The original idea of the collinear principle in the antenna design comes from Franklin (Franklin, 1925). He faced the problem of resonant long wire antennas. In principle, the standing wave current distribution on the long straight wire produces n radiation lobes of the same level, depending on the number n of half-wave antenna sections. Employing nonradiating quarter-wave stubs Franklin converted the original out-phase current distribution into an in-phased distribution of currents on collinear segments (represented by solid red arrows in Fig. 1), thus producing only one major radiation beam. A key advantage of such arrangement is represented by the high gain of the antenna with the properties of series antenna array, whereas the simplicity of the single feeding point is maintained. All antenna structures based on this principle are known as collinear arrays (CoA). The latter are composed of in-phase fed radiating elements that lie in the straight line. Their radiation is typically broadside and perpendicular to the axis of collinear elements. Since Franklin’s times many collinear antenna structures have been proposed. The principle representatives of the CoA are described later on.