized antennas for surface and space wave propagation.

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11
Q

What are sky waves?

A

Radiated upward and refracted or reflected back to Earth by the ionosphere.

Can travel great distances, even across the world.

High Frequency (HF) is extensively used by military and amateur radio operators for long distance sky wave communications.
Radiated upward and refracted or reflected back to Earth by the ionosphere.

Can travel great distances, even across the world.

High Frequency (HF) is extensively used by military and amateur radio operators for long distance sky wave communications.

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12
Q

What is SATCOM?

A

Satellite Communication

Radiated upward through the atomsphere to an orbiting satellite

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13
Q

When is SATCOM used?

A

SATCOM is used by military radio operators for long distance communications

UHF SATCOM is narrowband

SHF and UHF SATCOM are wideband

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14
Q

What is Electromagnetic Noise?

A

Like audio noise, but for radio frequencies

Makes its way into radio receivers from a myriad of sources of electromagnetism

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15
Q

What is noise floor?

A 
The “background noise” for radio frequencies. 
Signal below (or quieter than) the noise floor will not be understood
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16
Q

What is the Signal to Noise Ratio?

A

A comparison between the strength of the received signal and the strength (or loudness) of the noise floor.

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17
Q

What are two sources of Electromagnetic noise?

A
  1. Man-made: Other radios & communications systems, electronic warfare devices, vehicles, generators, etc.
    1. Natural: Thunderstorms, snow storms, cosmic radiation, etc.
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18
Q

What is electromagnetic interference?

A

When the Signal to Noise Ratio (SNR) is not good enough to receive a signal, those sources of electromagnetic noise become EMI.

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19
Q

What is the electromagnetic spectrum?

A

The entire range of electromagnetic waves arranged in order of their frequencies.

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20
Q

What frequency does the USMC use?

A 
USMC uses from 1.6 MHz to 300 GHz on the electromagnetic spectrum.
Extremely High Frequency (EHF)
Super High Frequency (SHF)
Ultra High Frequency (UHF)
Very High Frequency (VHF)
High Frequency (HF)
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21
Q

What is bandwidth?

A

Bandwidth refers to “amount of spectrum used”

kHz, MHz, GHz, etc.

22
Q

What is data rate?

A

Information carrying capacity refers to “data rate”

bps, kbps, Mbps, etc.

23
Q

What is the difference between narrowband and wideband?

A

Narrowband :
<64 kbps
Voice communications
Text-based data

Wideband:
> 64 kbps
Streaming video
Internet web browsing
File sharing
24
Q

What is the VHF frequency?

A

Officially ranges from 30-300 MHz

25
Q

What are used for VHF?

A

Use: Foundation for USMC tactical voice communications

26
Q

What are the pros and cons of VHF?

A

Advantage: Smaller antenna for mobile applications
Disadvantages:
Freqs above 30 MHz normally limited to line-of-sight (LOS)
VHF & up: not normally refracted by atmosphere and ground wave is minimal
Very crowded across the world for multiplicity of applications

27
Q

What is the range of Ultra High Frequency? (UHF)

A

Officially ranges from 300 MHz to 3 GHz

USMC often includes freqs between 225-300 MHZ

28
Q

What are some uses for UHF?

A

Uses: USMC uses for ground-to-air comm., tactical SCR SATCOM, and wideband terrestrial

29
Q

What are some pros and cons of UHF?

A

Advantages: UHF antennas are small with high information carrying capacity, and penetrate somewhat through foliage and into buildings
Disadvantages: Reliant on line of sight (LOS)

30
Q

What is the range of super high frequency? (SHF)

A

Ranges from 3-30 GHz

31
Q

What is the use of SHF by USMC?

A

Used by USMC for wideband terrestrial and satellite communications.

32
Q

What are the pros and cons of SHF?

A

Advantages: high data throughput and all weather
Disadvantages: high power and complexity, usually stationary

33
Q

What is the range of Extremely High Frequency (EHF)?

A

30-300 GHz

34
Q

What are some uses of EHF?

A

Used by USMC for satellite communications

35
Q

What are the pros and cons of EHF?

A

Advantage: High data carrying capacity
-Small beam width
-Resilient to nuclear attack
Disadvantages: highly susceptible to rain fade

36
Q

What is the ionosphere?

A

An upper region of Earth’s atmosphere approximately 30-400 miles (50-600 km) above the Earth’s surface comprised of electrically charged atoms called ions.

37
Q

What is the range of High Frequency waves?

A

Officially ranges from 3-30 MHz.
USMC includes frequencies as low as 1.6 MHz
-Our HF radios are capable down to that freq
-Easier to include in HF than to start talking about Medium Frequency (MF)

38
Q

What are the pros and cons of High Frequency waves?

A

Advantage: long haul communications
Disadvantage: Ideal HF antennas are large and time consuming to erect and susceptible to noise interference.

39
Q

What are the best used of HF?

A

Long haul narrowband on the pause

40
Q

What are the layers of the ionsphere?

A

D layer = Closest to the earth 30-55 miles
E layer = 55-90 mi above disappears at freqs during daytime, mostly absorbs
F layer = 90-375 miles (or 145-600 km) above, two sub-layers (F1 & F2) during day, one at night, most used for long-distance tactical HF communications.

41
Q

What are the factors affecting refraction and reflection in the ionsphere?

A
  1. Density of ionization
    - Time of day
    - Time of year
    - Solar cycle
    - Sunspot number
    - location
  2. Radio Wave frequency
  3. Angle of incidence
42
Q

What occurs when low density ionization goes to high density ionization?

A

waves bend back towards the earth

43
Q

What occurs when high density ionization goes to low density ionization?

A

waves bend away from Earth

44
Q

What is the skip zone in the ionsphere?

A

The zone of silence between the point where the ground wave becomes too weak for reception and the point where the sky wave is first returned to Earth

45
Q

What is the skip distance?

A

The distance from the transmitter to the point where the sky wave is first returned to Earth

46
Q

What is scintillation?

A

Rapid manipulation of UHF and L-band (GPS) radio waves caused by densely collected ions in the ionosphere that affect communicating with satellites.

47
Q

What areas are marines most concerned with satellite comms?

A

Middle East
Central America
S. E. Asia

48
Q

What are three frequencies need to be planned for?

A
  • Max Usable Freq (MUF)
  • Lowest Usable Freq (LUF)
  • Optimum Working Freq (FOT)
  • FOT = Freq of Optimum Transmission
49
Q

What are freq planning considerations of Max Usable Freq? (MUF)

A

The highest frequency that can be used for skywave communications between two locations, at a given angle of incidence and time of day.

Freqs above MUF refract too slow for given distance and will pass through ionosphere into space.

The day’s hourly MUFs will be highest in mid-day

Don’t plan on actually using a freq near the MUF. Use it to judge which lower freqs you will use.

50
Q

What are planning considerations for Lowest Usable Freq? (LUF)

A

The lowest frequency that can be used for skywave communications between two locations, at a given angle of incidence and time of day.

Freqs below LUF refract too quickly for a given distance or are absorbed by lower layers of the ionosphere.

The day’s hourly LUFs will be lowest in the middle of the night.

Don’t plan on actually using a freq near the LUF. Use it to judge which higher freqs you will use.

51
Q

What are the planning considerations for Freq Optimum Transmission?

A

Neither MUF nor LUF practical for transmissions; too unpredictable.

Roughly 85-90% of the MUF.

Closer to MUF because the lower the frequency the more signal-to-noise ratio present and the greater absorption losses.

Many different acronyms and expansions exist to describe this; OWF, FOT, etc.

BCOC - Annex B (5 decks)

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