Low Pass Filter Construction

Background

Next step after building a functioning transmitter: clean up the signal. 

Moving Forward

To create a Low Pass Filter, I used information readily available at the G-QRP site. A good page is the A Short Guide to W3NQN Harmonic Filters for QRP Transmitter Output. - George Dobbs G3RJV. This is a good overview, but I found the older version that I think, well, would be an easier for the time being. A copy of George, G3RJV first paper can be found here: A Complete Do-It-Yourself Kit ....... with just a few simple calculations (George Dobbs G3RJV)

Design & Build

Here's the design:

And here's my parts:

• 3 x T37-6 (for L2, 4, 6) 
• 2 x 220 pF and 47 pF capacitors. These will be in parallel for a 267pF cap substituting for the 270 pF caps for C1 & C7
• 2 x 470 pF and 220 pF capacitors. These will be in parallel for a 690pF cap substituting for the 680 pF caps for C3 & C5

Finished Product

Here's what the final product looks like:

Top Side

Underneath

Results/Performance

I listened for oscillation at 2x (14.260 MHz), 3x (21.369 MHz), and 4x (28.492 MHz). I never really heard anything on 14.260, but certainly did on 21.369 and less so on 28.492. The following video shows the test on my receiver and scope plus a quick voltage calculation on the dummy load.

Next Steps

• Finalize capacitor value
• Assemble into an enclosure. Add BNC antenna connector and 3.5mm jack for a straight key
• Make some QSOs