Tuning an Airframes Cavity Filter for Meshtastic or MeshCore

Tools & Materials

• nanoVNA
• Two short, quality 50 Ω coax jumpers(typically comes 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 cable/adapters identical during calibration and measurement.
• Work in tiny moves: 1/16–1/8 turn per step. Never force a hard stop.

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 freq.
2. Perform a full 2-port SOLT calibration (O/S/L on both ports, then THRU).
3. Save the calibration to a slot for reuse.

Recommended nanoVNA Trace Setup

Configure four 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 for Meshtastic LongFast or 910.525 for MeshCore

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 Smith:
      • 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).

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 Smith shows capacitive (−jX), back the probe/screw out slightly; if inductive (+jX), advance slightly. Stop when the dot sits near the Smith center and 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).

Troubleshooting Patterns

• Two peaks / “camel back”: resonators are off-frequency or over-coupled. Re-align 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 still off; balance with tiny opposing moves.

Targets & Reference Values

• Center frequency: your Meshtastic channel/center (e.g., 910.5 MHz).
• Bandwidth: 1-2 MHz for the Airframes
• Insertion loss (S21@center): ≤~3 dB is 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!