40 m 7th-order Chebyshev Low-Pass Filter (7.5 MHz)

What it does

It cleans up a 40 m QRP transmitter's output. It passes 7.0–7.3 MHz with almost no loss and cuts the harmonics (14, 21, 28 MHz) by 50 to 100 dB.

How it works

It's a 7-element Chebyshev low-pass: four shunt capacitors and three series toroid inductors, designed for 50 Ω at both ends. The odd 0.07 dB ripple was chosen so standard capacitor values fit (EMRFD p. 1.20). The LC filter designer gives 467 pF / 1.521 µH / 859 pF / 1.696 µH for the same specification, within 1% of the book.

Building it

• Silver mica capacitors; each 860 pF is a 680 pF and a 180 pF in parallel.
• L4 and L6: 19 turns on T50-6 (1.52 µH). L5: 21 turns on T50-6 (1.7 µH). Spread or squeeze the turns to tune.
• The book housed it in a small aluminum box with coax connectors and found the shielding important.

Tuning and testing

Sweep with a 50 Ω generator and detector, or a NanoVNA, from 1 to 30 MHz. Expect under 0.25 dB loss at 7 MHz and a return loss better than about 17 dB across 40 m.