Tuning your antenna, step by step

The theory lives in Antennes; here we act. Goal: start from a standard telescopic whip and get the best possible reception on the band you're listening to — measured, not guessed.

1. The right length

A quarter-wave antenna receives best when the whip is a quarter of the wavelength, shortened by about 5% (the wave "slows down" in metal — velocity factor):

L (cm) ≈ 7125 / f (MHz)

Band	Frequency	Whip length	Telescopic?
FM radio	100 MHz	71 cm	almost fully extended
Airband (voice)	125 MHz	57 cm	three quarters out
2 m amateur	145 MHz	49 cm	two thirds out
ISM 433	433 MHz	16.5 cm	2–3 segments
ISM 868 (LoRa, sensors)	868 MHz	8.2 cm	just the first segment
ADS-B (aircraft)	1090 MHz	6.5 cm	first segment, or a dedicated antenna
2.4 GHz (WiFi, drones)	2440 MHz	2.9 cm	too short — use a dedicated antenna

Remember the gesture: frequency up → antenna shorter. A fully extended telescopic on 868 MHz receives worse than one pulled out 8 cm: too long, it's simply out of tune.

2. Placement (often more important than length)

High and in the clear: near a window; better, outside. Walls and reinforced concrete eat UHF.
Vertical: almost everything you'll listen to (local FM, airband, ISM, ADS-B) is vertically polarised. A horizontal whip halves your signal.
Away from noise: switch-mode power supplies, screens, USB hubs and routers pollute the noise floor for several metres. A shielded USB extension that moves the SDR away from the PC is the best value upgrade there is.
A ground plane helps: a magnetic-base antenna sitting on a metal plate (tin box, car roof, radiator) completes the quarter wave and easily gains a few dB.

3. Measure instead of believing

Open the Explore console and use the "Signals heard" panel:

Tune to the target band (preset or manual frequency).
Note the SNR of the strongest signal.
Change one thing only (length, position, orientation), wait two seconds, read the SNR again.
Keep whichever configuration wins. Repeat.

Reference points: < 10 dB is painful listening, 15–25 dB is comfortable, > 25 dB excellent. If the whole spectrum jumps at once, that's not better reception — that's too much gain saturating the receiver; see Bien l'utiliser en live : la méthode.

4. Quick troubleshooting

Symptom	Likely cause	Fix
Nothing rises above the noise	gain too low, antenna unplugged	raise gain in steps, check the connector
Very high noise floor	USB/power-supply interference nearby	move the SDR away, shielded extension, ferrites
Strong station but choppy audio	saturation (gain too high)	lower the gain until the floor is flat
Good FM, bad 868 MHz	antenna too long for the band	shorten to ~8 cm or swap antennas
Repeating ghost signals	images (overloaded front end)	lower the gain, move away from strong transmitters

5. Going further

When you want purpose-built antennas (V-dipole for satellites, ADS-B collinear, directional Yagi), matching and SWR theory is in Lignes, ROS et adaptation, and hardware choices in Quel matériel pour débuter ?.