This paper illustrates and applies the recently established Q-balancing condition in frequency-scanning periodic leaky-wave antennas (LWAs) for achieving a frequency independent leakage factor and input impedance. The unit cells of such LWAs can be modeled by an equivalent two-port network comprising two independent circuit elements, a series resonator and a shunt resonator, which are characterized by their resonance frequencies ( fse, fsh) and quality factors (Qse,Qsh), where the quality factors correspond to the reactive energies stored over the radiated and dissipated powers in each respective resonator. The Q-balancing condition consists in achieving equal quality factors for the series and the shunt resonator: Qse = Qsh. Here, we propose two general concepts –the resistive loading concept– and –the asymmetry concept– to design LWAs based on the Q-balancing criteria. Furthermore, we apply the aforementioned concepts to the specific example of a series-fed patch array (SFPA) LWA to obtain a frequency independent input impedance and hence a constant gain for the LWA radiation through broadside.