SondeHardware guide
SondeFox is deliberately minimal on hardware: one Android phone, one Airspy, one antenna. No root, no drivers, no laptop in the passenger seat. Here is exactly what to buy, what to plug into what, and the practical details that make the difference between a smooth chase and a dead spectrum.
Requirements at a glance
| Item | Requirement | Notes |
|---|---|---|
| Phone / tablet | Android 10+ (API 29+) with USB-OTG host support | Most phones from the last several years qualify. No root, ever. |
| SDR receiver | Airspy Mini or Airspy R2 | The only supported hardware today — see RTL-SDR below. |
| Cable | USB-OTG cable or adapter | A powered OTG hub is recommended — the Airspy draws significant current, more than some phones will supply on their own. |
| Antenna | Anything resonant across 400–406 MHz | A ~17 cm quarter-wave whip covers most chasing; a small Yagi helps for the last kilometre. |
| Drivers / root | None | SondeFox talks to the Airspy directly through Android's USB host API. |
| Internet | Optional | Used for SondeHub data and map tiles; tiles cache for offline use, and a chase continues without a connection. |
Why Airspy?
SondeFox is built Airspy-first, and the choice is about signal quality and a clean software path rather than brand loyalty:
12-bit samples
The Airspy delivers 12-bit resolution — welcome dynamic-range headroom when a strong pager or telemetry carrier sits near a faint sonde at the edge of the band.
3 / 6 MSPS
3 MSPS is the stable default for chasing; 6 MSPS is offered as a wideband mode for scanning more of the 400–406 MHz band at once.
Direct USB host API
The app opens the Airspy itself over Android's standard USB host API — no root, no kernel driver, no vendor app. Plug in, grant the USB permission, receive.
Both the Mini and the R2 present the same USB identity; SondeFox tells them apart automatically at runtime, so there is nothing to configure. Gain comes as presets — Quiet rural, Urban / interference, External LNA — plus a full Manual mode with individual LNA, mixer, and VGA sliders if you like to tinker.
Antennas
Radiosondes transmit between 400 and 406 MHz, so anything resonant there works. In practice a two-antenna kit covers the whole chase:
Quarter-wave whip — the driving antenna
A quarter-wave whip is about 17 cm at these frequencies: cheap, omnidirectional, and hard to break. It is the right antenna for the drive — the sonde is high in the sky for most of the flight and you don't yet care about direction, only about keeping decode lock while you move.
Magmount tips: a magmount whip in the centre of the car roof uses the roof as a ground plane and comfortably outperforms an antenna lying on the dashboard behind glass. Route the coax through a door seal rather than pinching it in the door frame, and keep the run short — every metre of thin coax at 400 MHz costs signal.
Small Yagi — the last kilometre
Once the sonde is down, the game changes: the signal is now weak, near the ground, and possibly behind terrain or in a crop field. A small hand-held Yagi gives you gain and a bearing — swing it until the SNR readout peaks and walk that way. It pairs naturally with the Signal Hunt heat-map, which builds a picture from your receiver position and signal-strength samples as you move on foot.
You don't need anything exotic: a 3–5 element Yagi cut for 400–406 MHz is compact enough to live in the boot next to the OTG hub.
Power notes
The Airspy is the hungriest thing on the bus. Some phones drive it happily from a bare OTG adapter; others brown-out the port and the device disappears mid-flight. A powered OTG hub (or a hub fed from a USB battery pack) removes the guesswork and also keeps your phone charging during a long chase — strongly recommended for anything beyond a quick test.
SondeFox can enable the Airspy's bias-tee to power a masthead LNA, and it ships behind an explicit warning toggle for a reason: switching it on feeds DC up the coax, which can damage an antenna or amplifier that isn't designed for it. The electrical behaviour of this path is untested in the current beta — only enable it if you know your LNA expects bias-tee power, and treat it as experimental.
What about RTL-SDR?
SondeFox currently supports the Airspy Mini and R2 only. The capture path — USB protocol, 12-bit unpacking, IQ synthesis, gain presets, fault detection — is purpose-built around the Airspy's interface rather than wrapped around a generic driver layer, which is what lets the app run it directly over the USB host API with no root and recover cleanly from unplugs. Supporting other SDRs would mean building and validating a second capture stack, and during the beta the focus is on making one path excellent. If RTL-SDR support matters to you, tell us at [email protected] — hearing demand helps prioritise the roadmap.
Phone tips
- Feed the phone too. GPS + screen + USB host duty drains a battery fast. A powered hub that passes charge through, or a separate battery bank, keeps a multi-hour chase alive.
- Dim the screen and relax. A foreground service keeps capture and decode running while the screen is dimmed — you don't need to keep the display awake to hold decode lock.
- Grant the USB prompt once per plug-in. Android asks for USB permission when the Airspy appears; accept it and SondeFox takes over from there.
- If the spectrum goes quiet, re-plug. The Airspy's USB control endpoint can occasionally wedge. SondeFox probes for this on open and shows a clear "unplug and re-plug" banner instead of a silently dead spectrum — do what the banner says and you're back in seconds.
- Unplugging is safe. Yanking the cable stops capture cleanly and the app returns to waiting for the Airspy; plug back in to resume.