B15 Wave Propagation Modes — Study Guide

Acronyms first

Key facts

1. A wave striking a surface larger than its wavelength and bouncing off:

   Reflection

2. A wave bending as it passes into a medium of different density:

   Refraction

3. A wave bending and spreading around an obstacle or through an opening:

   Diffraction

4. Diffraction over a sharp ridge or building corner:

   Knife-edge diffraction

5. Law of reflection:

   Angle of incidence = angle of reflection

6. Scenario: your signal bounces off a mountain face back toward a valley.

   Reflection

7. Scenario: changes in air temperature, pressure and water vapor bend your signal.

   Refraction

8. Scenario: you get a weak signal behind a sharp ridge with no line of sight.

   Knife-edge diffraction

9. Cost of knife-edge diffraction:

   diffraction loss (attenuation)

10. Ground wave travels…

    along or near Earth's surface, following its curvature

11. Ground wave is mainly used for…

    LF and MF signals

12. Ground wave works mainly by…

    diffraction and induced ground currents

13. Signals below about ____ diffract well around Earth's curvature.

    2–3 MHz

14. 'Wave tilt' happens because…

    ground currents slow the bottom of the wavefront, tilting it to hug the terrain

15. Typical VLF ground wave distance:

    1,000–5,000 miles

16. Typical LF ground wave distance:

    500–2,000 miles

17. Typical MF ground wave distance:

    100–1,000 miles

18. Which surface lets ground waves travel farthest?

    Saltwater (high conductivity)

19. For ground wave range, lower frequency gives…

    better range

20. Military advantage of ground wave:

    Hard to jam over large areas; works in all weather day or night

21. Order the atmosphere layers from the surface upward.

    Troposphere → Stratosphere → Mesosphere → Thermosphere → Exosphere

22. Layer where most weather occurs:

    Troposphere

23. Layer containing the ozone that absorbs UV:

    Stratosphere

24. Layer where meteors burn up:

    Mesosphere

25. Layer with auroras and satellites:

    Thermosphere

26. The troposphere extends from the surface to about…

    7 miles (12 km)

27. The troposphere holds about what share of the atmosphere's water vapor?

    99%

28. The ionosphere spans mainly the…

    Mesosphere and Thermosphere

29. Order the ionospheric layers from lowest to highest.

    D → E → F1 → F2

30. D-layer altitude:

    30–56 miles (50–90 km)

31. E-layer altitude:

    56–93 miles (90–150 km)

32. F1-layer altitude (daytime):

    ~130 miles (210 km)

33. F2-layer altitude:

    190–280 miles (300–450 km)

34. Which layer completely disappears at night?

    D-layer

35. Which layer is highest, most dense, and the main driver of long-distance HF?

    F2

36. During the day the F-layer…

    splits into F1 and F2

37. Which layer ABSORBS lower frequencies during the day?

    D-layer

38. HF rule of thumb:

    Higher frequencies by day, lower frequencies at night

39. Scenario: it's 0200 and your daytime HF frequency isn't reaching. Best move?

    Drop to a lower frequency

40. Sky wave propagation typically uses which band?

    HF (3–30 MHz)

41. Thin, highly ionized patches in the E-layer that unexpectedly reflect signals long distances:

    Sporadic-E

42. Tool that gives vertical soundings of critical frequencies:

    Ionosonde

43. The LOWEST frequency for reliable skywave between two points:

    LUF

44. The HIGHEST frequency the ionosphere will refract back over a path:

    MUF

45. The best reliable working frequency, about 85% of MUF:

    OWF

46. LUF is set mainly by…

    absorption in the D-layer

47. Scenario: your HF frequency is ABOVE the MUF. What happens?

    It passes through the ionosphere into space — contact lost

48. Scenario: your HF frequency is BELOW the LUF. What happens?

    The D-layer absorbs it — weak or no signal

49. Scenario: MUF is 20 MHz. Calculate the OWF.

    17 MHz

50. Scenario: MUF is 10 MHz. OWF?

    8.5 MHz

51. VHF/UHF signals trapped in an atmospheric layer and carried far beyond LOS:

    Tropospheric ducting

52. Signals scattered off turbulence, water vapor or temperature gradients for BLOS links:

    Tropospheric scattering (troposcatter)

53. Troposcatter typical distance:

    60–620 miles (100–1000 km)

54. Space wave (direct wave) travels…

    in a straight line from transmit to receive antenna

55. Ideal propagation mode for satellite communications:

    Space wave

56. A space wave requires ionospheric refraction.

    False

57. Terrestrial LOS range for antennas 10–100 m high:

    25–62 miles (40–100 km)

58. What limits LOS range the most?

    Earth's curvature (and obstacles)

59. Scenario: a ridge blocks your UHF link. Standard fixes:

    Elevate antennas or use a repeater