›
PortaPack › Reference › SDR++ Cheatsheet
SDR++ Cheatsheet
HackRF source settings · mode & bandwidth table · squelch · DC spike fix · keyboard shortcuts
HackRF source settings
| Setting | Recommended start | Range | Notes |
|---|---|---|---|
| Sample Rate | 8 – 20 MHz | 2 – 20 MHz | Higher = wider view in waterfall. 20 MHz shows the entire FM band at once. Lower for narrow-band work. |
| LNA Gain | 16 – 24 dB | 0 – 40 dB (8 dB steps) | Primary quality control. Raise until signal visible. Lower if strong signals distort or bleed. |
| VGA Gain | 20 – 30 dB | 0 – 62 dB (2 dB steps) | Volume/brightness fine-tune. Does not cause overload. Adjust after LNA is set. |
| Amp (ext. LNA) | OFF | ON / OFF | Enable for weak signals above ~500 MHz. Provides ~14 dB extra via bias-tee. OFF for FM broadcast. |
| IQ Correction | ON | ON / OFF | Reduces DC spike at centre frequency. Always enable unless it causes issues. |
| Bias Tee | OFF | ON / OFF | Powers the external LNA (⑤) via the coax. Only enable when the LNA module is physically connected — will damage passive antennas. |
Mode & bandwidth reference
| Mode | Full name | Bandwidth | De-emphasis | Used for |
|---|---|---|---|---|
| WFM | Wide FM | 150 – 200 kHz | 50 µs (EU) / 75 µs (US/JP) | FM broadcast. The wide bandwidth is what makes it sound like a proper radio station. |
| NFM | Narrow FM | 12.5 – 25 kHz | — | Airband (AM preferred), PMR446, marine VHF, amateur radio, repeaters. |
| AM | Amplitude Modulation | 6 – 12 kHz | — | Airband (always AM), shortwave, MW broadcast. |
| USB | Upper Sideband | 2.7 – 3 kHz | — | HF ham radio (above 10 MHz), VOLMET weather broadcasts, some maritime. |
| LSB | Lower Sideband | 2.7 – 3 kHz | — | HF ham radio (below 10 MHz). |
| DSB | Double Sideband | 6 kHz | — | Rarely needed. Older aviation beacons, some utility stations. |
| CW | Continuous Wave (Morse) | 50 – 500 Hz | — | Morse code transmissions, beacons, time signals. |
| RAW | Raw IQ | Full sample rate | — | Piping IQ data to external decoders (dump1090, multimon-ng, etc.). |
Airband in WFM sounds like continuous static. FM broadcast in NFM sounds like thin buzzing. AM in NFM sounds robotic. Always match mode to the signal type first.
Squelch
Squelch cuts audio when the signal drops below a threshold — useful for intermittent transmissions so you don't hear constant noise between messages.
| Scenario | Squelch setting | Why |
|---|---|---|
| FM Broadcast | OFF / minimum | Signal is always present — squelch just gets in the way. |
| Airband | –30 to –40 dB | Silence between transmissions. Set just above noise floor so you hear every transmission. |
| PMR446 / Marine VHF | –30 to –40 dB | Same as airband. Adjust until noise floor is muted but real signals break through. |
| Satellites (NOAA, etc.) | OFF | Signal is weak and continuous during pass — squelch will cut it. |
DC spike fix
The HackRF produces a bright vertical line at the exact centre frequency. This is LO leakage — the local oscillator signal bleeding into the output. It's not a real signal.
Method 1 — IQ Correction (recommended)
In the HackRF source panel in SDR++, enable IQ Correction. This applies a software correction that significantly reduces or eliminates the spike. Works for most situations.
Method 2 — Offset tuning
Tune your centre frequency a few MHz away from your target. For example, to receive 100.0 MHz, set the centre to 98.0 MHz. The 100 MHz station appears in the waterfall but the DC spike sits at 98 MHz, away from your signal. Click the station in the waterfall as usual.
Method 3 — Ignore it
For FM broadcast and other wideband work, the DC spike is narrow and falls on the centre frequency only. If you're not tuned exactly to the centre, it doesn't interfere with audio. Many users simply tune slightly off-centre and ignore it.
Keyboard shortcuts
Quick issue checklist
HackRF not detected in SDR++
1. Run hackrf_info in terminal — if no output, it's a driver or USB issue, not SDR++.
2. Try a different USB port (USB 3.0 preferred) or a shorter cable.
3. On Linux: check if the user is in the plugdev group (groups $USER).
4. On Windows: use Zadig to install the WinUSB driver for the HackRF.
Waterfall is entirely bright / no signals distinguishable
Front-end overload from a very strong nearby signal (usually FM broadcast). Lower LNA to 0–8 dB, shorten or disconnect the antenna, then re-tune.
Audio stutters or drops out
USB bandwidth issue. Try: lower sample rate (2–4 MHz), close other USB devices, use a USB 3.0 port directly on the motherboard (not a hub), use a shorter/better cable.
Airband sounds like noise even though planes are overhead
Airband is AM, not FM. Check: mode set to AM · bandwidth 6–12 kHz · frequency on an active channel (check ATIS, approach freq for your nearest airport). Also: raise LNA if the signal is weak.
NOAA pass expected but nothing visible
NOAA APT is weak. Requirements: outdoors or near a window with sky view, loop antenna (⑥) aimed at the sky, high LNA (24–32 dB), external LNA enabled. Use a pass predictor (heavens-above.com or Stellarium) to confirm the satellite is actually above your horizon.