Term
| 213-1. Which radio wave travels near the earth's surface of the earth and are greatly affected by the earth’s conductivity? |
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Definition
| 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. |
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Term
| 213-2. What are the limiting factors for direct wave communications? |
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Definition
| 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. |
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Term
| 213-3. What determines the earth’s conductivity? |
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Definition
| The type of soil and water in the propagation path. |
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Term
| 213-4. Name the radio wave used in long-distance communications. |
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Definition
| Sky wave transmissions are very effective for long-distance communications in the HF range (3 – 30 MHz). |
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Term
| 214-1. What frequency is used for ground wave propagation? |
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Definition
| Low and very low frequencies |
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Term
| 214-2. How we extend LOS distance? |
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Definition
| Increasing the height of the transmitting antenna, the receiving antenna, or both. |
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Term
| 214-3. What is the best type of surface for surface wave transmission? |
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Definition
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Term
| 214-4. What gives sky wave propagation its ability to communicate beyond the optical LOS? |
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Definition
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Term
214-5. Match the terminology in sky wave communications in column B with the characteristic it is associated with in column A. Items in column B may be used once.
Column A
Column B
____ (1) Highest frequency that allows reliable long-range HF radio communication.
____ (2) The point of the ionosphere from which a radio wave appears to have been refracted.
____ (3) The closer you operate to this frequency, the noisier is the signal.
____ (4) Roughly 85% of the MUF.
____ (5) Highest frequency at which a vertical signal is returned to earth.
____ (6) The angle of an antenna above the horizon through which an antenna radiates the largest amount of its RF energy.
a. Virtual height.
b. Critical angle.
c. Critical frequency.
d. Maximum usable frequency.
e. Lowest usable frequency.
f. Frequency of optimum transmission. |
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Definition
(1) d.
(2) a.
(3) e.
(4) f.
(5) c.
(6) b. |
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Term
| 214-6. Describe the skip zone. |
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Definition
| 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. |
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Term
| 214-7. Define skip distance. |
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Definition
| The distance from the transmitter to the point at which the refracted sky-wave first returns to earth. |
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Term
| 214-8. What is the primary loss from multihop transmission? |
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Definition
| Each time a hop is made, considerable signal strength loss occurs. This loss results primarily from absorption. |
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Term
| 214-9. What causes the cancellation and summation effects of the received signal to occur at the receiver? |
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Definition
| Because of the relative amplitude and phase differences of these various signals. |
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Term
| 214-10. Which type of fading is a function of frequency? |
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Definition
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Term
| 215-1. List the five basic regions that make up the atmosphere. |
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Definition
| The troposphere, stratosphere, ionosphere, mesosphere and thermosphere. |
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Term
| 215-2. How are long-distance HF communications made possible? |
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Definition
| By reflections/refractions of radio waves from ionized layers in the ionosphere. |
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Term
| 215-3. What causes the different ionospheric layers? |
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Definition
| The different wavelengths of ultraviolet rays expanding their energy at different heights within the atmosphere. |
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Term
| 215-4. The recombination process is dependent on what? |
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Definition
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Term
| 215-5. Name the different layers within the ionosphere? |
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Definition
| D, E, F1, F2 and the topside. |
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Term
| 215-6. How is the E layer broken down? |
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Definition
| It is broken down into a thick E2 layer and a highly variable thin layer called Sporadic E. |
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Term
| 215-7. Describe the difference between the two general types of ionospheric variations. |
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Definition
| 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. |
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Term
| 215-8. What happens to the ionospheric layers when solar activities are no longer present? |
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Definition
| 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. |
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Term
| 215-9. What is the length of the sunspot cycle? |
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Definition
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Term
| 215-10. Solar flares produce what |
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Definition
| A burst of radiation across the EM spectrum. |
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Term
| 215-11. Which layer do ionospheric storms mainly affect? |
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Definition
| The higher F2 layer, reducing its ion density. |
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Term
| 215-12. What is the exact cause of the Sporadic E? |
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Definition
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Term
| 216-1. For communications purposes, what is the usable frequency spectrum? |
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Definition
| 3 Hz through 300 GHz, and up to about 100 THz Tera Hertz. |
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Term
| 216-2. What organization regulates the use of the frequency spectrum by all nations? |
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Definition
| International Telecommunications Union (ITU). |
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Term
| 216-3. What is the audio frequency range? |
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Definition
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Term
| 217-1. How are ELF transmissions propagated? |
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Definition
| Through the earth’s substrate. |
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Term
| 217-2. What determines the range of MF propagation? |
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Definition
| Transmit output power and atmospheric conditions. |
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Term
| 217-3. What is the frequency range of the HF frequency band? |
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Definition
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Term
| 217-4. What determines the distance HF sky waves can propagate? |
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Definition
| Atmospheric conditions and the frequency used. |
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Term
| 217-5. Why are HF communications not considered suitable for critical C2 systems? |
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Definition
| The inherent vulnerability of intercept and jamming. |
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Term
| 217-6. What is the general rule to remember when using the VHF frequency band? |
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Definition
| The higher the frequency, the less power required to transmit VHF signals over a given distance. |
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Term
| 217-7. Name a satellite system that is currently using the EHF frequency range? |
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Definition
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Term
| 218-1. What is the general rule of thumb for radio wave propagation as frequencies increase from HF to VHF? |
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Definition
| Propagation takes on more of the characteristics of LOS. |
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Term
| 218-2. What is the approximate air-to-ground range of VHF/UHF communications? |
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Definition
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Term
| 219-1. What is the primary transmission path for frequencies in the SHF/EHF range? |
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Definition
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Term
| 219-2. Give an example of a SHF communications system. |
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Definition
| Defense Satellite Communications System (DSCS) III. |
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Term
| 220-1. Explain the reason for the difference in distance between the optical and radio horizon. |
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Definition
| The slight bending of the transmitted waves in the lower atmosphere. |
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Term
| 220-2. Which principle of communications was developed to overcome the LOS distance disadvantage? |
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Definition
| Forward propagation by tropospheric scatter (FPTS). |
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Term
| 220-3. Why is the principle of reflection very useful in the design of directional antennas? |
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Definition
| Reflection allows the waves to be focused into a beam. |
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Term
| 220-4. What happens to the speed of a propagated wave as the atmosphere becomes less dense? |
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Definition
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Term
| 220-5. How much farther is the radio horizon than the true horizon in a standard atmosphere? |
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Definition
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Term
| 220-6. What effect happens to the radio horizon if K experiences a significant decrease in value? |
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Definition
| It decreases along with the value of K. |
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Term
| 220-7. Which propagation characteristic permits communications in shadow regions behind obstacles? |
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Definition
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Term
| 220-8. What causes absorption of a radio signal? |
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Definition
| The presence of moisture particles such as rain, snow, and clouds in the transmission path. |
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Term
| 220-9. In super refraction, what will decrease from the standard? |
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Definition
| 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. |
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Term
| 220-10. Describe how ducting can occur. |
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Definition
| 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. |
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Term
| 220-11. What is free space loss? |
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Definition
| 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. |
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Term
| 220-12. Why is the free space path loss increases with the square of the frequency? |
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Definition
| 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. |
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Term
| 220-13. What is the major loss in satellite and tropospheric communications? |
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Definition
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Term
| 220-14. How do receivers overcome Doppler shift? |
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Definition
| The receiver needs to accommodate the maximum expected Doppler shift. It needs either sufficient bandwidth or a means of following the frequency shift. |
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Term
| 220-15. What is the average distance between stations using direct wave communications? |
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Definition
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