I would like to know why are side lobes formed along with main beam in an antenna.

I asked one of my colleague and he explained me that its due to radiation of frequencies near to the required center frequency. I'm not sure about this explanation and would like to about this in detail.

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    $\begingroup$ Welcome to hamSE, Apama B. I look forward to the myth-busting answers you will receive. $\endgroup$ – Brian K1LI Apr 4 at 16:03

Similar to optics or a water tank, waves passing through a finite aperture will produce a diffraction pattern.

To over-simplify: In the far field, the difference between the distance to the left and right side of a dipole's aperture will depend on the angle from the main beam. That difference can produce a constructive or a destructive interference, again depending on the angle. The sum of this interference pattern we call side lobes.

For the size of an antenna's aperture, see this question: Effective area of a dipole

To get rid of these side lobes, you would need something like an infinitely small radiator with no gain (which seems to have a name).

  • $\begingroup$ So, it doesn't have anything to do with out of band frequency? $\endgroup$ – Aparna B Apr 5 at 2:32
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    $\begingroup$ @AparnaB nothing at all. You get lobes at the design center frequency. $\endgroup$ – hobbs - KC2G Apr 5 at 3:18
  • $\begingroup$ A little bit. Wavelengths of different frequencies will produce different diffraction patterns. The effective aperture size also changes with frequency. So any spurious spectrum (harmonics) might have different side lobes. $\endgroup$ – hotpaw2 Apr 5 at 4:27
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    $\begingroup$ The theoretical, infinitely small radiator you describe is the "isotropic" point source. This is the "i" in "dBi" - dB over an isotropic radiator. By definition, it radiates equally in all directions, so it has zero directivity. $\endgroup$ – Brian K1LI Apr 5 at 8:54
  • $\begingroup$ Frequencies different from the design frequency will have different side lobes, but it is rare for the design frequency itself to not have side lobes too. $\endgroup$ – user10489 Apr 5 at 11:26

Here is an easy way to visualize what's going on with those pesky side lobes.

We take the case of a centerfed resonant dipole with a wire a little longer than the dipole mounted behind it by some fraction of a wavelength. That wire is intended to reflect the output of the dipole in the "back" direction (by absorbing and immediately re-radiating it) so it adds to the output in the "forward" direction.

We find that by playing with the offset distance, we can make the reflected wave be in phase with the wave train being radiated by the dipole, and get perhaps +3dB of gain in the forward direction, on-axis.

But things are different as we move off-axis. If we model each element as a point source and sweep a receiving antenna through 180 degrees in front of the element pair, there will be certain angles in that sweep where the direct and reflected waves are 180 degrees out-of-phase, and cancel out. What remains is the on-axis "main lobe" and two off-axis "minor lobes", separated from the main lobe by those "nulls" where the cancellation happens.

By adding director elements in front of the active dipole, we can further concentrate the output of the antenna in the desired direction by squeezing the side lobes inwards, further strengthening the main lobe- but at the expense of adding even more "nulls" to the radiation pattern where the phases of the waves in the beam interfere and cancel each other out. So instead of two side lobes, now we have three or four flanking the main beam.

So we see that by trying to further and further "confine" the radiation from the dipole into a collimated beam that gets stronger and stronger, we obtain more and more side lobes squeezed together around the main lobe.


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