QRP CMC Choke Test Jig

The QRP CMC Choke Test Jig is a compact PCB test fixture that makes evaluating small common-mode chokes like FT140 and below fast, repeatable and convenient. Designed for use with a handheld VNA (e.g. NanoVNA), the board provides dedicated SMA test ports and simple wiring options to measure the choke’s common-mode attenuation (S21), insertion loss (S21) and reflection / matching (S11 / VSWR) under consistent conditions.

A built-in ~50 Ω termination (two 100 Ω SMD resistors in parallel) and configurable female headers remove the need for ad-hoc fixtures or awkward cable rigs, so you can quickly compare different choke constructions, spot resonances, and quantify how the choke affects the desired signal path. Intended for QRP choke testing, the jig speeds prototyping, helps validate designs for real-world use, and standardizes measurements so results are easier to reproduce and share.

Gerber files

Gerber package (manufacturing files): /gerbers/QRP_CMC_Choke_Test_Jig_v1_0.zip

Bill of Materials (BOM) — key items

Qty	Part	Value / Type	Notes
5	SMA connector, PCB mount (female)	SMA-F, PCB-mount	5 positions for test ports
6	Female header (single-row)	2.54 mm female socket	For test jumpers / optional wiring
2	SMD resistor	100 Ω 0805	Two in parallel → ~50 Ω termination
1	PCB	QRP CMC Choke Test Jig v1.0	Use Gerbers in /gerber

Termination note: Two 100 Ω resistors in parallel produce approximately 50 Ω (R_eq = 1 / (1/100 + 1/100) = 50 Ω). This is a practical, low-power test termination for S11/SWR checks at QRP power levels. Verify resistor power rating if higher transmit power is expected.

Assembly & soldering

• Solder the two SMD 100 Ω resistors first.
• Install and secure SMA PCB-mount connectors; solder all pads and mechanical anchors.
• Fit and solder the female headers used for optional wiring or test leads.
• Inspect joints for cold solder or shorts before connecting the VNA.

CMC Choke measurement procedures

General

• Always perform a full VNA calibration (open / short / load) on the frequency range you will measure.
• Use short, good-quality SMA cables between VNA and jig. Ensure connectors are tight.

S21 — Common-Mode Attenuation

1. Place the choke in the intended common mode attenuation configuration.
2. Connect VNA Port 1 → jig Port 1, VNA Port 2 → jig Port 2.
3. Measure S21 (dB) across the frequency range. More negative dB values mean better common-mode suppression.

S21 — Insertion Loss

1. Place the choke in the intended insertion loss configuration.
2. Connect VNA Port 1 → jig Port 1, VNA Port 2 → jig Port 2.
3. Measure S21 (dB) and determine insertion loss introduced by the choke.

S11 — VSWR / Reflection

1. Place the choke in the intended VSWR configuration.
2. Connect VNA Port 1 → jig Port 1
3. Sweep the frequency range and record S11 / VSWR. Low |S11| (dB) and VSWR close to 1:1 indicate good matching.

Contribution & license

Contributions welcome:

• Open issues for bugs or feature requests.
• Send pull requests for improvements (BOM updates, footprints, documentation).
• If you submit board changes, include updated Gerbers and a brief validation plan.