β

9 cm

Figure 7.190 An implemented self-complementary antenna on the cone ([93], copyright C 2008 IEEE).

cone, not only better axial ratio performance, but also impedance matching over a wide frequency range, was made possible. An implemented antenna is shown in Figure 7.190. The input impedance and axial ratio, respectively, are shown in Figure 7.191(a) and

(b). Radiation of the antenna is uni directional as shown in Figure 7.192, where the performances at five bands are depicted along with the simulation software such as MoM, FEM, and (MoM + SEP) used for calculations.

7.2.4.2Conjugate structure

7.2.4.2.1 Electrically small complementary paired antenna

An antenna system composed with two thick, short monopoles, which were fed through a complementary network, was studied theoretically and experimentally [94]. As Figure 7.193 shows, the complementary network, a 180-degree hybrid tee, was used, by which impedance of the antenna is modified to be complementary to that of another antenna. With the phase reversal, the capacitive impedance of a short monopole was changed to the inductive impedance, which is conjugate to that of the other short monopole so that the resonance condition could easily be obtained and matching was made feasible without extra matching network, even though the antenna size was very small. A developed model consisted of two identical monopoles, each of which had the height of one eighteenth of the wavelength at the lowest frequency, and 90-degrees conical section having one-half the total height for a length-to-diameter ratio of unity as Figure 7.194 shows. The antenna referred to as ESCP (Electrical Small Complementary Antenna) was designed for acceptable VSWR at the lowest frequency. Figure 7.195 illustrates VSWR of an ESCP in comparison with an individual monopole. In the paper [94] theoretical analyses were shown, in which the geometrical parameters to optimize the antenna performances were discussed. It was shown that an optimized configuration could yield substantial improvement in gain–bandwidth product at the expense of having a directional pattern over much of this bandwidth. The radiation

Figure 7.192 Far-field patterns for five bands computed by MoM and FEM for lower bands and by MoM and MoM+SEP for higher bands ([93], copyright C 2008 IEEE).

180-deg HYBRID

Σ

z∑ = z0

Figure 7.193 Basic circuit for complementary feed ([94], copyright C 1976 IEEE).