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Here Albert Helfrick speaks about 40% modulation depth for each tone in a localiser radio signal. However latest manuals of ILS manufacturers and google source speaks about 20% modulation depth, not 40%. Albert also says that modulation depth becomes 20% at the centreline of runway.

My question is whether Albert is correct or manufacturers like NORMARC, Thales and SELEX are correct? If Albert is correct then how does 16,20,24 element log periodic dipole array help in achieving this modulation depth variation?

I would appreciate if someone with proper explanation comes up. And this question is not duplicate so please do not confuse it with other modulation varying behaviour of questions asked on this forum.

  • $\begingroup$ have you looked at @mins answer to this question? $\endgroup$
    – Gerry
    May 25, 2018 at 15:48

1 Answer 1


Mod Depth (more commonly called modulation index in electrical engineering) is the amount a carrier signal is modulated. Mod depth has nothing to do with antenna configuration and is set by the transmitter (or more precisely the modulator). The log periodic antenna controls the width of the various beams. A larger antenna means a smaller beam and smaller beams mean you will have more precision on an ILS approach. As far as the discrepancy between the 40% and 20% mod depth for ILS systems I will always believe the manufacture. It seems like the documentation you posted is describing a simple hypothetical setup to help you understand the concept. I believe most ILS systems use more than two beams. The degree of mod depth shouldn't matter too much as long as your Signal to Noise Ratio is high so I wouldn't get caught with the amount of mod depth in ILS systems.

  • $\begingroup$ DLH sorry but its navigation my friend and this concept is different to your electrical equipments stuff !!! I fully understand the wider the antenna the narrower the beam but feed of antenna in ils has strong role in creating mod depth changing effect with respect to azimuth. I am very certain about it. Donot confuse this concept with general modulation index my friend.people dealing with navigation stuff would second my opinion i am sure. $\endgroup$
    – Rumi
    Apr 19, 2018 at 17:05
  • $\begingroup$ Rumi. Perhaps I misunderstood your question. If you are talking about the mod depth variation between the two beams (i.e. space modulation) Then the characteristics of the antennas does matter. I thought you were referring to the transmitted signals of the two beams and their mod depth (which is different than space modulation). Antennas themselves cannot modulate a signal, that was all I was trying to say. I am sorry if I misunderstood your question. $\endgroup$
    – DLH
    Apr 19, 2018 at 18:04
  • $\begingroup$ DLH i appreciate now lets focus and hope someone can comeup with best explanation. $\endgroup$
    – Rumi
    Apr 19, 2018 at 18:20

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