4 Horizontally Polarized Wideband Antenna with Conical Radiation Pattern
To create a horizontally polarized wideband antenna with conical radiation pattern, four magneto-electric dipoles are arranged in a ring, as shown in Fig.11 and Fig.12 . The prototype is operated at 2 GHz. The dimensions shown in Table 1 were selected for excellent performance. The four magneto-electric dipoles are excited in-phase by a tapered power divider mounted underneath the ground plane. The upper part of the antenna is an electric dipole comprising a pair of sector-shaped horizontal plates. It is connected to the lower part of the antenna, which functions as a folded magnetic current. The magnetic current is provided by a vertically-oriented shorted patch antenna of height H=28 mm and that has a Γ-shaped strip feed. The separation between the two vertical plates is G =10.7286 mm. Each Γ-shaped strip has a transmission line section and a coupled strip section. The transmission line has a characteristic impedance of 50 Ω. The thickness of the transmission line is 0.3 mm, and the separation between the transmission line and nearby vertical plate is 1 mm.
The SWR and antenna gain are shown in Fig.13 . The measured impedance bandwidth (SWR≤2) is about 38%, ranging from 1.61 GHz to 2.38 GHz. This is much larger than the bandwidth of many other designs of horizontally polarized conical beam antennas. The measured operating frequency band is slightly lower than the simulated operating frequency band by about 0.2 GHz, which is acceptable in practice. The measured gain is about 5 dBi on average, and the 3 dB gain bandwidth covers (and is larger than) the impedance bandwidth. The radiation patterns of the antenna at 1.6 GHz, 1.8 GHz, and 2 GHz are plotted in Fig.14 . These are stable over the operating band. Compared with the simulation results, the measured radiation patterns have slightly higher cross-polarization and back lobe levels.
This paper begins by discussing the design of a magneto-electric dipole antenna. This structure has several advantages, including stable radiation pattern with low cross-polarization, low radiation, nearly identical E-and H-plane patterns, and stable antenna gain over the entire operating frequency range. The proposed antenna has more than 52% impedance bandwidth (for SWR≤2) with a stable gain of 8dBi and low back radiation. A dual-polarized antenna element based on the magneto-electric dipole is also presented. This antenna has about 67% (for SWR≤2) impedance bandwidth, and its isolation is more than 36 dB over the impedance bandwidth. The maximum gain is 9.5 dBi. Furthermore, a horizontally polarized conical beam wideband antenna is discussed. This antenna is low in profile (H=0.1867 λ0) and has about 38% impedance bandwidth (for SWR≤2) and an average antenna gain of 5 dBi.
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