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

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fc_mhz 7.5 book
ripple_db 0.07 book
il_db 0.11 at 7 MHz book
fc_mhz 7.5 simulated
f3db_mhz 8.08 simulated
ripple_db 0.076 simulated
il_db 0.24 at 7 MHz simulated
rl_db 17.6 simulated
stopband_db 51.4 at 14 MHz simulated
stopband_db 78.9 at 21 MHz simulated

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.

Where it comes from

The low-pass filter for the beginner's experimental transmitter in Experimental Methods in RF Design, Chapter 1.

Parts list

Ref	Value	Qty
C1,C6	470p	2
C2,C4	680p	2
C3,C5	180p	2
J1	IN 50R	1
J2	OUT 50R	1
L4,L6	1.52u	2
L5	1.7u	1

SPICE netlist

References

EMRFD Fig 1.37, p. 1.20