3D1X3 Self Test Questions Vol. 2 Unit 2

Front 1

Which radio wave travels near the earth's surface of the earth and are greatly affected by the
earth’s conductivity?

Back 1

Since ground waves travel near the surface of the earth, they’re greatly affected by the earth’s conductivity
and by any obstruction (such as mountains or buildings) on its surface.

Front 2

What are the limiting factors for direct wave communications?

Back 2

Direct waves continue to travel in a straight line until they are interrupted by an object or weaken over a
great distance. The average distance of direct wave communications is therefore limited by the height of the
transmit or receive antenna.

Front 3

What determines the earth’s conductivity?

Back 3

The type of soil and water in the propagation path.

Front 4

Name the radio wave used in long-distance communications.

Back 4

Sky wave transmissions are very effective for long-distance communications in the HF range (3 – 30 MHz).

Front 5

What frequency is used for ground wave propagation?

Back 5

Low and very low frequencies.

Front 6

How we extend LOS distance?

Back 6

Increasing the height of the transmitting antenna, the receiving antenna, or both.

Front 7

What is the best type of surface for surface wave transmission?

Back 7

Seawater.

Front 8

What gives sky wave propagation its ability to communicate beyond the optical LOS?

Back 8

Refraction.

Front 9

Describe the skip zone.

Back 9

The area between the most distant point reached by the ground waves of a particular signal and the point at
which the ionospheric wave first returns to the earth. In this zone, you would experience a zone of silence
because no radio signals are received.

Front 10

Define skip distance.

Back 10

The distance from the transmitter to the point at which the refracted sky-wave first returns to earth.

Front 11

What is the primary loss from multihop transmission?

Back 11

Each time a hop is made, considerable signal strength loss occurs. This loss results primarily from
absorption.

Front 12

What causes the cancellation and summation effects of the received signal to occur at the
receiver?

Back 12

Because of the relative amplitude and phase differences of these various signals.

Front 13

Which type of fading is a function of frequency?

Back 13

Selective fading.

Front 14

List the five basic regions that make up the atmosphere.

Back 14

The troposphere, stratosphere, ionosphere, mesosphere and thermosphere.

Front 15

How are long-distance HF communications made possible?

Back 15

By reflections/refractions of radio waves from ionized layers in the ionosphere.

Front 16

What causes the different ionospheric layers?

Back 16

The different wavelengths of ultraviolet rays expanding their energy at different heights within the
atmosphere.

Front 17

The recombination process is dependent on what?

Back 17

Time of day.

Front 18

Name the different layers within the ionosphere?

Back 18

D, E, F1, F2 and the topside.

Front 19

How is the E layer broken down?

Back 19

It is broken down into a thick E2 layer and a highly variable thin layer called Sporadic E.

Front 20

Describe the difference between the two general types of ionospheric variations.

Back 20

Regular variations can be predicted in advance with reasonable accuracy. Irregular variations are those that
result from abnormal variations and cannot be predicted in advance.

Front 21

What happens to the ionospheric layers when solar activities are no longer present?

Back 21

When solar activity is no longer present the D, E, and F1 layers disappear, leaving only the F2 layer. The
F2 layer decreases in altitude with the setting sun, and combines with the remnants of the F1 layer to form a
single nighttime F layer.

Front 22

What is the length of the sunspot cycle?

Back 22

Every 11 years.

Front 23

Solar flares produce what?

Back 23

A burst of radiation across the EM spectrum.

Front 24

Which layer do ionospheric storms mainly affect?

Back 24

The higher F2 layer, reducing its ion density

Front 25

What is the exact cause of the Sporadic E?

Back 25

Its not known.

Front 26

For communications purposes, what is the usable frequency spectrum?

Back 26

3 Hz through 300 GHz, and up to about 100 THz Tera Hertz.

Front 27

What organization regulates the use of the frequency spectrum by all nations?

Back 27

International Telecommunications Union (ITU).

Front 28

What is the audio frequency range?

Back 28

15 Hz and 20,000 Hz.

Front 29

How are ELF transmissions propagated?

Back 29

Through the earth’s substrate.

Front 30

What determines the range of MF propagation?

Back 30

Transmit output power and atmospheric conditions.

Front 31

What is the frequency range of the HF frequency band?

Back 31

3–30 MHz.

Front 32

What determines the distance HF sky waves can propagate?

Back 32

Atmospheric conditions and the frequency used.

Front 33

Why are HF communications not considered suitable for critical C2 systems?

Back 33

The inherent vulnerability of intercept and jamming.

Front 34

What is the general rule to remember when using the VHF frequency band?

Back 34

The higher the frequency, the less power required to transmit VHF signals over a given distance.

Front 35

Name a satellite system that is currently using the EHF frequency range?

Back 35

Milstar.

Front 36

What is the general rule of thumb for radio wave propagation as frequencies increase from HF to
VHF?

Back 36

Propagation takes on more of the characteristics of LOS.

Front 37

What is the approximate air-to-ground range of VHF/UHF communications?

Back 37

150 miles.

Front 38

What is the primary transmission path for frequencies in the SHF/EHF range?

Back 38

The direct wave.

Front 39

Give an example of a SHF communications system.

Back 39

Defense Satellite Communications System (DSCS) III.

Front 40

Explain the reason for the difference in distance between the optical and radio horizon.

Back 40

The slight bending of the transmitted waves in the lower atmosphere.

Front 41

Which principle of communications was developed to overcome the LOS distance disadvantage?

Back 41

Forward propagation by tropospheric scatter (FPTS).

Front 42

Why is the principle of reflection very useful in the design of directional antennas?

Back 42

Reflection allows the waves to be focused into a beam.

Front 43

What happens to the speed of a propagated wave as the atmosphere becomes less dense?

Back 43

It increases.

Front 44

How much farther is the radio horizon than the true horizon in a standard atmosphere?

Back 44

33 percent.

Front 45

What effect happens to the radio horizon if K experiences a significant decrease in value?

Back 45

It decreases along with the value of K.

Front 46

Which propagation characteristic permits communications in shadow regions behind obstacles?

Back 46

Diffraction.

Front 47

What causes absorption of a radio signal?

Back 47

The presence of moisture particles such as rain, snow, and clouds in the transmission path.

Front 48

In super refraction, what will decrease from the standard?

Back 48

If the atmosphere’s temperature increases with height (inversion) and/or the water vapor content decreases
rapidly with height, the refractivity gradient will decrease from the standard.

Front 49

Describe how ducting can occur.

Back 49

During a temperature or humidity inversion, thicker air is on top instead of on the bottom. Instead of
downward, a radio wave entering this inversion is bent upwards, out of the LOS transmission path. If
propagated radio waves encounter another atmospheric layer above the inversion layer, they could be
refracted (bounced) back and forth between the boundaries of the two layers.

Front 50

What is free space loss?

Back 50

If radio waves could originate at a center source in free space, they would spread out in ever-growing
spheres from the source. In free space the intensity of the field of the radio wave decreases directly with the
distance from the source. This decrease in field strength is caused by spread of wave energy over larger and
larger spheres as the distance from the source in increased.

Front 51

Why is the free space path loss increases with the square of the frequency?

Back 51

The frequency dependence solely based on the decreasing effective aperture of the receiving antenna as the
frequency increases. This is because the physical size of an antenna type is inversely proportional to
frequency.

Front 52

What is the major loss in satellite and tropospheric communications?

Back 52

Free space loss.

Front 53

How do receivers overcome Doppler shift?

Back 53

The receiver needs to accommodate the maximum expected Doppler shift. It needs either sufficient
bandwidth or a means of following the frequency shift.

Front 54

What is the average distance between stations using direct wave communications?

Back 54

20 to 30 miles.