How to design a Low Pass ladder filter for 80m amateur radio band?

| February 3, 2026 | Amateur Radio (Ham Radio) | No Comments

To design a Low Pass Filter (LPF) for the 80-meter band, the most common approach is the Pi-network ladder filter. This design uses a “ladder” of capacitors and inductors to create a steep roll-off, ensuring your transmitter doesn’t spray harmonics into the higher bands. For an 80m transmitter, a 5-pole (or 7-pole) Chebyshev filter is the gold standard.

1. Defining Your Parameters

To keep your 80m signal clean while protecting the 40m band (7 MHz), we need to set the Cutoff Frequency (fc) slightly above the band edge.

  • Design Cutoff (fc): 4.2 MHz (This gives you “breathing room” for the 4.0 MHz edge in case of the 80m band as in most regions; here the upper end is 3.7 MHz).
  • System Impedance (Z0): 50 Ω (Standard for most transceivers and antennas).
  • Filter Type: 5-Pole Chebyshev (3 capacitors, 2 inductors), like the LPF in VU2EVQ linear amplifier which I am homebrewing.

2. The Circuit Topology

A 5-pole ladder filter for 50 Ω systems usually starts and ends with a capacitor to ground.

The arrangement looks like this:

  1. C1: Input capacitor to ground.
  2. L1: Series inductor.
  3. C2: Center capacitor to ground.
  4. L2: Series inductor.
  5. C3: Output capacitor to ground.

3. Calculated Component Values

For a 5-pole Chebyshev filter with a 4.2 MHz cutoff at 50 Ω:

ComponentCalculated ValueNearest Standard Value
C1 & C3~820 pF, 1 kV 820 pF (Silver Mica or NP0/C0G)
C2~1500 pF, 1 kV1500 pF (Silver Mica or NP0/C0G)
L1 & L2~2.8 μH2.8 μH (approx. 25 turns on a T50-2 core)

Pro-Tip: Use Silver Mica or high-voltage NP0/C0G ceramic capacitors. Standard cheap ceramic disks will heat up and drift under RF power, which shifts your cutoff frequency.

4. Building the Inductors

We can wind these our self. For a compact, high-efficiency build, T50-2 (Red) iron powder toroids are ideal for the 80m band, for 100W SSB.

To achieve 2.8 μH on a T50-2 core:

  • Turns: ~25 turns of #22 AWG enameled wire.
  • Spreading: Ensure the turns are spread evenly around about 3/4 of the core to minimize stray capacitance.

If long duty cycle as in digital modes are being planned, T68-2 or T80-2 cores may have to be used to handle the heat generated. Good ventilation should also be there. T68-2 core needs 23 turns for 2.8 μH. Heat generation will be more if SWR is high and with long ragchews!

5. Testing and Tuning

Once assembled, you should ideally test the filter before hooking it up to your transmitter:

  1. NanoVNA: This is the best tool for the job. Look for the “S21” (LogMag) plot to see where the signal starts to drop off. It should stay flat until 4 MHz and be significantly “in the basement” by 7 MHz.
  2. SWR Check: Connect a 50 Ω dummy load to the output. SWR should be near 1.1:1 across the entire 80m band. It is a good idea to seal the toroids after confirming SWR, with a dab of hot glue or clear nail polish to prevent the wires from shifting.