Antenna types

In radio systems, many different antenna types are used whose properties are especially crafted for particular applications. Antennas can be classified in various ways. The list below groups together antennas under common operating principles, following the way antennas are classified in many engineering textbooks.^(p4)

• The dipole, monopole, and large loop antenna types (simple antennas) discussed below typically function as resonant antennas; in resonant antennas, waves of current and voltage bounce back and forth between the ends, or circulate around a closed loop, and their overlapping sections add and subtract to form standing waves along the elements. These are sometimes further subdivided into linear antennas (dipoles, monopoles, etc.) and loop antennas (large loops, small loops, and halo antennas).
• Array and aperture antennas are both types of composite antennas. Array antennas are made out of combinations of several simple antennas that function as a single antenna. Aperture antennas made of an outer, surrounding reflective surface whose shape concentrates waves striking it onto a small inner simple antenna; the inner antenna can be either resonant or non-resonant.
• Traveling wave antennas are not resonant: Current and voltage waves induced by RF radiation travel through them in opposite directions along the antenna element(s), but are delivered to the feedpoint only on one end; on the end opposite the feedpoint, the waves raised by radio signals arriving from the direction of the feedpoint are absorbed by a terminating resistor, to prevent the waves from reflecting backward, towards the feedpoint. The termination on the antenna makes it deliver to the feedpoint signals received from only one direction.