LNA & VGA Gain Guide

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What LNA and VGA actually do

The HackRF has two internal gain stages plus an optional external LNA. Each one amplifies the incoming signal — but each one also amplifies noise. The goal is to add enough gain for a clean signal without adding so much that strong signals distort or noise drowns out weak ones.

LNA — Low Noise Amplifier

Applied at the RF front end, before mixing. 0–40 dB in 8 dB steps.

Controls signal quality. Amplifies the raw antenna signal. Too low → signal is weak. Too high → front-end overload: strong signals clip and bleed into adjacent frequencies. Set this first.

VGA — Variable Gain Amplifier

Applied in baseband, after mixing. 0–62 dB in 2 dB steps.

Controls volume. Amplifies the already-mixed signal. Does not cause front-end overload. Use this to fine-tune once LNA is set. Think of VGA as a volume knob and LNA as a pre-amp.

⑤ External LNA — TQP3M9037

Placed between antenna and HackRF. Powered via bias-tee (HackRF amp ON).

Adds ~20 dB gain with very low noise figure before the signal even reaches the HackRF. Most effective for weak signals above 500 MHz: airband, ADS-B, NOAA satellites. For FM broadcast (already very strong), this module is not needed and may cause overload.

Tuning method

Start with low gain. LNA: 0 dB · VGA: 20 dB. Get the signal visible in the waterfall first.
Raise LNA in steps of 8 dB until the target signal is clearly visible. Stop before the waterfall shows distortion or strong signals bleeding into each other.
Adjust VGA to bring audio to a comfortable level or sharpen the waterfall contrast. VGA doesn't affect overload.
Enable external LNA (⑤) by turning Amp ON in PortaPack. Only for weak signals above ~500 MHz.

⚠ FM overload is the most common mistake

FM broadcast stations are extremely strong. With LNA at 24+ dB and the telescopic antenna extended, FM signals will bleed across the entire spectrum and make everything around 87–108 MHz unusable. For FM itself this isn't a problem — lower the gain instead of raising it.

Settings by scenario

Scenario	LNA	VGA	Amp (ext. LNA)	Notes
FM Broadcast	8–16 dB	20–30 dB	OFF	Signals are very strong. Lower LNA if stations bleed together.
Airband (AM)	16–24 dB	20–30 dB	ON	Moderate strength. External LNA helps for distant aircraft.
NOAA Satellites	24–32 dB	30–40 dB	ON	Weak signal — fast-moving target. Use loop antenna (⑥), outdoors.
ADS-B (1090 MHz)	16–24 dB	20 dB	ON	Pulsed signal. External LNA significantly increases range.
ISM 433 MHz	16–24 dB	20–30 dB	OFF	Short-range transmitters. No external LNA needed unless source is distant.
Marine VHF	16–24 dB	20–30 dB	ON	Moderate strength near water. Loop antenna (⑥).
PMR446	16–24 dB	20 dB	OFF	Close range. Too much gain causes desensitisation with nearby devices.
Wardriving (WiFi/GSM)	8–16 dB	20 dB	OFF	Dense urban environment — lots of strong signals. Keep gain low to avoid overload.

Visualising gain on the waterfall

A correctly-set waterfall has a dark noise floor with clearly visible signals rising above it. Use these symptoms to diagnose:

What you see	Cause	Fix
Waterfall is all one colour (solid bright)	Gain too high — front-end saturated	Lower LNA first, then VGA
Signal barely visible, waterfall is dark noise	Gain too low	Raise LNA in 8 dB steps
Bright vertical stripe at centre frequency	DC spike — normal LO leakage	Enable IQ Correction in SDR++, or offset tune
Strong stations show horizontal bleed across band	FM overload / LNA too high	Lower LNA to 8 dB, shorten antenna
Signal present but audio is distorted/noisy	LNA high enough but VGA too low, or wrong mode	Raise VGA; verify mode (WFM vs NFM vs AM)

✓ Rule of thumb

LNA sets quality. VGA sets loudness. Start with LNA just high enough to see the signal, then use VGA to bring it to a comfortable level.