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Neither of my two BITX 40 meter boards would meet current FCC spurious emission regulations of 43 dB below the fundamental signal (dBc). The 5th order Butterworth low pass filter doesn't provide enough suppression of the second harmonic.

That is easily remedied by paralleling L4 with a 100 pF capacitor. The voltage rating and temperature coefficient of the 100 pF capacitor is not important. There is little harmonic energy at this point and tuning is not critical. I used a 100 volt ceramic disk. A silver mica would be nice.

The final section of the filter is now an elliptical or Cauer configuration. Picture the new capacitor paralleled with L4 as a trap. It's tuned close to the frequency of the second harmonic near 15 MHz.

Instead of 39 dBc second harmonic, I now have more than 60 dBc. All with just the addition of a single, cheap, capacitor.

Don Cantrell, ND6T