Before this:What is software-defined radio?
SDR hardware — RTL-SDR, HackRF, Airspy
Key takeaways For trunk-tracking, an RTL-SDR (~$30, ~24 MHz–1.7 GHz, ~2.4 MHz bandwidth, receive-only) is the best place to start and enough for most systems. Airspy adds sensitivity and bandwidth; the Airspy HF+ targets the lower bands. HackRF One is a wideband transceiver (can transmit) but overkill for scanning. The radio mostly sets your frequency range and how much spectrum you capture at once — and GopherTrunk can drive a pool of radios, even remote ones, at the same time.
What is SDR? made the case that the software does the clever part and the hardware is almost interchangeable. “Almost” — there are real differences worth knowing before you buy. This lesson keeps it practical.
What actually matters
When comparing SDRs for trunk-tracking, three specs dominate:
- Frequency range — can it tune the band your targets live in? (VHF/UHF/700-800 MHz for most trunked systems.)
- Bandwidth — how much spectrum it captures at once (sample rate); more lets you follow channels spread further apart from one radio.
- Sensitivity & dynamic range — how well it hears weak signals without overloading on strong ones (tied to gain headroom).
Price and “receive vs. transmit” round it out. For receiving trunked voice, you never need transmit.
RTL-SDR — the cheap entry point
The RTL-SDR (an RTL2832U-based dongle, originally a TV tuner) is the reason this hobby exploded. A good modern unit costs around $30, tunes roughly 24 MHz–1.7 GHz, and captures about 2.4 MHz. It’s receive-only with modest dynamic range — but it’s more than enough to learn on and to follow most VHF/UHF trunked systems in GopherTrunk. Buy a reputable one (quality varies); cheap no-name clones can drift and overload.
Airspy R2 / Mini and HF+
Airspy R2 and the smaller Airspy Mini offer better front ends, higher sensitivity, and wider bandwidth (R2 up to ~10 MHz) than an RTL-SDR — useful when a system’s channels are spread across a band, or in tough RF environments. The Airspy HF+ is a different beast, optimised for the lower bands (HF and low VHF) with excellent dynamic range — the one to reach for if shortwave or low-band is your goal (an RTL-SDR can’t reach HF without a direct-sampling mode or upconverter).
HackRF One
HackRF One is a wideband transceiver: a huge range (~1 MHz–6 GHz) and the ability to transmit. That breadth is great for experimentation, but for receiving trunked voice it’s overkill, has only 8-bit sampling (less dynamic range than Airspy), and transmit is irrelevant to scanning. Worth it if you have broader SDR ambitions; not the obvious trunk-tracking pick.
Comparison at a glance
| Radio | Range | Bandwidth | TX? | Best for |
|---|---|---|---|---|
| RTL-SDR | ~24 MHz–1.7 GHz | ~2.4 MHz | No | Starting out, most trunked systems |
| Airspy Mini/R2 | ~24 MHz–1.8 GHz | up to ~6–10 MHz | No | Wider spans, weak signals |
| Airspy HF+ | HF–low VHF | ~0.66 MHz | No | HF / low-band reception |
| HackRF One | ~1 MHz–6 GHz | up to ~20 MHz | Yes | Wideband experimentation |
Remote backends
The radio doesn’t have to sit next to the computer running GopherTrunk. rtl_tcp and SoapyRemote stream IQ over the network, so you can put a dongle on a Raspberry Pi right at the antenna (short coax = less loss) and decode on a beefier machine elsewhere. GopherTrunk treats these like any other radio.
Matching hardware to your targets
- Identify your systems’ bands (see finding systems).
- Check coverage — an RTL-SDR covers nearly all VHF/UHF trunked work.
- Check span — if your channels fit in ~2 MHz, RTL-SDR is fine; wider may want Airspy or a second dongle.
- Going to HF? Choose Airspy HF+.
- Start cheap. An RTL-SDR teaches you everything; upgrade only when you hit a real limit.
The Hardware guide is the authoritative, GopherTrunk-specific list of supported radios and gotchas.
Quick check: you want to follow a local 800 MHz P25 system on a budget. Best first radio?
Recap
- The radio mainly sets frequency range and capture bandwidth.
- RTL-SDR is the ideal, cheap starting point for VHF/UHF trunk-tracking.
- Airspy adds sensitivity/bandwidth; HF+ for the low bands; HackRF for wideband/TX.
- Remote backends let the dongle live at the antenna.
- Match the radio to your targets’ band and span — and start cheap.
That completes Module 3. Next module digs into the DSP that turns IQ into bits — starting with the FFT and the waterfall.
Frequently asked questions
What is the best SDR for beginners?
An RTL-SDR (a modern RTL2832U dongle, ideally a quality “v3/v4”-class unit) is the best starting point. It costs around $30, covers roughly 24 MHz to 1.7 GHz, captures about 2.4 MHz of bandwidth, and is enough to follow most VHF/UHF trunked systems with GopherTrunk. You can always step up later.
What's the difference between RTL-SDR, HackRF, and Airspy?
RTL-SDR is the cheap, receive-only entry point with modest bandwidth. Airspy radios offer better sensitivity and wider bandwidth for more demanding reception, and the Airspy HF+ specialises in the lower (HF) bands. HackRF One is a wideband transceiver that can also transmit and covers a very large frequency range, but isn’t needed for scanning. For trunk-tracking, RTL-SDR or Airspy are the usual picks.
How much SDR bandwidth do I need for trunked radio?
Enough to cover the channels you follow. If a system’s control and voice channels fit within about 2 MHz, an RTL-SDR is sufficient. If they’re spread wider, an Airspy’s larger bandwidth — or a second dongle covering another chunk — helps. GopherTrunk can drive a pool of radios for exactly this reason.
Can the SDR be on a different computer from GopherTrunk?
Yes. Remote backends like rtl_tcp and SoapyRemote let the radio live on one machine (say, a Raspberry Pi at the antenna) and stream IQ to GopherTrunk on another. This keeps the dongle close to the antenna with a short coax run while you run the decoder elsewhere.