An antenna is a device for recieving and /or transmiting (mainly radio) electromagnetic waves trough natural mediums (air, water, earth, etc.). It is a system of conductors, by which the energy of elelctromagnetic radiation is converted to HF currents recieving antenna) and vice-versa (transmiting antenna).
Off Center Fed Antennas known as OCF use a specific feed point on the line, where favourable conditions from several SW bands meet to connect a coaxial feed thru an UNUN.
Loop antennas are sometimes called "quiet". They are favoured because of their interesting radiation pattern, which helps eliminating all kinds of noise and they have a higher gain against the common dipole (1,29 dBd). A loop antenna needs a bigger space and 3 suspension points for instalation.