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 .
For the low-pass filter, I am following a PDF guide  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)
- Solder + iron
- An online toroid calculator  (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) 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 arp-labs.com . They have a good selection and ship anyplace (also check-out their other kits)