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In the 4NEC2 program, I observed the radiation patterns for the antenna array composed of two half-wave dipoles.

I researched for different distances between the dipoles: λ/4, λ/2, λ and 2λ.Both dipoles are supplied with 1 A DC.

I found that for λ/2 the energy gain is the greatest.

Why?

EDIT Screenshots from 4NEC2

λ/4

enter image description here

λ/2

enter image description here

λ

enter image description here

2λ

enter image description here

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  • \$\begingroup\$ A picture would help plus a link to whatever 4NEC2 is. \$\endgroup\$
    – Andy aka
    Commented Dec 29, 2021 at 11:09
  • \$\begingroup\$ @Andyaka ok, I edited the post. \$\endgroup\$
    – Chuckyy
    Commented Dec 29, 2021 at 11:16
  • \$\begingroup\$ It would have been far better if you'd have kept the scaling the same on all plots. Now, it makes it really hard to differentiate. You still haven't linked to 4NEC2. \$\endgroup\$
    – Andy aka
    Commented Dec 29, 2021 at 11:47
  • \$\begingroup\$ @Andyaka "It would have been far better if you'd have kept the scaling the same on all plots." what do you mean? "You still haven't linked to 4NEC2." I added screenshot Pattern window (F4) from 4NEC2. What can I add to make my post understandable? \$\endgroup\$
    – Chuckyy
    Commented Dec 29, 2021 at 11:59
  • \$\begingroup\$ "Both dipoles are supplied with 1 A DC." Are you sure? It seems more likely that the dipoles are supplied with AC at whatever frequency makes them λ/2 long. You should also specify the orientation of the dipoles (which seem to be vertical) and the phasing of the two feeds (which seem to be in phase). Best distance depends on what polar pattern you are trying to achieve. You can also change the polar pattern by introducing a phase shift in the feeds to the two copies. You should get an interesting result for λ/4 spacing by feeding the two dipoles with a 90° phase shift between them. \$\endgroup\$
    – Graham Nye
    Commented Dec 29, 2021 at 12:00

1 Answer 1

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Because for spacings greater than lambda/2, you start to get grating lobes. These are the smaller lobes that show up away from the main axis of the beam, + and - 90 deg in these plots.

enter image description here

The grating lobes are at ~ -35 deg, + 35 deg, -145 deg, and + 145 deg.

These grating lobes take energy away from the main lobe and so reduce the gain of the main lobe. They don't come for free, and are usually undesirable in scanned array antenna systems.

Why do sidelobes appear?

Because when you combine energy from multiple antenna elements, the gain is greatest in the direction where the incoming energy is in phase when the two (or more) elements are combined. With separations greater than lambda/2, there are 2 (or more, depending on the spacing between the elements) directions in which the incoming energy is at the same phase - phase coherered is a term that's sometimes used. It's a modulo 180 deg phase thing. Hence the lambda/2 (180 deg) restriction for grating lobe free performance across the scan volume, -90 deg to + 90 deg, or - 180 deg to 0 deg in these plots.

There have been numerous posts here on this topic. Look for questions with tags like "antenna array" or "phase array". For example, look at my response here: how to understand grating lobe problem in broadside array

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  • \$\begingroup\$ Yes, I also noticed sidelobes appearing. But why? And why for λ/2 sidelobes don't appear and the gain is the greatest compared to the gain in other distances? \$\endgroup\$
    – Chuckyy
    Commented Dec 29, 2021 at 13:15
  • \$\begingroup\$ @Chuckyy The side lobe formation has to do with destructive-constructive interference from the two antennae. The gain has to do with the directivity index, or focusing the energy in a particular direction. Of course, you need to take in to consideration the wasted energy in sidelobes (no sidelobes occur in a 2-element array) and grating lobes - assuming you consider energy in side & grating lobes wasted energy. \$\endgroup\$
    – qrk
    Commented Dec 29, 2021 at 22:40

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