Explanation: In a self-excited oscillator, the frequency is determined by a resonant tank circuit (L and C components). If an antenna is directly connected to, or forms part of, this frequency-determining tank circuit, its electrical characteristics become critical to the oscillation frequency. A swinging antenna, due to its physical movement, will change its capacitance to ground and surrounding objects, and potentially its effective inductance. Since the resonant frequency of an LC circuit is given by $f = 1 / (2\pi\sqrt{LC})$, any change in the antenna's capacitance (C) or inductance (L) will directly alter the oscillator's output frequency. This direct coupling means even slight movements will cause noticeable and serious frequency shifts, making the output signal unstable. Therefore, a swinging antenna will cause serious frequency instability.