A Multiband Monopole Antenna With an Embedded Reactance-Cancelling Transmission-Line Matching Network

A series reactance-canceling (SERCA) transmission-line (TL) matching network is proposed, which replaces the series resonant circuit of a double-tuned Wheeler matching network with a λ_g/² TL in order to achieve a broadband double-tuned matching response around the design frequency f₀ while also introducing additional matching bands at harmonic frequencies of f₀. The reactance cancellation property of the λ_g/² TL is first outlined, followed by simple equations for the design of multiband SERCA-TL matching networks. A SERCA-TL matching network is subsequently embedded within a printed circular monopole antenna, and it is demonstrated that the lowest frequency of operation can be reduced from 4.43 GHz for the unmatched monopole to 2.1 GHz. Thus, a compact, multiband, and highly efficient antenna is achieved that operates in the frequency ranges of 2.1-4.31 and 6.35-6.78 GHz. The total size of the antenna is 20 × 50.3 × 1.59 mm³, and the measured efficiency is on the order of 80%-90% within the bands of operation.