Recommended Band Pass Filter (BPF) applied power with antenna VSWR <1.5:

  • 200W CW/SSB ICAS
  • 100W 100% Duty Cycle

Enclosure is Hammond 1411MU :

  •  6″ x 3″ x 3″ (16 x 7.6 x 7.6 cm) without connectors
  •  7″ x 3″ x 3″ (18 x 7.6 x 7.6 cm) with connectors

In general, schematics is the 3th order Cauer (Elliptic) type BPF.

20Cauer

On a circuit above 1.282 uH is a trifilar winding coil on T130-17 Micrometals Iron Powder core.  On a toroid I made 5 turns of a primary winding and other 10 turns as a secondary one over the initial 5 turns. The same technique used for a classic W3NQN BPFs.

As a result of that transformation, I designed the BPF for 450 Ohm( 50 * 9 =450). For 40 and 15 meter band LC traps the required values should be recalculated, multiplying by 9 a capacitor values and dividing by 9 inductance values.

These are a final values I used for 20M BPF:

  • 14.4 pF * 9 = 130 pF
  • 34.17uH / 9 = 3.85 uH  (T130-17 29 turns)
  • 8.8 pF * 9 = 80 pF
  • 6.33uH / 9=0.7 uH   (T130-17 11 turns)

For part values calculated above I used two/three in parallel ceramic capacitors and any Iron Powder cores can be used for coils , like T130, T106 or T94 of a size (types 2 or 17).

I used 98pF good quality and able to withstand 200W output power capacitors from Charcroft.com. A few good quality ceramic or Mica capacitors can be used as well.

20_LP_View

This BPF compare to a very popular W3NQN BPF  has much better band attenuation (with the same number of required parts ).

Final results are very good for such a simple schematics.

  • Insertion Loss < 0.3dB
  • Band Isolation ( 40M > -60dB, 15M > -65dB, 10M > -60dB)
  • VSWR <1.1 (RL >26 dB)

Below is a 20M BPF graph showing adjacent band isolation.

20_LP

Below is a In-Band graph showing VSWR and Insertion Loss.

20_LP_InBand