In practice, a conical structure on which simple antenna elements such as dipole, disk, cone, slot, or patch are mounted to excite a cone is often used. The infinite cone excited with an axial dipole at the tip has shown (14) that the strong radiation occurs along the small-angle cone unless the dipole length is about a half-wavelength. Figure 8 shows a finite wide-angle cone excited with a quarter-wavelength long and a half-wavelength long dipole (17). Note that the maximum radiation can be directed toward the horizontal plane by proper choice of a and в0 at a desired frequency. A cone excited with an axially symmetric circumferential slot close to the tip (18) shows radiation characteristics similar to those of a dipole-excited cone, since a small circumferential slot (magnetic current loop) is equivalent to a small axial electric dipole. When the circumferential slot is not too close to the tip, that is, apart by 2.5 wavelength (2ф = 30°), the radiation pattern shows a rather complicated lobe structure (18). The cone excited with a circular disk at the tip is called a discone antenna (19). This antenna is fed with a coaxial cable whose inner conductor terminates on the center of the disk and whose outer conductor terminates on the tip of the cone. The radiation pattern of the discone is similar to that of a dipole antenna, but its input impedance bandwidth is exceedingly broad compared with an ordinary dipole antenna.
Figure 9 shows the measured input VSWR (for 57.6 П cable) versus frequency of the finite conical antenna with different exciting elements at its tip (17). The cone excited with a conical element (an asymmetric biconical antenna) indicates very broadband characteristics, particularly in lower-fre — quency regions, without affecting the radiation pattern.