Why are there multiple side lobes in the glide slope signal from an ILS?

  • $\begingroup$ Hi and welcome to the site. I have edited your question to be a bit more adequate. You can also try the physics or amateur radio stack exchange sites, as this question is also firmly in their domains (the reason has to do with antenna radiation patterns) $\endgroup$ – AEhere supports Monica May 14 '19 at 17:16

All directional antennas have sidelobes (and all real antennas are at least somewhat directional). I won't get into detail of the fundamental physics reasons behind that, as that's beyond the scope of the aviation stack exchange, but briefly it's related to diffraction and the Fourier relationship between the aperture distribution and the far-field radiation pattern.

Various design techniques can reduce the amplitude of the sidelobes, sometimes to the point where they're undetectable or at least insignificant. However, these all involve tradeoffs that compromise other performance parameters, size or cost, so if the sidelobes aren't problematic or can be easily avoided operationally, it's not worth trying to suppress them. Also, most glideslope antennas use passive phased-array techniques, and phased arrays are especially prone to sidelobe generation (called grating lobes in that case).

  • $\begingroup$ If all antenna have side lobes then why we don’t have it in localiser signal in same strength as they are in glideslope antenna. In localiser, side lobes strength is quite less as compared to glideslope . Why? $\endgroup$ – Vikas Singh May 16 '19 at 0:38
  • $\begingroup$ @Vikas All antennas have sidelobes but as I stated, some are much worse than others. Localizer antennas and glideslope antennas operate at significantly different frequencies and have very different construction from each other. I'd suggest asking on the Electrical Engineering stack exchange if you'd like to get more into the details of antenna design (and do some background reading beforehand, perhaps Stutzman or Carr) $\endgroup$ – pericynthion May 16 '19 at 1:02
  • $\begingroup$ Thanks buddy for clarification $\endgroup$ – Vikas Singh May 16 '19 at 16:35

The ILS transmitting antennas looks to be an array of yagi antennas. Those antennas should have a tight beamwidth, but the penalty for being able to "squirt" energy in a single direction is that you have multiple sidelobes coming off the side and back side of the antenna. The more antenna elements you add to a single antenna, the more lobes you'll get.

Look at figure 2.1, then compare figure 3.2 and 3.3 at http://www.researchinventy.com/papers/v2i12/E0212026035.pdf

You'll probably get a more in depth answer from the amateur radio guys who are more experienced that I am.


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