A notch filter is a resonant circuit, or rather an automatic switching inductor, used in antenna engineering to automatically extend or shorten the length of the antenna based on the frequency of the signal. A filter specifically used in radio receivers to eliminate certain useless signals and reduce interference with useful signals. Used in circuits to filter out signals of unwanted frequencies, such as adding a notch filter at the edge of the passband of a bandpass filter. Usually, a parallel resonant circuit is connected in series, or a series circuit is connected in parallel. Their resonant frequency is the frequency to be filtered out, which is common in the mid frequency part of television circuits. The notch filter in the antenna: As mentioned by the moderator, the wavelength of 7MHZ is 40 meters. If a dipole antenna is used, each side should be 10 meters long, and the antenna will resonate. The wavelength of 14MHZ is 20 meters, which is also a dipole antenna, and each side should be 5 meters. If the same antenna needs to operate at both a wavelength of 40 meters and a wavelength of 20 meters, a notch filter needs to be added at a distance of 5 meters from the dipole antenna at 10 meters on each side, which is usually a parallel circuit. It is an instrument used to measure the resonant frequency of a resonant circuit. Its principle is that it is an LC oscillator. When its coil is close to another resonant circuit and the oscillation frequency is close to the resonant frequency of the tested circuit, the oscillation will weaken due to the absorption of oscillation energy. The oscillation frequency of the notch meter will be changed until the oscillation weakens to the lowest point. The oscillation frequency is the resonant frequency of the tested circuit