A Study for Improving Antenna Performances in LTE/Cellular/WLAN Applications
Since the wireless communication services have been dramatically increasing and saturating in the LTE/Cellular/WLAN bands, new approaches for antenna design are proposed and analyzed for improvements of performances in this dissertation. The proposed antennas are a new short pyramidal horn antenna for LTE/Cellular applications and a printed collinear dipole array antenna for WLAN applications.
For assessing electromagnetic compatibility and radiation characteristics of the electromagnetic-wave transmitting devices and equipment, it is required that the standard gain horn antenna, referred to as a reference gain antenna, has the practical size and light weight with good performances such as a high antenna gain and a good front-to-back (F/B) ratio. The proposed short pyramidal horn antenna is integrated with metal- strips/rods to control the edge diffraction. As the length of horn which has the fixed aperture size is shortened, the flare angle of horn is increased. The phenomenon results in increasing the level of backward radiations. By inserting the metal- strips/rods into the shortened horn, the phenomenon can be solved. The newly proposed horn antenna has improved the average F/B ratio by at least 8 dB in the frequency range from 690 to 960 MHz. The fabricated horn length, including a waveguide flange, is only about 0.6 λ at 800 MHz. Therefore, the proposed antenna can replace the dipole antenna as a reference gain antenna for both indoor and outdoor measurement systems below the L-band to monitor LTE/Cellular wireless services.
The two-radiation elements composed of a printed half-wavelength dipole antenna on the opposite sides of the dielectric substrate are collinearly arrayed with a parallel feed network to enhance the antenna gain and to generate the broadside radiation with the horizontally omnidirectional pattern. The radiation element comprises two pairs of a combination of long and short dipoles, which are printed back-to-back on the opposite sides of the dielectric substrate, for operating in the both bands of 2.45 and 5.5 GHz. For feeding to the radiation elements with 180˚ phase difference, the broadside coupled microstrip line (BCML) is applied to the parallel feed network to exclude external balun structures. The BCML, whose physical widths of bi-faced striplines are identical, has brought the differential signals to the balanced structure such as dipole antennas. The two-section impedance transformer is realized on the BCML for dual-band operation. The dual-band impedance transformer is used to connect the parallel feed network with the radiation elements. As expected, the proposed array antenna with a dual-band parallel feed network achieves better performances such as the reflection coefficients and antenna gains in the two WLAN bands. The fabricated collinear array antenna yields an antenna gain of 3.89 – 4.48 and 5.79 – 7.18 dBi in WLAN dual-bands with the antenna size of 16 × 130 × 0.5 mm3. The radiation patterns are relatively omnidirectional in the azimuth plane regardless of frequency bands. The maximum differences of the radiation level are 0.22, 3.24, 2.82, and 2.65 dB at the 2.45, 5.2, 5.5, and 5.8 GHz in the azimuth plane, respectively. The proposed collinear dipole array antenna is profitable for dual-band WLAN access points.
