Explanation: A shorted stub that is 1/4 wavelength long at the fundamental frequency presents a very high impedance (an open circuit) at its input. When attached in parallel to a transmission line, it allows the fundamental frequency and its odd harmonics (which also see an effective open circuit, e.g., 3/4 wavelength for the 3rd harmonic) to pass with minimal attenuation. However, at the second harmonic, this same stub is effectively 2 * (1/4 wavelength) = 1/2 wavelength long. A 1/2 wavelength shorted stub acts as a short circuit at its input. This shunts and absorbs the second harmonic from the transmission line. This principle applies to all even harmonics (e.g., the 4th harmonic sees a 1-wavelength shorted stub, which is also a short circuit), making the 1/4 wavelength stub ideal for suppressing them.