The airwaves are divided up like a street map: every frequency has an owner. Here you'll see who talks on each one (CB, walkie-talkies, emergency networks, aircraft, ships, satellites), how they sound, what to listen with, what is legal to listen to and, at the end, everything about antennas.
An invisible street map
The spectrum map
From 100 kHz to 12 GHz, arranged by who uses it. The scale is logarithmic: each mark multiplies the frequency by ten. Tap a band to find out what it is and where it's explained.
100 kHz1 MHz10 MHz100 MHz1 GHz10 GHz
Choose a band on the map.
Approximate ranges based on Spain's national frequency allocation table and European agreements. Each band holds more services than fit in a drawing.
For anyone
Licence-free radio
Some bands are for common use: anyone may transmit on them, with no exam or paperwork, as long as they use type-approved equipment and stick to the power limit. They are many people's way into radio.
27 MHz · common use
CB, Citizens Band
It was born in the United States after the Second World War, became a craze there in the 1970s and in Spain in the 1980s, when lorry drivers filled it with nicknames and road talk. It's still alive among lorry drivers, country folk, DX fans (long-distance contacts) and anyone who wants to talk on the radio without an exam.
How it works
40 channels of 10 kHz between 26.960 and 27.410 MHz.
Three modes: AM (the classic), FM (cleaner over short distances) and SSB, single sideband (goes further with the same power; the favourite for long-distance contacts).
Power: 4 W in AM and FM; 12 W peak in SSB.
Channel 9 (27.065 MHz): distress. It is always kept clear when there's an emergency.
Direct only: radio to radio, with no repeaters and no links to the internet or the phone network.
How far it goes
With a radio in the car, 5 to 20 km depending on the terrain; from a house with a good antenna, more. As it sits right next to the 10-metre amateur band, it behaves the same way: in the good years of the solar cycle the ionosphere bounces it back to earth and you hear stations from other countries, and in summer there are sudden openings thanks to sporadic E.
The equipment
A type-approved mobile or base radio (CE marking, European standard EN 300 433) and, above all, a good antenna: the wavelength is 11 metres, so a quarter-wave vertical is about 2.7 m long. Car antennas are usually shorter, with a coil that "lengthens" them electrically at some cost in performance. More in antennas.
Not allowed on CB in Spain: amplifiers, exceeding the power limit, repeaters, connecting it to the phone network and transmitting a continuous carrier. Since 2015 no licence or call sign is needed (Orden IET/614/2015, note UN-3 of the national frequency allocation table).
446 MHz · common use
PMR446 walkie-talkies
The walkie-talkies from toy shops, hiking stores and building sites. 16 analogue FM channels between 446.0 and 446.2 MHz, 0.5 W and a fixed antenna (it can't be changed). There are digital versions (dPMR446 and DMR446) in the same band, which can't talk to the analogue ones.
Range: from a few hundred metres among buildings to several kilometres with a clear line of sight; from a summit, much further.
"Privacy codes" are not private. They are sub-tones (CTCSS or DCS): your walkie only opens its speaker when the chosen tone arrives, but anyone on the same channel without a tone hears you perfectly. They stop you hearing others, not others hearing you.
433 MHz and more · short range
Short-range devices
Garage remotes, wireless doorbells, home weather stations, sensors and 10 mW LPD walkie-talkies use bands such as 433 MHz (433.05–434.79 MHz) or 868 MHz, at very low power and with no licence.
Wifi and Bluetooth are licence-free radio too, at 2.4 and 5 GHz. With a cheap SDR receiver you can see all of this on the waterfall: short bursts every time someone opens a garage or a sensor sends the temperature.
With a business licence
Professional and emergency networks
Taxis, ambulances, police, fire brigades, metros, ports or a big building site use radio because it's instant: press and your whole group hears you at once, with no dialling or waiting, and it works even if the mobile network goes down. Each organisation has frequencies assigned by the authorities. These networks are not intended for the public (see what you may listen to).
Analogue
Analogue PMR
Good old radio: FM on 12.5 kHz channels in VHF (around 146–174 MHz) and UHF (400–470 MHz), with repeaters on high ground. Cheap and simple; used by private security, building sites, councils and transport. More and more of it is going digital.
Digital · 12.5 kHz
DMR
The European (ETSI) standard most used by businesses: each 12.5 kHz channel carries two conversations at once, taking turns in 30-millisecond slots. It has three tiers: DMR446 (licence-free), conventional and trunked (a network that shares out the channels). Radio amateurs use it too, on their own bands.
Digital · 6.25 kHz
dPMR and NXDN
DMR's siblings which, instead of taking turns, use narrower 6.25 kHz channels. Popular with small businesses, hospitality and events.
Digital trunked · mission-critical
TETRA, the radio of the emergency services
TETRA (Terrestrial Trunked Radio) is a European standard from 1995 designed for the emergency services. It doesn't work like a walkie-talkie but like a mobile phone network for walkie-talkies: many base stations, a central system that shares out the channels, and terminals that move from one cell to another without dropping.
What makes it special
Instant group calls: press and everyone in your group (a police station, a fire crew) hears you in under a second.
Four conversations per channel: each 25 kHz channel is split into four time slots (TDMA), with digital phase modulation (π/4-DQPSK) at 36 kbit/s gross.
Emergency button that overrides any other call.
Direct mode: with no network (a forest fire, a tunnel), terminals talk to each other like walkie-talkies.
Encryption over the air and end to end: police conversations can't be heard.
Slow data: messages, GPS position and status, but no video (about 28.8 kbit/s at most).
Who uses it in Spain
Regional emergency networks, in the 380–400 MHz band that Europe reserves for public safety. For example RESCAT in Catalonia: over 60,000 users (regional police, local police, firefighters, paramedics, civil protection) and 321 base stations.
Local police, fire brigades and civil protection in many towns.
Transport and large sites: metros, airports, ports, on other 400 MHz bands.
The national network, SIRDEE (National Police, Guardia Civil and others, over 150,000 users), uses TETRAPOL, a sibling system of French origin that shares channels out by frequency rather than by time slot.
What it looks like
On an SDR receiver's waterfall, a TETRA base station is a 25 kHz-wide block that never switches off: it transmits its control channel non-stop so terminals know there's a network. Tuned in FM, it sounds like a continuous raspy buzz. You can hear an imitation in how they sound.
The future: emergency networks are starting to move to mission-critical broadband (push-to-talk, video and data over 4G and 5G with priority). TETRA will stay for years as the backup voice network, because it keeps working when everything else fails.
Feature
Analogue PMR
DMR
TETRA
TETRAPOL
Type
Analogue FM
Digital
Digital trunked
Digital trunked
Channel width
12.5 kHz
12.5 kHz
25 kHz
12.5 kHz
Conversations per channel
1
2 (slots)
4 (slots)
1
Encryption
No
Optional
Yes
Yes
Typical use
Building sites, private security
Businesses, radio amateurs
Regional and local emergency services, metros
SIRDEE (National Police, Guardia Civil)
Volunteering
When everything fails: amateur radio in emergencies
If power, mobile and internet go down, a battery-powered radio and a wire antenna keep working. That's why Spain's civil protection has REMER (the emergency radio network), made up of volunteer radio amateurs, and many clubs run drills. It's one of the best reasons to get licensed: get started.
What flies and what sails
Aircraft and ships
Aviation and shipping have used voice radio for a century, and today they also broadcast their position automatically, so other aircraft, other ships and anyone with a receiver can see them.
118–137 MHz · AM
✈️ Airband
Pilots and controllers talk in AM, not FM. That's deliberate: if two transmit at once, in AM you hear both on top of each other (and someone notices); in FM the stronger one would blot out the other without anyone knowing. In Europe channels are 8.33 kHz apart.
121.5 MHz is the international emergency frequency, which every aircraft monitors.
ADS-B, 1090 MHz
Every airliner broadcasts its identity, position, altitude and speed every second. Flight-tracking websites are fed by thousands of volunteers' receivers: with a €30 SDR and a 7 cm antenna you can be one of them.
156–162 MHz · FM
⚓ Marine VHF
Ships, ports and rescue services talk on numbered FM channels. Channel 16 (156.800 MHz) is for distress and calling: everyone listens to it and moves to another channel to talk. Channel 70 is reserved for digital selective calling (DSC), the "red button" on ship radios.
AIS, 161.975 and 162.025 MHz
Ships broadcast their name, position, course and speed in very short digital bursts. That's how live ship-tracking maps work, also built from volunteers' receivers along the coast.
Signals falling from the sky
Satellites and balloons
Some of the easiest and most exciting signals to receive come from above. Amateur satellites and the Space Station are in Understand; here, the weather ones.
137 MHz
🛰️ Weather satellites
For decades the American NOAA satellites sent pictures of the clouds on 137 MHz that anyone could receive with a home-made antenna: they sounded like a rhythmic "tick-tock". They were switched off in 2025 (NOAA-18 in June, NOAA-19 on 13 August and NOAA-15 on 19 August).
The Russian Meteor-M satellites still transmit on the same band, digitally (LRPT): with an SDR, a satellite antenna and the right software you get colour images of your region as the satellite passes over.
400–406 MHz
🎈 Radiosondes
Twice a day, at 00 and 12 UTC, Spain's AEMET and weather services around the world release balloons carrying a radiosonde: a little box that measures temperature, humidity, pressure and wind as it climbs to 30 km, transmitting every second.
You can follow them live on collaborative maps such as SondeHub, fed by hobbyists. When the balloon bursts, the sonde falls by parachute and some people go and look for it: it's called "radiosonde hunting".
For everyone
Broadcasting
Radio "as we know it": stations meant to be heard by everybody. It's the only part of the spectrum explicitly intended for the public.
Band
Frequencies
What you'll hear
Range
Long wave
148.5–283.5 kHz
Few stations left; some European ones at night
Hundreds of km by ground wave
Medium wave (AM)
531–1,602 kHz
National and local stations; at night, half of Europe
By day, your region; at night, the continent
Shortwave
Bands between 2.3 and 26 MHz (49, 41, 31, 25, 19 metres…)
International and religious stations, from other continents
Tune through these bands with a receiver and you don't hear voices: you hear buzzes, screeches and bursts. Each system has its own "signature". Tap a signal to hear an imitation generated in your browser and see how it's drawn.
Choose a signal
0 Hz2,0004,0006,000 Hz
This is what comes out of an FM receiver's speaker when you tune to each signal: you can't make out the content (and on encrypted networks you can't decode it). Real recordings are behind each signal's link.
The signals are generated by imitating their rhythm and shape. Identifying signals is a hobby in itself: there are catalogues with hundreds of them.
The gear
What to listen with
You don't need a licence to listen (you do to transmit, except on the common-use bands). These are the receivers, from simplest to most versatile. Prices are a rough guide.
Receiver
What it picks up
Price
Who it's for
Web receiver (WebSDR, KiwiSDR)
Whatever someone else's antenna picks up, anywhere in the world
Free
Starting today, without buying anything
Portable multiband radio
Medium wave, shortwave and FM; with SSB, radio amateurs too
€30–150
Listening to world stations from the sofa
USB SDR (RTL-SDR type)
From about 24 MHz to 1.7 GHz; recent models also shortwave. You see the waterfall on your computer
€30–40
The best way in: aircraft, ships, satellites, radiosondes, walkie-talkies…
Quality SDR
Shortwave and more, with much less noise and more headroom
€100–300
Serious shortwave listening and DX
Scanner
Sweeps analogue VHF and UHF channels; digital ones understand some unencrypted systems
€150–500
Anyone wanting something pocket-sized with no computer
Walkie-talkie or CB radio
Its common-use band
€30–200
To talk, not just listen
Amateur radio transceiver
Many receive almost everything, from long wave to UHF
From €30 (handheld) to thousands
With a licence, to transmit
With an SDR you use free programs such as SDR++, SDR# or GQRX. And remember: the antenna matters more than the receiver. A cheap one with a good antenna hears more than an expensive one with a poor antenna.
Before you switch on the receiver
What you may listen to (and what not)
Owning a receiver is legal. What matters is what you listen to and what you do with it. In Spain the Constitution protects the secrecy of communications, and the General Telecommunications Act treats intercepting communications not intended for the general public without authorisation as a very serious offence, and also disclosing what was heard. Most countries have similar rules.
✅ Free
Broadcasting: long, medium and short wave, FM, DAB+.
Radio amateurs: they talk in the open and like being listened to.
CB and PMR446: common use.
Beacons, amateur and weather satellites.
Radiosondes, aircraft ADS-B and ship AIS: position broadcasts designed for anyone to receive.
⚠️ Service communications
Aviation and marine voice, taxis, businesses, private security. Plenty of people tune in with scanners, but they are not intended for the public and Spanish law makes no exceptions: don't record them, don't share them and don't use what you hear.
⛔ No
Police, emergency and health services (TETRA, TETRAPOL, service DMR): they are also encrypted, and trying to decrypt them is far more serious.
Mobile phones, cordless phones and any private communication.
Seeing a signal on the waterfall isn't listening to its content: recognising a TETRA network by its shape is signal identification, not interception.
This is general guidance, not legal advice. And to transmit, outside the common-use bands, you need a licence: how to get one.
More range
Repeaters: how a signal gets further
On VHF and UHF, waves travel in a straight line and stop at the horizon or behind a hill. The solution is to put something very high up that receives and transmits again.
The repeater listens on one frequency and retransmits on another at the same time, with much more power and from a hilltop. Your handheld transmits on the input and listens on the output: your radio switches by itself if you set the offset.
Service
Repeaters?
What they're like
Radio amateurs
Yes, hundreds
Analogue FM (on 2 m you transmit 600 kHz lower; on 70 cm, 7.6 MHz lower in Spain) with an access tone, and digital ones (DMR, C4FM, D-STAR) linked over the internet. Clubs look after them. How to use them and calculator
Amateur satellites
Yes, in orbit
FM satellites are repeaters 500–800 km up: with a handheld you can talk to someone 2,000 km away. Satellites
Businesses (PMR and DMR)
Yes
Their own licensed repeaters on high ground; in DMR, one repeater carries two conversations.
TETRA and TETRAPOL
A network of base stations
Not standalone repeaters but a network like the mobile phone one, with a central system and coverage repeaters in tunnels and buildings.
CB and PMR446
No
Not allowed: only direct radio-to-radio contact.
The most important part of the station
Antennas
A radio without an antenna is deaf and dumb. The antenna decides how far you get and what you hear, far more than the radio does. And it's the most fun thing to build: with a well-cut piece of wire you can talk to other continents.
What is an antenna?
A piece of metal with just the right shape and size to turn an electric current flowing back and forth very fast into radio waves that travel through the air (when transmitting), and the other way round (when receiving). The same antenna does both.
It works like a guitar string: it vibrates best at the frequency it's sized for. That's why an antenna is roughly half a wavelength or a quarter long, and why shortwave antennas are huge while the one in your phone fits inside it.
Wavelength (m) = 300 ÷ frequency (MHz)
📏 How long is the antenna?
Theoretical lengths in free air; in practice you cut them about 5% shorter and trim. Detailed calculator in your first station.
What you need to know about any antenna
🎯 Resonance
An antenna "resonates" at one frequency, like the string. Away from it, the energy doesn't get out and comes back to the radio.
🔌 Impedance
The resistance it presents to the cable. Radios and cables use 50 Ω: if the antenna does too, all the power goes through.
📉 SWR
Standing wave ratio: how much energy bounces back. 1:1 is perfect, under 1.5 is very good, over 3 is bad.
🔦 Gain
An antenna doesn't create energy: it concentrates it in some directions, like a torch reflector. Measured in dBi (against an ideal antenna that radiates equally everywhere) or dBd (against a dipole). +3 dB is double.
🌸 Radiation pattern
The drawing of where it radiates. Omnidirectional: equally all round the horizon. Directional: strongly one way, weakly the others.
↕️ Polarisation
The orientation of the wave: vertical (handhelds, cars, repeaters), horizontal (long-distance VHF, TV) or circular (satellites). If it doesn't match, you lose a lot of signal.
📶 Bandwidth
How far you can move from the ideal frequency before the SWR rises too much. Thick antennas and broadband ones are more forgiving.
🏔️ Height and surroundings
On VHF and UHF every metre of height counts: the signal goes as far as the horizon. On shortwave, height above ground changes the angle at which it heads for the ionosphere.
👂 Receiving and transmitting
To listen, almost any wire will do. To transmit, the antenna must be tuned, or the radio protects itself by cutting power (or gets damaged).
How many types are there?
Hundreds of variants, but almost all belong to seven families: dipoles, verticals, directional antennas, wire antennas, loops, broadband antennas and reflectors. These are the 18 you'll see most. Filter by what you want to do:
Dipoles
Half-wave dipole
Two equal wires in a line, fed in the centre. The reference antenna: simple, cheap and effective.
Bands
Shortwave to UHF
Gain
2.15 dBi
Radiates
In a figure of eight, to the sides
Ideal for
Your first shortwave antenna, strung between two points
Dipoles
Inverted V
A dipole with its centre at the top of a single mast and the arms sloping down in a V. Needs less space and one support.
Bands
Shortwave
Gain
≈ 2 dBi
Radiates
Almost in all directions
Ideal for
Gardens and portable use
Verticals
Quarter-wave vertical
A vertical rod and some wires (radials) that act as a "mirror" on the ground. Small footprint.
Bands
Shortwave to UHF
Gain
≈ 2–5 dBi
Radiates
Equally all round, low angle
Ideal for
Long-distance contacts, CB, VHF base
Verticals
Whip and handheld antenna
The flexible rubber antenna on a walkie-talkie or the rod on a car: a shortened vertical with a coil. Convenient, but less efficient.
Bands
VHF and UHF, CB in the car
Gain
Negative: −2 to 0 dBi
Radiates
In all directions
Ideal for
Carrying around
Verticals
J-pole and Slim Jim
A half-wave vertical that needs no radials, shaped like a J. Made at home from copper pipe or twin-lead ribbon.
Bands
VHF and UHF
Gain
≈ 2–3 dBi
Radiates
In all directions
Ideal for
Reaching the repeater from home, a first project
Verticals
Collinear
Several half-waves stacked inside a fibreglass tube. It squashes the radiation towards the horizon: more range with no preferred direction.
Bands
VHF, UHF, 1090 MHz
Gain
≈ 5–9 dBi
Radiates
In all directions, very flat
Ideal for
Rooftops, repeaters, receiving aircraft
Directional
Yagi
The "fishbone" on rooftops: a driven element, a reflector behind and directors in front. The more elements, the more gain and the narrower the beam.
Bands
Upper shortwave to microwave
Gain
6–17 dBi
Radiates
Forwards
Ideal for
Long-distance VHF, hand-held satellite work
Directional
Moxon and quad
Compact directional antennas: the Moxon is a folded wire rectangle; the quad, one or more wire squares. Good front-to-back ratio in little space.
Bands
Upper shortwave, VHF
Gain
≈ 5–8 dBi
Radiates
Forwards, almost nothing behind
Ideal for
A light home-made beam
Broadband
Log-periodic
It looks like a Yagi, but its elements grow gradually: each pair works on part of the band. Directional and very wideband.
Bands
For example 100 MHz–1.3 GHz
Gain
≈ 6–8 dBi
Radiates
Forwards
Ideal for
Listening to many bands with one antenna
Broadband
Discone
A disc on top of a cone of rods. It receives decently from 25 MHz to over 1 GHz with no tuning at all.
Bands
≈ 25–1,300 MHz
Gain
≈ 0–2 dBi
Radiates
In all directions
Ideal for
A scanner or SDR on the roof
Wire
End-fed half-wave
A half-wave wire with a transformer at one end. It works on several shortwave bands and hangs from a tree in minutes.
Bands
Several shortwave bands
Gain
≈ 2 dBi
Radiates
Varies with the band
Ideal for
Portable, mountains, parks (POTA)
Wire
Long wire
A wire as long as will fit, as high as possible. For listening to shortwave, unbeatable for the price. To transmit it needs a tuner.
Bands
Long wave to shortwave
Gain
Variable
Radiates
Depends on the length
Ideal for
Your first listening antenna
Loops
Full-wave loop (delta)
A triangle or square of wire one wavelength round. A little more gain than a dipole and less noise.
Bands
Shortwave
Gain
≈ 3–4 dBi
Radiates
To both sides of the loop
Ideal for
Anyone with a bit of land
Loops
Magnetic loop
A ring of copper pipe one or two metres across with a variable capacitor. Very small for shortwave and quiet, but it has to be retuned every time you change frequency.
Bands
Shortwave
Gain
A little less than a dipole
Radiates
Out of the sides of the ring
Ideal for
Balconies and flats
Receiving
Active antenna
A short rod with an amplifier. It only receives, but covers long wave to shortwave on a half-metre stick.
Bands
Long wave to 30 MHz
Gain
From the amplifier
Radiates
Doesn't transmit
Ideal for
Shortwave listening with an SDR
Receiving
Ferrite rod
The one inside medium-wave radios: a coil on a ferrite bar. That's why turning the radio makes a station louder or quieter.
Bands
Long and medium wave
Gain
Very low
Radiates
Doesn't transmit; receives from two sides
Ideal for
Portable radios
Satellites
Quadrifilar helix (QFH) and helical
Wire helices with circular polarisation that receive the satellite wherever it is without moving them. The QFH is the classic for 137 MHz weather satellites.
Bands
VHF to microwave
Gain
≈ 2–12 dBi
Radiates
Towards the sky (QFH) or forwards
Ideal for
Weather satellites, GPS
Reflectors
Dish
A dish that focuses the signal on one point, like a mirror. At high frequencies it gives huge gain at a reasonable size.
Bands
Microwave (GHz)
Gain
25–40 dBi
Radiates
A very narrow beam
Ideal for
QO-100, satellite TV
Which one should I choose?
I want to…
Antenna
Listen to shortwave from a flat
A long wire out of the window, an active antenna or a magnetic loop on the balcony
Start with an SDR (aircraft, ships, FM, walkie-talkies)
The telescopic dipole that usually comes with the SDR, near a window; later, a discone on the roof
Track aircraft (ADS-B)
A quarter-wave vertical for 1090 MHz (6.9 cm) or a home-made collinear, as high as possible
Weather satellites
A QFH or V-dipole for 137 MHz, outdoors with a clear sky
Amateur satellites with a handheld
A small hand-held Yagi for 2 m and 70 cm, pointed at the satellite
CB in the car
A vertical on a magnetic mount, the longer the better (a quarter-wave is about 2.7 m)
2 m and 70 cm repeaters from home
A collinear or J-pole on the roof
Licensed shortwave at home
A dipole or inverted V for one band; an end-fed half-wave for several
QO-100
A 60 to 120 cm dish with a TV LNB to receive
⚡ Safety first
Never near power lines: neither while putting it up nor if it falls. That's the cause of serious antenna accidents.
Earthing and lightning protection for outdoor antennas. In a storm, disconnect the antenna from the radio.
Don't touch an antenna while it's transmitting: high-power ones can burn.
Roofs and neighbours: in Spain, licensed radio amateurs have the right to put up antennas on their building, with conditions. We explain it in your first station.
Sources: Spain's national frequency allocation table (CNAF); Orden IET/614/2015 (CB); ECC Decision (15)05 (PMR446); ETSI EN 300 392 (TETRA) and TS 102 361 (DMR); NOAA/NESDIS (end of the NOAA satellites, 2025); Ara newspaper (RESCAT) and Spain's Ministry of the Interior (SIRDEE). Checked on 2 November 2026.