Low-Profile Planar Filtering Dipole Antenna With Omnidirectional Radiation Pattern

This paper presents a low-profile planar dipole antenna with omnidirectional radiation pattern and filtering response. The proposed antenna consists of a microstrip-to-slotline transition structure as the feeding network and a planar dipole as the radiator. Filtering response is obtained by adding nonradiative elements, including a coupled U-shaped microstrip line and two I-shaped slots, to the feeding network. Within the operating passband, the added nonradiative elements do not work, and thus the in-band radiation performance of the dipole antenna is nearly not affected. However, at the side stopbands, the added elements resonate and prevent the signal passing through the feeding network to the dipole antenna, suppressing the out-of-band radiation significantly. As a result, both satisfactory filtering and radiation performances are obtained. For demonstration, an omnidirectional filtering dipole antenna is implemented. Single-band bandpass filtering responses in both the reflection coefficient and realized gain are obtained. The measured in-band gain is similar to 2.5 dBi, whereas the out-of-band radiation suppression is more than 15 dB.