An iterative technique to adjust ripple in a bandpass filter has been developed by curve‐fitting numerical data. It gives an approximate relation of ripple against filter coupling coefficients and resonator resonant frequencies instead of finding a Jacobian matrix for each individual filter, in exchange for making more iterations. Numerical examples show reasonable convergence for Chebyshev filters up to the 16th‐order, coupled quadruplets, coupled triplets and folded cross‐coupled filters. A bandpass filter for a radio telescope in Yebes, Spain is designed and measured, a tenth‐order superconducting spiral filter with 7% bandwidth, 2295 MHz centre frequency, <0.1 dB ripple and overall substrate size 30 mm × 8 mm. The 11 iterative simulations totalled 8.5 h of computer processing time.

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