Understanding dB, dBm and dBFS

The decibel (dB) is a logarithmic unit used to express ratios - usually power or amplitude ratios - in a compact and human-friendly way. Instead of saying "this signal is 10,000 times stronger than that one", you can just say "it's 40 dB stronger". Decibels are relative by design - you always need to know what the reference point is.

Here are a few common variants:

dB: A relative unit without a fixed reference. You’ll often see it used in comparisons like “signal-to-noise ratio = 50 dB”.
dBm: This is a power measurement referenced to 1 milliwatt. A signal of 0 dBm means it's 1 milliwatt of power. This unit is useful for real-world measurements and requires proper calibration.
dBFS: Short for decibels relative to full scale. It’s used in digital systems, where 0 dBFS is the maximum level that the system can represent. All other values are negative (e.g., -10 dBFS, -40 dBFS), since they are below this maximum.

Why the App Uses dBFS, Not dBm

You might expect the signal strength in the app to be shown in dBm, but here’s the catch: dBm requires exact knowledge of the hardware’s RF chain - including gain stages, losses, antenna efficiency, and calibration constants. Every SDR device is different, and many don’t provide accurate power calibration out of the box.

Instead, the app shows signal levels in dBFS - decibels relative to the full scale of the incoming digital signal from your SDR. This is a digital measurement, and it’s consistent across devices because it’s based on the signal’s amplitude relative to the device’s own maximum input range.

What this means for you:

The values are still very useful for comparing signal strengths (e.g., “this signal is 20 dB stronger than the noise floor”).
However, they are not absolute - you can’t use them to determine whether a signal is actually -73 dBm at the antenna, for example.

Understanding the Fast Fourier Transformation

The Fourier Transformation is a powerful tool used throughout signal processing, and it’s at the heart of what your SDR app shows in the spectrum and waterfall displays. In simple terms, it allows us to "look inside" a complex signal and break it down into its individual frequency components.

Imagine you are listening to an orchestra: although all the instruments play together, your brain can often distinguish the violins from the trumpets. The Fourier Transform does something similar for signals - it shows how much of each frequency is present at any given moment.

In practice, we use a computational method called the Fast Fourier Transform (FFT). It's an efficient algorithm that performs the Fourier Transform on discrete digital data. It’s what allows your device to visualize the signal spectrum in real time without needing supercomputers.

FFT Size

The FFT size determines how many signal samples are analyzed at once when transforming the signal from the time domain to the frequency domain. This directly affects the frequency resolution - how precisely we can distinguish between different frequencies.

Importantly, frequency resolution depends on both:

the FFT size
and the sample rate of the input signal.

The formula is simple:

Frequency Resolution = Sample Rate / FFT Size

This means that higher sample rates - like those used with wideband devices such as the HackRF - require larger FFT sizes in order to achieve useful frequency resolution. If you're trying to analyze narrow signals such as SSB (Single Sideband) or CW (Morse code), using a small FFT size with a high sample rate can result in a blurry spectrum where individual signals are difficult to distinguish.

In contrast, if you're using a lower sample rate or analyzing wideband signals (like FM broadcast), smaller FFT sizes may already provide sufficient detail.

Ham Radio

Amateur radio, often simply called ham radio, is a globally licensed hobby and service that allows individuals to explore the airwaves, communicate with others across the world, and experiment with radio technology.

Amateur radio operators use specific frequency bands allocated by international agreements. They are allowed to communicate for non-commercial purposes, engage in emergency services, and even bounce signals off the moon or satellites!

Unlike commercial radio or unlicensed bands (like CB or PMR), operating as a ham requires passing an exam and obtaining a callsign. This may sound daunting - but it’s also incredibly rewarding.

Ham radio blends technical learning, practical skills, and a global community. Whether you are interested in voice contacts, Morse code, digital data modes, or just tuning the bands to listen, there’s something in it for everyone.

Learn more:

Want to get inspired? Try tuning into an amateur band using this app. You’ll often hear contacts in progress, and different modes like USB or CW depending on the band.

Best regards (or as we hams like to put it: 73), Dennis (callsign: DM4NTZ)