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If we can increase the directivity of the single element antenna by increasing its electrical size then why do we need array antennas?

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    \$\begingroup\$ Arrays are just one way of increasing the size of the antenna. \$\endgroup\$
    – Dave Tweed
    Commented Jul 22, 2020 at 12:14
  • \$\begingroup\$ Also, you cannot change the direction of the beam unless you physically move the element. A multi-element antenna allows you to move the beam electronically by changing the relative phase in the element's path. \$\endgroup\$
    – SteveSh
    Commented Jul 23, 2020 at 0:52

3 Answers 3

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Array antennas are the basis for beam forming. With array antennas, you can control the directivity by driving each antenna of the array with a different phase.

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If we can increase the directivity of the single element antenna by increasing its electrical size then why do we need array antennas?

Increasing the size of a single element means it will certainly receive unwanted lower frequencies and this might make the electronics more complex.

It also means that the impedance of the antenna at the "wanted" frequency changes from the straightforward near-resistive impedance of the half-wave dipole antenna (for instance) to a less useful higher impedance as shown below: -

enter image description here

So, when the dipole is dealing with a frequency corresponding to half a wavelength, it's impedance is around 74 ohms resistive but, if the length doubled the impedance sky-rockets and this becomes a bigger problem to receive.

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Long wire antennas can do that, but only in two of 3 axis.

And the usual narrow band response of a dipole has been ruined.

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