Antenna array is a broad term and it encompasses at least the following concepts:
- MIMO or massive MIMO: for spatial multiplexing
- beamforming: increasing the SNR (at transmitter or receiver) by aligning the signals properly
- phased array: special case of beamforming for narrowband signals in which the phase can be adjusted to align the signals. Mainly used in transmitters to stear a beam into a certain direction without mechanical movements. Used in RADAR.
Please correct this list if it is wrong or if I have forgotton something.
I could also think of cases like increasing bandwidth by having many narrowband antennas in parallel – not sure if this is done in practice?
It is well known that the size of an antenna has to be on the order of lambda/2 but conventionally larger than ~lambda/10.
Suppose for a moment I would have a hypothetical isotropic radiator (an infinitely small antenna able to radiate at any desired frequency).
Are there upper (or lower limits) on the distance between the individual antenna elements in antenna arrays (for the cases listed above)?
Stated differently, can antenna arrays be decreased in size if the antenna elements themselves can be decreased in size?
Best Answer
The antenna array spacing and thus size is usually based on the wavelength of the signals of interest, not the radiating element length.
It's a geometry problem in the far field, with the ratio between the antenna separation and the wavelength being the interesting parameter.
Constructive and destructive interference contributes to the pattern and gain that can be accomplished with the array. If the elements are much closer together than lambda/10, the geometric patterns of wavefronts for each antenna are too similar to create useful interference patterns, and the array acts more like a single antenna at those separations and frequencies. As the antennas are separated into each others far-field, the number of lobes in the pattern increases, thus managing and aiming the pattern becomes far more complicated.