Willy, W1LY, decided to try using the standard unshielded 1 uH axial inductor that the Pixie uses for it’s current 3-pole filter (C5=47o pf, L2=1 uH, C6=470 pF) as the starting point for a 5-pole filter that could easily fit on the existing board. Two inductors are soldered in place of L2, standing up vertically and soldered together at the top. A cap is then soldered from that bridge down to the grounded side of C6 (Willy paralleled two caps to get the required value). See the area indicated below.
Component values are:
- C5 changed to 820 pF
- C6 changed to 820 pF
- New Cap (bridge of L2A/L2B to ground at left side of C6) 1150 pF (680+470)
- L2A (1 uH)
- L2B (1 uH)
Measuring the output of the modified Pixie indicates a passing value! The 2nd harmonic, worst of the lot, was -43 dBc. Power output was 0.3 watts using a P2n2222 for Q2.
We suspect that original 5-pole Elsie values of 1.2/1.2 uH and 910/1300/910 pF would produce even better 2nd harmonic suppression and work just fine at the lower end of 40 meters.