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.
of 39 dBc second harmonic, I
now have more than 60 dBc. All with just the addition of a single,
Don Cantrell, ND6T