Batos Coaxial 1/4 Wave Notch Filter (video1)

Coaxial 1/4 Wave ( Lambda) Stub if intraduced in a circuit acts as Notch Filter with atenuation of anything up to -32dB of unwanted, interfering signal (amount of att depends on the quality of the Coax)
Notch Filters can be aded in parallel close by exsisting filter in order to increase attenuation from -3dB and making attenuating bandwith of the signal narrower
Adding more will result in further attenuation of the signal but at lower order ( less than 3 dB) .