Custom 400-406 MHZ bandpass filter?

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Custom 400-406 MHZ bandpass filter?

Post by leechunsonnn » Tue Oct 09, 2018 7:37 am

Im trying to receive Radiosondes with my RTL-SDR and im getting alot of strong interference from a nearby transmitter (@ 409.824 MHZ.). Is there any way i could easily build (or buy) a band pass filter for the 400-406 MHZ range?


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Re: Custom 400-406 MHZ bandpass filter?

Post by Airwind » Tue Oct 09, 2018 9:48 am

As an entry-level RF enthusiast I've tried to build my own filters, but I was told that it's much better to just buy already made filters instead because they are going to be of much better quality than anything I'm ever going to come up with. If you're into filters and want to learn stuff then sure go ahead and experiment, but if you just want to have a working filter then perhaps just buy an existing component (e.g. SAW filter) from Mouser, Digikey or Farnell as they have a wide range of stuff available. You'll have to solder the component and the SMA connectors to a custom PCB though.

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Re: Custom 400-406 MHZ bandpass filter?

Post by m_a_x » Tue Oct 09, 2018 6:43 pm

It's not easy to make a filter so sharp to attenuate a strong signal only a few Mhz away. You should try to increase the received signal using a Yagi and lower the rf gain.
Also try to play with the polarization of the Yagi to see if it helps to remove the interferences even if attenuating the received signal.

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Re: Custom 400-406 MHZ bandpass filter?

Post by aurgathor » Sun Oct 14, 2018 5:44 pm

As it was already mentioned, making a suitable filter is not easy, but in case you still want to try it, you probably want an elliptic (Cauer) filter since that's the steepest, or an inverse Chebyshev filter. Unfortunately, the math for filter design is kinda (?) ugly.

Methinks the best way to reduce interference is by using a directional antenna aligned in a way to minimize signal pick-up from the nearby transmitter

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