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Exam Feedback 7, Q48 (NDBs)

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  • Exam Feedback 7, Q48 (NDBs)

    Which of the following is correct regarding the range of an NDB?

    (a) The range is limited to the line of sight
    (b) Aircraft height is not limiting for the reception of signals from the NDB
    (c) The range of an NDB will most likely increase at day time compared to night time
    (d) The transmitter power of an NDB station has no effect on the range

    I went for (c) ('Night effect'), answer stated as (b). ADF is more effective over smooth, flat surfaces, and at height. Does this not imply that range will impeded for aircraft flying at low altitudes?

    Ex-BGS student, fATPL, DCFI(A) Atlantic Flight Training, Coventry

  • #2
    I agree with you. Answer b does not cater for dead space/skip distance.


    • #3
      What they're going for here is an understanding that the primary propogation path of NDB signals is by surface wave, not line-of-sight.

      The issue of sky waves is thorny. In the daytime there aren't any significant sky waves because the signals (lower frequency that HF comms) are attenuated. At night time there are sky waves but they tend to arrive beyond the surface wave (weaker ionosphere, less refraction and less attenuation) leading to a dead space. At dawn and dusk they are just strong enough to interfere with the surface wave and just bent enough to interfere with the surface wave, night effect. You could argue that night effect reduces the effective range at dawn and dusk but you could alspo argue that this doesn't, paradoxically, happen at night where the range might be considered to be extended by the skywaves.


      • #4
        I am a bit confused!
        I have three theory questions, if someone can help!
        Question 1) Night effect is caused by :
        a) The interaction between the ground wave and the sky wave when out of phase hit the antenna?
        b) The "Strike" of only the sky wave on the antenna? Theory says that skywaves tend to strike the top of the loop aerial giving a current flow where none would otherwise be present giving a current flow. This causes, the ADF needle hunting and reduced bearing accuracy.
        c) a) an b)

        Question 2) The hunting of the ADF needle and the fading of the audio signals is caused by only b)? by a)? or a) and b) cause needle hunting?

        Question 3) Since the skywave propagation is longer than the ground wave (and that's why we have the dead space), when or where the two waves interact? At dawn and at dusk theory says. What happens then? Dead space is nill?

        Thanks in advance