How it's designed
The filter starts from a standard low-pass prototype, a ladder of normalised values (the g values) for a Butterworth or Chebyshev response. Each shunt capacitor in the prototype becomes a parallel LC resonator to ground, and each series inductor becomes a series LC resonator, all tuned to the centre frequency. The bandwidth sets the ratio of L to C in each resonator. Butterworth filters have the flattest passband, and Chebyshev filters roll off faster outside the passband at the cost of a small ripple inside it.
Practical tips
- Bandwidth matters. Below about 5% of the centre frequency, the series inductors become huge and the shunt ones tiny, and losses climb. Narrow filters are better built as coupled resonators, such as top-coupled or helical designs.
- Use the nearest standard capacitors plus a trimmer, and wind the inductors on toroids with the toroid calculator.
- Tune with a NanoVNA. Real parts have tolerance and stray capacitance, so adjust each resonator for the best passband.
- Shield and separate input and output so signals can't leak around the filter.
For harmonic filtering after a transmitter, a low-pass design is simpler: see the low-pass filter calculator.
Questions
When do I need a bandpass filter?
To protect a receiver or SDR from strong signals on other frequencies, such as broadcast stations, or to run two stations on different bands side by side.
Butterworth or Chebyshev?
Chebyshev gives steeper skirts for the same number of parts. Butterworth has the flattest passband. 0.1 dB Chebyshev is a good default.
How many resonators do I need?
Three is enough for most receiver protection. Five or seven give much steeper skirts but more loss and harder tuning.
Why are my inductor values so extreme?
The bandwidth is narrow compared with the centre frequency. Widen it or use a coupled-resonator design.