Before this:What is a radio wave?
Frequency, bands & the spectrum
Key takeaways The radio spectrum runs from a few kilohertz to hundreds of gigahertz, divided into bands — VLF, LF, MF, HF, VHF, UHF, SHF — each with its own propagation behaviour and uses. Regulators publish band plans that say what lives where: broadcast, aviation, marine, public safety, amateur. Knowing the band plan tells you where to point your SDR and what to expect. Most trunked-radio scanning happens in VHF, UHF, and the 700/800 MHz bands.
In lesson 1 you learned that frequency is how fast a wave cycles. This lesson zooms out: how the whole spectrum is measured, named, and carved up — so you know which slice of it to go looking in.
How is frequency measured?
Frequency is counted in hertz (Hz) — cycles per second — but radio frequencies are large, so we lean on prefixes:
| Prefix | Hertz | Example |
|---|---|---|
| kHz (kilo) | 1,000 | AM broadcast, navigation beacons |
| MHz (mega) | 1,000,000 | FM radio, airband, public safety |
| GHz (giga) | 1,000,000,000 | Wi-Fi, GPS, satellite, radar |
When you set GopherTrunk to 460.025 MHz, you’re naming a precise point on this ruler. The spectrum is continuous, but in practice it’s organised into the named bands below.
What are the radio bands?
Each band spans a decade of frequency (a ten-fold range) and behaves differently. The names are an international convention:
| Band | Range | Wavelength | Typical uses |
|---|---|---|---|
| VLF | 3–30 kHz | 100–10 km | Submarine comms, time signals |
| LF | 30–300 kHz | 10–1 km | Longwave broadcast, navigation |
| MF | 300 kHz–3 MHz | 1 km–100 m | AM broadcast, marine |
| HF | 3–30 MHz | 100–10 m | Shortwave, amateur, long-distance |
| VHF | 30–300 MHz | 10–1 m | FM radio, airband, marine, public safety |
| UHF | 300 MHz–3 GHz | 1 m–10 cm | TV, cellular, public-safety trunking, GPS |
| SHF | 3–30 GHz | 10–1 cm | Wi-Fi, satellite, radar |
Notice the wavelength column — it’s just λ ≈ 300 ÷ MHz from the last lesson. It shrinks as you climb, which is why a UHF antenna is a handspan while an HF antenna can be metres long.
Why does the band change how a signal behaves?
The band sets the propagation — how far and by what path a signal travels (the next lesson goes deeper):
- HF waves can refract off the ionosphere and skip thousands of kilometres — that’s how shortwave reaches across oceans.
- VHF and UHF are essentially line-of-sight: they travel to the radio horizon and don’t bend far around the Earth. Local public-safety, airband, and marine all live here because they’re meant to cover a region, not the globe.
- SHF is even more line-of-sight and easily blocked, but carries huge bandwidth.
For GopherTrunk the practical upshot: the trunked systems you’ll follow are regional, so they live in the line-of-sight VHF/UHF/700/800 MHz range.
What is a band plan, and why should I care?
Within each band, regulators and standards bodies divide the spectrum into channels and allocations — a band plan. It’s the master map that keeps broadcasters, aircraft, ships, police, and hams from stepping on each other.
A rough tour of where common things live (exact allocations vary by country):
| Roughly | What you’ll find |
|---|---|
| 88–108 MHz | FM broadcast radio |
| 108–137 MHz | Aircraft (VHF airband, AM) |
| 144–148 MHz | Amateur 2-metre (APRS, repeaters) |
| 156–162 MHz | Marine VHF (and AIS at 161.975 / 162.025) |
| 150–174 MHz | VHF public safety / business |
| 420–450 MHz | Amateur 70-cm |
| 450–470 MHz | UHF public safety / business |
| 700 / 800 MHz | Digital trunked public safety (P25, etc.) |
| 1090 MHz | Aircraft ADS-B transponders |
You don’t memorise this — you look up specific systems in a database. But knowing the shape of the band plan means that when you see a signal on the waterfall, you already have a guess about what it is.
Channel spacing: the grid within a band
Zoom into an allocation and it’s divided again into evenly spaced channels. The spacing is the step between adjacent channel centres, and it tells you how wide each signal is allowed to be:
| Channel spacing | Era / use | Example |
|---|---|---|
| 25 kHz | Older “wideband” land-mobile | Legacy analog FM |
| 12.5 kHz | Modern “narrowband” | P25 Phase 1, DMR, NXDN (wider) |
| 6.25 kHz | Very narrow | NXDN, dPMR |
So a P25 control channel listed as 851.0125 MHz sits on a 12.5 kHz grid — the adjacent channels are at 851.000 and 851.025. This matters in practice: it sets the filter width GopherTrunk needs and explains why mistuning by even a few kilohertz lands you between channels. Regulators have steadily pushed users from 25 → 12.5 kHz (and narrower) to fit more systems in the same band — a big reason analog gave way to digital.
How does this map to my SDR?
Every SDR has a frequency range it can tune. A basic RTL-SDR covers roughly 24 MHz–1.7 GHz — enough for nearly all VHF/UHF trunked scanning, airband, marine, and ADS-B, but not the HF shortwave bands (you’d want an Airspy HF+ or an upconverter for those). Matching your hardware’s range to the band your targets live in is the first hardware decision — see the SDR hardware lesson and the Hardware guide.
Quick check: a regional police trunked system is most likely found in which bands?
Recap
- Frequency is measured in Hz, scaled by kHz / MHz / GHz.
- The spectrum is divided into bands (VLF…SHF); VHF and UHF carry most scanner traffic.
- The band sets propagation — HF can skip worldwide; VHF/UHF are line-of-sight.
- A band plan tells you what lives where, so you know where to look.
- Match your SDR’s tuning range to the band your targets are in.
Next: why two antennas at the same frequency can perform completely differently.
Frequently asked questions
What are the main radio frequency bands?
The radio spectrum is split into bands by frequency. The common ones are VLF (3–30 kHz), LF (30–300 kHz), MF (300 kHz–3 MHz), HF (3–30 MHz), VHF (30–300 MHz), UHF (300 MHz–3 GHz), and SHF (3–30 GHz). Each has different propagation behaviour and is allocated to different uses. Most scanner and trunked-radio activity lives in VHF and UHF.
What frequencies do police and fire use?
It varies by country and region, but public-safety radio commonly sits in VHF (around 150–174 MHz), UHF (around 450–470 MHz), and the 700/800 MHz bands, increasingly as digital trunked systems. There is no single answer — you look up the specific system in a database like RadioReference for your area.
What is a band plan?
A band plan is the agreed division of a frequency band into channels and uses. Regulators (like the FCC in the US or Ofcom in the UK) and standards bodies publish band plans so broadcasters, public safety, aviation, marine, amateur, and other users do not interfere with each other. It tells you what kind of signal to expect at a given frequency.
Why does the band matter for what I can receive?
Different bands propagate differently and need different antennas. HF can bounce off the ionosphere and travel worldwide; VHF and UHF are mostly line-of-sight. The band also tells you what is there — aircraft on VHF airband, marine on VHF marine, trunked public safety on UHF and 700/800 MHz — so knowing the band plan tells you where to point your SDR.