| FM band notch filter made with coaxial cable. |
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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 a nearby FM broadcasting station transmitting on 89.5 MHz. - The 1/4 wavelength open lines are: (75 / 89.5 MHz x Cable_velocity_factor) = 0.55m each. - The 1/2 wavelength line is: (150 / 89.5 MHz x Cable_velocity_factor) = 1.10m. |
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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 running 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. |
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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. |
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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. |
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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. Keep the interconnections as short as possible. |
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