I’ve been playing with the DRA818V modules that have been making quite a stir in the amateur radio world at the moment. I haven’t gotten one on a spectrum analyzer yet, but I have reason to believe that it will require a low pass filter to be RF legal. I’ll write more about that once I get a look at it, but figured I’d first built myself a low pass filter in case I need it (if not for these modules, but some other VHF project in the future).
My process for building a low pass filter went as follows:
- Select the type of filter and cutoff frequency desired
- Look up normalized coefficients in the ARRL Handbook
- Divide these coefficients by the cutoff frequency
- Convert the inductances into turns on some core and capacitors into the nearest values
- Build the filter.