Although it is risky to form any conclusions about the state of a technology in relation to possible future development, it appears that all types of antennas were invented in the 20 th century or shortly before. The four antenna types discussed here are summarized in Table 1. That is, small, low gain antennas are usually electrically small. Narrow band, but impedance matched, antennas with simple geometrices are usually resonant. Antennas with wide bandwidth are a special class of antennas.
High gain is achieved using aperture antennas.
It is likely that the categorization in Table 1 will also apply through the next century. This, of course, does not mean that the pace of innovation will slow any. To illustrate for resonant antennas, the half-wave dipole antenna appeared early in the century and the microstrip antenna was invented late in the century. Horns Reflectors Antennas can also be divided by geometric continuity. Much as signals are either analog or digital, antennas are either continuous or discrete.