Insertion loss of 0.2 dB in a Ka-band cavity filter directly affects whether a payload downlink closes at the required data rate, and achieving it alongside better than 20 dB of rejection 1.7 GHz away is the design problem this filter addresses. The configuration has flown on NASA science missions, with hardware on orbit on PACE and delivered into the Roman Space Telescope programme. Qualification follows NASA GEVS Table 2.4-3 with 14.1 GRMS overall random vibration, minimum resonant frequency verification, and a 0.25 g low-level sine sweep before and after vibration to detect any shift. Thermal cycling is to mission requirements, typically -50 to -110 °C at the low end and +50 to +90 °C at the high end. S-parameter testing runs from 1 to 40 GHz, with workmanship inspection, acceptance testing before shipment and a Certificate of Conformance available on request. Custom bandpass, low-pass, high-pass and notch filters from L-band through Ka-band are developed to mission-specific electrical, mechanical and interface requirements, in coaxial or waveguide. Low-volume custom programmes are the norm, and short lead times can be accommodated.