Hardware & RF

Tuning a Cavity Filter

This guide shows how to identify the unlabeled screws on a small cavity band-pass filter and tune it for the bands used by a mesh repeater — for example 906.875 MHz for Meshtastic LongFast or 910.525 MHz for MeshCore — using a nanoVNA. It's written for the Airframes filter, but the empirical method works on most small cavity cans.

Preparation

Tools, Materials & Setup

Tools & Materials

  • nanoVNA
  • Two short, quality 50 Ω coax jumpers (typically included with a VNA)
  • 50 Ω terminations (for calibration and optional port-match checks)
  • Non-metallic surface to work on
  • Tape/marker to label screws A, B, C, D
  • Screwdriver, wrench/pliers

Before You Start

  • Let the VNA and filter sit a few minutes to reach room temperature.
  • Keep cables and adapters identical during calibration and measurement.
  • Work in tiny moves: 1/16–1/8 turn per step. Never force a hard stop.

Note: Exact screw roles and rotation directions vary by manufacturer. The method below is intentionally empirical so you can deduce each screw's function safely without a datasheet.

Calibration

Calibrate the nanoVNA

  1. Choose a span that fully covers your target band:
    • Start wide at 50 MHz, later zoom to ~8 MHz centered around your desired frequency.
  2. Perform a full 2-port SOLT calibration (Open / Short / Load on both ports, then THRU).
  3. Save the calibration to a slot for reuse.

Recommended nanoVNA Trace Setup

Configure these traces so you rarely need to toggle views:

  1. Trace 1: S11 LOGMAG (REFL) — input return loss (match quality).
  2. Trace 2: S21 LOGMAG (THRU) — insertion loss; the “passband hill.”
  3. Trace 3: S11 Smith set to R + jX — shows input impedance relative to 50 Ω.

Place markers on your desired center frequency — e.g. 906.875 MHz for Meshtastic LongFast, or 910.525 MHz for MeshCore.

Step 1

Identify the Screws

  1. Label the four screws A, B, C, D.
  2. For each screw:
    1. Note the current traces, then rotate the screw +1/8 turn.
    2. Watch S21 LOGMAG, S11 LOGMAG, and the Smith chart:
      • Resonator tuning screw: the S21 peak frequency slides left/right; phase slope shifts.
      • Inter-resonator coupling screw: passband bandwidth/ripple changes more than center frequency.
      • Port (input/output) coupling screw: S11 depth / Smith position changes strongly; S21 shape changes little.
    3. Return the screw to its original position (−1/8 turn) and log what you saw.
  3. After testing A–D, you should have two tuning screws and two coupling screws identified (usually one inter-resonator + one port coupling).
The Airframes cavity filter with screws labeled A–D for identification.
Step 2

Core Tuning Procedure

  1. Find and zoom the passband: With S21 LOGMAG visible, sweep wide to locate the “hill,” then narrow the span around your target (e.g., 904–914 MHz for a 910.5 MHz center).
  2. Align the resonators to center frequency:
    1. Adjust Tuning Screw 1 with tiny turns until the S21 peak moves toward your center.
    2. Adjust Tuning Screw 2 to “pull” the top into a single tall peak centered on your target.
    3. Iterate 1–2 until the highest S21 point sits at your center. A smooth, monotonic S21 Phase trace indicates good alignment.
  3. Set the bandwidth (inter-resonator coupling):
    • Increase coupling → wider passband, lower center loss, may add ripple.
    • Decrease coupling → narrower passband, steeper skirts, sometimes higher center loss.
  4. Optimize input match (port coupling):
    1. Watch S11 LOGMAG and the Smith point at center. Target ≤ −15 dB S11 with impedance near 50 Ω + j0.
    2. If the Smith chart shows capacitive (−jX), back the probe/screw out slightly; if inductive (+jX), advance slightly. Stop when the dot sits near the Smith center and the S11 dip deepens.
  5. Final polish:
    • Re-maximize S21 at center with tiny alternating tweaks on both tuning screws.
    • Confirm −3 dB bandwidth, insertion loss at center (ideally ~1–2 dB; some cans land ~3 dB), and S11 (≤ −10 dB acceptable, ≤ −15 dB preferred).
A cleanly tuned passband: a single tall S21 peak centered on the target frequency.
Troubleshooting

Common Patterns

  • Two peaks / “camel back”: resonators are off-frequency or over-coupled. Re-align the tuning screws and slightly reduce inter-resonator coupling.
  • Wide but lossy top: likely over-coupled. Reduce coupling, then re-center both resonators.
  • Great S11 but high loss: resonators not exactly centered; re-maximize S21 at the target frequency.
  • Asymmetric skirts: one resonator is still off; balance with tiny opposing moves.
Reference

Targets & Reference Values

Parameter Target
Center frequency Your channel/center (e.g., 906.875 MHz Meshtastic, 910.525 MHz MeshCore)
Bandwidth 1–2 MHz for the Airframes
Insertion loss (S21 @ center) ≤ ~3 dB acceptable
Return loss (S11/S22 @ center) ≤ −10 dB acceptable; ≤ −15 dB preferred

Save Your Results

  1. Save a calibration slot and the final trace state on the nanoVNA.
  2. Tighten the nuts while maintaining screw position. Keep an eye on the VNA while you do this!

More hardware and RF write-ups are on the Learn page.