SondeHardware guide
SondeFox is deliberately minimal on hardware: one Android phone, one SDR, 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 / R2, or an RTL-SDR dongle (R820T/R820T2/R828D tuner) | Both drive directly over USB; see choosing an SDR and RTL-SDR details. |
| Cable | USB-OTG cable or adapter | A powered OTG hub is recommended: these radios draw 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 SDR directly through Android's USB host API. |
| Internet | Optional | Used for SondeHub data and map tiles; the 2D map caches tiles for offline use, and a chase continues without a connection. The 3D map beta is online-only. |
Choosing an SDR
SondeFox supports two families of receiver, and both run the same feature set: the spectrum and auto-scan, the sonde decoders, and the experimental chirp monitor. The practical difference is dynamic range versus price:
Airspy: 12-bit headroom
The Airspy Mini and R2 deliver 12-bit resolution at 3 or 6 MSPS: welcome dynamic-range headroom when a strong pager or telemetry carrier sits near a faint sonde at the edge of the band.
RTL-SDR: the cheap way in
RTL2832U dongles with R820T/R820T2/R828D tuners (including the RTL-SDR Blog V3/V4) are fully supported: same decoders, same maps, same no-root setup, for a fraction of the price.
Direct USB host API
The app opens the radio itself over Android's standard USB host API: no root, no kernel driver, no vendor app. Plug in, grant the USB permission, receive.
There is nothing to configure: SondeFox scans the USB bus and drives whichever supported radio it finds, and if an Airspy and an RTL dongle are both attached, the Airspy wins. Gain comes as presets (Quiet rural, Urban / interference, External LNA) plus a full Manual mode. On the Airspy the manual sliders drive the LNA, mixer, and VGA stages individually. On an RTL dongle the presets and the LNA slider map onto the R82xx tuner's single gain axis, and any non-Manual profile also runs a closed-loop optimizer that can step the gain by itself; pick Manual for exact control.
If you already own either, start there. For chasing in a city, where strong pagers and telemetry carriers sit next to a faint sonde, the Airspy's 12-bit headroom and automatic overload back-off earn their price. For a first chase rig or a glovebox spare, an RTL-SDR Blog V3/V4 is good value.
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 SDR 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 bias-tee (on the Airspy, and on RTL-SDR Blog dongles that provide one) to power a masthead LNA. It is off by default and sits 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. Only enable it if you know your LNA expects bias-tee power. The setting applies live while streaming and is remembered for the next chase start.
RTL-SDR details
SondeFox drives RTL2832U dongles at the USB register level, the same direct, no-root approach as the Airspy path, not a wrapper around a generic driver. What that means in practice:
- Supported tuners: R820T, R820T2, and R828D, which covers most dongles sold for SDR use, including the RTL-SDR Blog V3/V4.
- Not supported: E4000 and FC0012/FC0013 tuners. Discovery is by USB identity, so a dongle outside the app's recognized list is never found at all rather than rejected with a message.
- Sample rates: selectable from 250 kHz to 3.2 MSPS. The top rate sits at the USB throughput limit and may drop samples on some phones, and a fresh install lands on it with an RTL attached (the 6 MHz Airspy default is clamped down to 3.2 MHz). Pick 2.4M on the Set tab before your first chase.
- Every feature works: the spectrum, auto-scan, all sonde decoders, and the chirp monitor run identically from RTL IQ.
- Where the Airspy keeps an edge: 12-bit versus 8-bit samples, finer manual gain staging, and automatic overload back-off in strong-signal environments (currently Airspy-only).
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 radio 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 sonde marker and trail stay on the map. By default the chase does not resume by itself: plug back in and tap Start Chase. Turn on Auto-restart chase on the Set tab (off by default) if you want the app to reopen the radio on its own.