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My homebrew low pass filter for SDR

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Nov 22, 2016
I've been scanning about with a RTL-SDR lately and noticed I'm getting hammered by FM-BCB (88-108), Pagers (148) and GSM (938-959) over different parts of the spectrum where there shouldn't be. The price of living almost in the centre of town. So I thought I'd have a play at making a 1/4 wave low-pass filter and see what affect it would make to the GSM band and above.

I all ready have a FM-BCB filter (the RTL-SDR_dot_COM one) and also just made a stub to test on the pagers. It does work but it takes out 2M Amateur and attenuates a heap of other frequencies.

Decided to use some junk box stuff to make it on the cheap.

Tuna can
Small length of telephone solid core wire approx. 300mm (to make the inductor and 2 capacitors)
2 x SMA male connectors
Bit of solder.

See the pictures of the finished filter (before the top went on) and the plot I generated with RTL-SDR Panorama and Calc (Excel). The Impedance of filter should be 75ohms and cutoff @ 600MHz. Attenuation seems deepest at 930MHz region.

I think it needs more poles (and more reading) to minimize to dip in the 400MHz region.


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