A planar quasi-Yagi antenna with cointegrated filtering functionality and flat in-band gain is reported. It exploits complementary-diplexer-based reflectionless-filtering techniques through the inclusion of an absorptive branch to realize the codesigned filtering functionality. This absorptive branch consists of a second-order bandstop-filter (BSF) unit loaded by a grounded resistor that is connected in parallel with the initial planar quasi-Yagi antenna. By designing the reflection coefficients of the basic antenna and the absorptive branch to be nearly complementary in frequency, the out-of-band RF power not radiated by the antenna is transmitted by the BSF to its loading resistor that dissipates it. In this manner, a quasi-absorptive filtering action is added to the antenna. Moreover, whereas the out-of-band radiation efficiency of the antenna is highly suppressed, its in-band behavior is kept as the in-band RF power is not transmitted by the BSF but radiated by the antenna as usual. For the experimental-validation purposes, a microstrip antenna prototype is developed and measured. Flat in-band gain with a ripple lower than 1 dB within the operational range of the antenna (2.6-3.06 GHz) is obtained as an added merit.

A novel microstrip dual-band filtering power divider (FPD) is presented in this letter. By introducing a proper coupling topology between the quarter-wavelength (λ/4) shortended microstrip line and two multi-mode resonators, the dualband FPD is constructed. Meanwhile, good isolation of this FPD is attained by installing an isolation resistor between the resonators. Additionally, two 3λ/4 open-ended stubs are attached in the output coupled lines to improve its frequency selectivity and obtain a wide upper stopband. For validation, a prototype FPD operating at 1.57 and 2.89 GHz is designed, fabricated and measured. Both the simulated and measured results are in good agreement. Results indicate that the proposed FPD exhibits not only a sharp frequency selectivity and good harmonic suppression with the help of the six inherent transmission zeros, but also better than 21 dB isolation within the entire dual operation bands.

A dual-band dual-circularly polarized (CP) all-metal antenna is proposed in this paper for vehicular communications to realize large signal coverage and high isolation over two bands. On the one hand, by combining two rings of three-dimensional (3-D) L-shaped stubs and a circular patch, opposite CP radiations, i.e., right-hand circular polarization (RHCP) and left-hand circular polarization (LHCP), can be achieved in lower and higher working frequency bands respectively, thus ensuring the isolation between different channels. On the other hand, both radiation patterns of two bands are conical-beam, i.e., omnidirectional, so as to realize large signal coverage, which is beneficial for vehicular communication especially when the car is moving or located on nonplanar plane. Besides, the proposed antenna is all-metal and can be easily manufactured by computer numerical control (CNC) machining technology. As such, it is expected to have higher power capacity than the conventional printed circuit board (PCB) structure. A prototype operating at 2.6 and 2.98 GHz is fabricated and measured, indicating that small frequency ratio between two operating bands can be realized. The proposed antenna is promisingly useful for application on vehicle to ensure simultaneous communications between different links without crosstalk, such as vehicle-to-satellite and vehicle-to-basestation communications.

This letter presents a simple microstrip dual-mode filtering power divider (FPD) with sharp frequency selectivity and good in-band isolation. A new topology is proposed to integrate only a single resonator and a resistor to realize the dual functions of the power division and filtering. In order to further improve its frequency selectivity and obtain wide upper stopband, three open-ended stubs are integrated into the input and outputs of the FPD, respectively. The presented FPD stands out from those in the literature, by both improved performance and simple design layout. For validation, a prototype FPD operating at 2.2 GHz with 3-dB fractional bandwidth of 18.2% is designed and fabricated. The experimental results agree well with the simulated ones. The results indicate that the new FPD exhibits better than 32-dB in-band isolation along with 24-dB harmonic suppression up to 6.28 GHz (2.85 f₀).

A new microstrip dual-wideband filtering power divider (FPD) is presented in this letter. The proposed device is mainly composed of a resonator loaded quarter-wavelength ( $\lambda $ /4) three-line coupled structure and a modified isolation network. To illustrate the design principle of the proposed FPD, equivalent circuits of the structure under the even-/odd-mode excitations are provided with the derived design equations. For demonstration, a prototype dual-band FPD operating at 2.3 and 3.64 GHz with 3-dB fractional bandwidths of 36.5% and 22.3% is designed, fabricated, and tested. Results indicate that the new FPD exhibits both good isolation and high frequency selectivity within the whole operation band.