A Planar Absorptive-Branch-Loaded Quasi-Yagi Antenna With Filtering Capability and Flat Gain

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.