Antenna Testing and Tuning
Edit on GitHubVerify an antenna is cut right: SWR, range checks and the 17.3 cm wire rule
Antenna Testing and Tuning
Overview
An antenna that is the wrong length, coiled or ground-shorted still transmits - just poorly. Testing takes minutes and catches the mistakes that silently halve your range.
The 17.3 cm Rule
A quarter-wave monopole at 433.5 MHz is 17.3 cm. If you solder a wire antenna:
- Cut it slightly long (~18 cm).
- Test, then trim 2-3 mm at a time.
- Stop when the signal is strongest (measured by a receiver or SDR).
Bench Tests Without an SWR Meter
| Test | What it tells you |
|---|---|
| Continuity antenna-to-ground | Should NOT beep (a short kills the antenna) |
| Continuity SMA center-to-shield | Should NOT beep |
| Receive a distant signal | Compare before/after changes |
| SDR waterfall amplitude | Rough relative comparison |
Range Check Method
- Put the beacon in a fixed spot, antenna vertical.
- Walk away with a receiver or SDR, note where the signal drops out.
- Repeat after each antenna change.
- Keep the change that gives the longest range.
Common Antenna Faults
| Fault | Symptom | Fix |
|---|---|---|
| Too short | Reduced range | Lengthen toward 17.3 cm |
| Too long | Slightly less efficient | Trim |
| Coiled inside case | Severe detuning | Straighten or move outside |
| Grounded to shield | Almost no radiation | Fix the short |
| Loose SMA | Intermittent signal | Tighten or re-solder |