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Two compact zeroth-order resonator (ZOR) antennas designed with inductor-loaded (IL) and capacitor-loaded (CL) coplanar waveguides (CPWs) are proposed. A generalized definition of the zeroth-order resonance is also presented, taking into account the losses of the host transmission lines. Analytic formulas are also derived to predict the reactance values needed to sustain a specified zeroth-order resonant frequency in a lossy host line. With these formulas, one may predict the input reflection coefficients for the proposed ZOR antennas. These antennas, using only two unit cells, are designed, fabricated, and tested, and they have a very compact size and a via-free uniplanar structure. The measured results agree very well with those acquired from the proposed method and full-wave simulations, thus verifying the accuracy of the proposed formulation. It is also demonstrated that the ZOR antenna using the CL CPW exhibits higher radiation efficiency.