Designing a 21 cm H I-line preselector that survives a Munich suburb.
Four cavity-filter topologies, 63 FDTD simulations, three tuning campaigns, and the physics of why you can't always have low insertion loss and strong GPS rejection in the same box.
01 The mission
The 21 cm hydrogen line sits in the ITU-protected 1400 – 1427 MHz band, but at a Munich-suburb site like Unterhaching the spectrum around it is anything but quiet. The filter's job is to admit the 10 MHz around 1420.4 MHz with negligible loss, and reject everything else — especially GPS L1 (155 MHz above) and Iridium (200 MHz above), both always-on satellite downlinks the receiver can never point away from.
02 Five design milestones
Each card is the final geometry of its column in the campaign log — click through to see the iteration journey that got there.
03 3D geometry viewer
Spin, pan and zoom the machined aluminium housing for each of the
five designs. Drag to rotate, scroll to zoom, right-click-drag to
pan. STL files are the same ones the HI_Filters/runs/
pipeline writes after each simulation — geometry in the viewer is
exactly the geometry the FDTD solver saw.
04 Interactive S-parameter comparison
Overlay any combination of the five designs. Toggle S11 or S21, zoom into the passband, and watch how the 1553 MHz spurious in the as-designed interdigital (run 035) sits right where GPS lives. The dashed vertical lines mark the key RFI frequencies.
05 Out-of-band rejection matrix
Simulated S21 (dB) at each interferer frequency, interpolated from the saved sweeps. The HI-line row is the passband; every other row is rejection — more negative is better. Red cells fail the ≥ 30 dB threshold typical for dense-RFI sites.
06 The iteration log
Click any campaign to see its full tuning trajectory — rod length and spacing for the coaxial designs, cavity length and iris gaps for WR-650, iris size for the interdigital+septa fixes. The scatter plot shows where each run landed in (f_peak, bandwidth) space; the table is the per-run parameter change.
07 Insertion loss — the 0.6 dB pinch point
Each filter's in-band IL is set by the product of unloaded Q and bandwidth. The iris-coupled interdigital is trapped: the aperture that suppresses the 1553 MHz spurious caps BW near 5 MHz, and bare-aluminium Qu then forces IL above 0.6 dB. The combline escapes this because its 10 MHz BW comes from rod spacing, not iris aperture.
The physics
For a 3-pole 0.1 dB-ripple Chebyshev, Σgk = 3.21. With f0 = 1420 MHz, meeting IL < 0.6 dB requires Qu × BW ≥ 33,000 MHz.
Realistic unloaded Q for a machined coaxial cavity at 1.4 GHz:
| Finish | Qu range |
|---|---|
| Bare 6061 aluminium | 3,500 – 4,500 |
| Silver plate / Cu strike (~2 µm) | 4,700 – 6,000 |
| Copper cladding or solid Cu | 6,000 – 7,500 |
| WR-650 waveguide (inherent) | 8,000 – 10,000 |
Interactive calculator
08 Verdict
Under the IL < 0.6 dB hard spec, the combline wins in bare aluminium. The interdigital + iris rescue is feasible but requires silver plating or copper cladding. WR-650 is the clear choice when size and budget aren't constraints.
Combline
Meets all specs in bare aluminium — IL ≈ 0.46 dB, GPS rejection −47 dB, post-fab tunable.
- 10 MHz BW native, no iris-coupling trap
- 3 cap-screws for post-fab centering
- €110 bill of materials, 0.6 kg
- 1495 MHz spurious (−8 dB) in an empty band
Interdigital + iris
Smaller footprint (0.5 kg), but the spurious-suppression iris forces BW down to 4.5–5.5 MHz. Plating required to meet IL spec.
- Run 047 (Cu-clad): IL 0.60 dB, GPS −30 dB
- Run 063 (Ag-plated): IL 0.61 dB, GPS −27 dB
- +€50–100 plating cost
- No tuning screws — tolerance-sensitive
WR-650 iris
IL ≈ 0.05 dB, structural cutoff at 908 MHz rejects FM, DAB, GSM 900 and Mode-S infinitely. 10 kg / 77 cm.
- Fixed-pier mount only
- €850–1100 inc. CPR-650 + coax transitions
- Highest unloaded Q of the bunch
- Spurious-free to ~1.8 GHz (next WG mode)
| Criterion | Winner | Notes |
|---|---|---|
| IL < 0.6 dB in bare AL | combline | Only bare-AL coaxial design that meets it. |
| f-peak centering accuracy | interdigital+iris (Δ = +0.1 MHz) | FDTD sampling resolution-limited. |
| Lowest achievable IL | WR-650 (0.05 dB) | 10× better than any coaxial. |
| Below-1-GHz rejection | WR-650 | Infinite below cutoff; coax drops monotonically. |
| GPS L1 rejection | combline ≈ WR-650 | Both adequate; interdigital+iris −30 dB. |
| Compactness | interdigital+iris (run 047) | 50 × 70 × 130 mm, 0.5 kg. |
| Post-fab tunability | combline | 3 cap screws, ±40 MHz trim range. |
| Cost in bare AL | combline (€110) | Interdigital+plating ≈ €200; WR-650 > €850. |
| Overall for Unterhaching hobby use | combline | Best IL/spec/€ balance in bare AL. |
| Research / observatory install | WR-650 | When the TSYS budget justifies the mass. |