Antennas

The antenna is the first link in the chain — no DSP can rescue a badly captured signal. A simple rule: an antenna resonates when its size relates to the wavelength of the target band.

The most common case is the quarter-wave: a whip of length λ/4. At 100 MHz (λ = 3 m) that's ~75 cm; at 868 MHz, ~8.6 cm; at 2.4 GHz, ~3 cm. That's why a long telescopic is perfect for FM but needlessly large for 2.4 GHz, where a short stub or a dedicated antenna does better.

Useful notions:

Gain & directivity: a directional antenna (Yagi, patch) concentrates reception in one direction → better SNR on a target, at the cost of having to aim it.
Polarisation: vertical vs horizontal; a 90° mismatch between transmitter and receiver loses a lot of signal.
Matching (SWR): an antenna poorly matched to its band reflects energy instead of capturing it. An antenna "tuned" to the band = maximum transfer.

In practice: keep one antenna per major band and swap per mission. Place it high, in the clear, away from noise (power supplies, USB, screens).

Which antenna for what?

Type	Typical gain	Pattern	Best for
λ/2 dipole	2.15 dBi	omni (⊥ plane)	starting out: FM, airband
λ/4 ground plane	~2 dBi	omni	fixed station, ISM
Telescopic whip	varies	omni	multi-band exploration
Discone	~2 dBi	omni, very wideband	scanner listening 25 MHz–1.3 GHz
Yagi	7–15 dBi	directional	DX, signal hunting, satellites
Collinear	5–8 dBi	flattened omni	ADS-B (1090 MHz), repeaters

λ/4 cheat sheet (element length)

Band	Frequency	λ/4
FM	100 MHz	75 cm
Aviation	125 MHz	60 cm
2 m	145 MHz	52 cm
ISM 433	433 MHz	17.3 cm
ISM 868	868 MHz	8.6 cm
ADS-B	1090 MHz	6.9 cm
WiFi	2440 MHz	3.1 cm

Placement golden rule: height beats gain. An average antenna, high and in the clear, beats an excellent antenna at the back of a room — and keep it away from switch-mode supplies, screens and USB cables that pollute the floor.