New WT Microwave WT-A11654-Qo4 1420MHz+/-2MHz Cavity Filter
This cavity filter from China has remarkably narrow band of only 2MHz either side of target frequency of 1420MHz (hydrogen).
Simulation curve from company (WT Microwave) below:
astronomy.network Lichfield Radio Observatory (LRO)
Andrew Thornett, Lichfield, England, M6THO, andrew (at) thornett (dot) net