Some days ago I received a parcel from my friend Silvio, DM9KS. There was a 20m QRP band-pass filter from QRP Labs inside. I then built the kit as it was there anyway. I did not wind the torroids very accurate, but I have an insertion loss of 2.1 db only on 20m and I have -53db attenuation on 40m and -32db on 15m. I would say great piece of kit!
Here you see the assembled and fine-tuned QRP-Labs 20m Bandpass-Filter connected to the network analyzer:
Peter, DC2ZO then asked me how bad the harmonics of the Pi really are, so I checked with the Spectrum Analyzer. The highest peak (marker point) was the 3rd harmonic on 42.83 MHz with about 1.6 dBm, which is 1,45mW:
With the filter connected the result is crazy. No QRM visible and -34.34 dBm on the 3rd harmonic now. What a clean signal:
With the filter and a lot of adapters connected, the Raspberry Pi makes 7.22 dBm output, which is about 5.2 mW:
Without the adapters and filter tha Pi makes 12.3 dBm (17 mW).
With the filter and 5.2mW I was heard by DL4VM in my city on 20m only, but I hope for some DX now.