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?