Nordintown Blog

Rubbish musings, projects and hackerings from Nordin

Building a Low-pass filter (LPF) for WPSR Tx ( e.g. a Raspberry Pi)

Nov 202015

I found a nice set of guidelines about how to build LPF board for your transmitter. Only if your radio license lets you, I recommend having a go with one of these and a Raspberry pi [0]. 

For the low-pass filter, I am following a PDF guide [1] authored by Revd. George Dobbs (G3RJV).  The kit list is as follows:

  • 3 toroid cores (type FT37-43)
  • 1m enamelled copper wire (AWG tba)
  • 1 Choc box electrical junction box 
  • 1 case mounted BNC connector (male)
  • 2m 50ohm Coax
  • Capacitors (varied) 
  • Stripboard
  • Solder + iron 
  • An online toroid calculator [2] (if I must, and even then only to check)

We have to calculate the windings from the core size, material and desired inductance. Our design calls for 3 coils and 4 caps.

W3NQN's 7 Element standard Value Capacitor low pass filters


Calculating number of N turns required on a Toroid for a given inductance:

N = 10 x SQUARE-ROOT ( L / L10)

N = Number turns.
L = Required inductance (uH).
L10 = Inductance at 10 Turns.

This design can be soldered onto strip board along with the antenna connectors of your choice. For the enclosure, I can recommend a choc-box connector housing from Screwfix (UK)[3] I have a few of these ready for projects and they work very nicely.  If you are very lazy, you can pick up complete LPF filters from [4]. They have a good selection and ship anyplace (also check-out their other kits)