Designing and Building a 2m Low Pass Filter

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).

Designing and building a 2m low pass filter

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.
Since I wanted this filter for 2m, the highest frequency I’m interested in passing is 148MHz, so I selected a cutoff frequency of 150MHz. In hind-sight, this was a poor choice, since a -3dB point only 2MHz above the band caused for a lousy insertion loss. A better choice would have been 10% higher than the top of the band, so 148MHz * 1.10 = 162MHz
I decided to build a 5 pole T configuration Chebyshev filter with 0.1dB of ripple. Looking this filter up in a random copy of the ARRL Handbook (1981, but any recent one will do), it gives the component values needed for a 50 ohm filter at 1MHz. I’m also building this for 50 ohms, so all I need to convert is the frequency by dividing by 162MHz.

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