FM band notch filter made with coaxial cable.
A simple and cheap FM band notch filter could be made with 1/4 wave length open line(s) connected as shown hereunder. With this example we want to reject a strong signal from an FM broadcasting station transmitting on 89.5 MHz.
The two open line cable length are: (75 / 89.5 MHz x Cable_velocity_factor) = 0.56m each.

Measurement: The cable going to the antenna and the cable going to the radio have been connected to the 'DUT' and 'DET' miniVNApro ports. The DL2SBA vna/J software used in 'transmission' mode display the following graph.

The graph below shows a measurement made with only one ¼ λ open line. This provide an attenuation of almost 40 dB at 89.5 MHz.


The graph below shows a measurement made with two ¼ λ open lines (like the above sketch). This provide an attenuation of more than 51 dB at 89.5 MHz.


The graph below shows a measurement made with three ¼ λ open lines. This provide an attenuation of more than 62 dB at 89.5 MHz.


The quality of the coaxial cable is very important. You will get better attenuation with low loss cables like LMR-400, Heliax or Cellflex. If you use another cable than RG-213 take in account the velocity factor of this cable, it might be different from 0.66.

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