What are smart antennas? How are they used in amateur radio, and how might they better than conventional antennas?
"Smart antennas" are antenna arrays with software-controlled phasing.
Antenna arrays are commonly used in the amateur radio hobby, such as in Yagis, four-square antennas, etc. However, the phasing for these arrays is either fixed (as in the Yagi) or sometimes selectable between a few configurations via a switch (as in the four-square). The individual antennas combine into a single feedline that runs to the receiver.
Traditionally these antennas are adjusted by manual control with a rotator, or by a manual switch. If this control is instead done by software, the antenna is said to be "smart". Software control allows the adjustment to be performed automatically by feedback from the receiver.
It's also possible to run a feedline to each element of the array, and then combine the signals from each in software. This allows for a much higher degree of flexibility and complexity. This is called an adaptive array.
One application is the development of software to dynamically determine the optimal weights for each array element. Numerous algorithms exist. In contrast with a four-square which typically allows selecting 1 of 4 possible arrangements to optimize reception in a specific direction, software phasing allows an infinite selection of weights which may not simply optimize gain in a particular direction. They may optimize to place a source of interference in a null, for example. Furthermore they are not limited to just one phasing at a time: the software demodulator may use one set of weights to receive a station in one direction, while simultaneously using a different set of weights for another station in a different direction.
Another application is spatial multiplexing. Software can find several orthogonal paths between stations, a trivial one being vertical versus horizontal polarization, though in practice multipath propagation creates opportunities for even more orthogonal paths. This allows a reduction in spectrum usage by sending multiple streams on the same frequency at the same time.
These technologies are ubiquitous even in consumer equipment, for example cell phones and Wi-Fi.
Unfortunately, application in the amateur radio hobby is not common. I've read of some people experimenting a little, for example LZ1AQ has some experiments switching between vertical and horizontal polarization. It should be possible to extend software like fldigi and wsjt to use inputs from multiple antennas to improve reception, though to my knowledge there's no commonly used amateur radio software that does so.
Smart antennas, the software controlled ones with directional phased arrays, are typically too expensive for the radio amateur.
Additionally, benefits of a smart antenna for simplex communication typically does not outweigh the cost of implementing such an antenna system. Variable phase adjusters are extremely expensive.
Typically they're used for detection purposes, since the beam is very quickly steerable.
The closest I've ever seen, closest to amateur radio, is here: https://hackaday.com/2015/04/07/build-a-phased-array-radar-in-your-garage-that-sees-through-walls/