Wireless Data Transmission

Legal Frequencies, Modulations And Protocols

Electromagnetic waves can invisibly transfer information through the air, and we all use this technology in Smartphones, WiFi, Walky Talkys, Wireless Headphones, and often even when we just grab a Remote Control and switch stations on the TV.


To give you a quick overview of available digital wireless transmission techniques, here is a quick comparison:

Technology Range Speed Requires Microcontroller
WiFi 40m fast yes (WiFi built into (most, not all) ESP microcontrollers)
Bluetooth 15m fast yes (Bluetooth built into many (not all) ESP microcontrollers )
ESPNow 100m fast legacy protocol for ESP microcontrollers only
nRF24 5-20m fast yes, typically SPI interface
SRD 5-20m medium no
LoRa 1-10km very slow typically they come as complete breakout boards with microcontroller

Range was estimated for inhouse use (excepz for LoRa where this makes no sense). Range depends on additional factors such as antenna, rf output, and rx sensitivity and are just rough estimates. Outside with free line of sight, the range can be 10-20x higher.


When you plan to use radio waves to transfer information as part of your DIY Project, it is important to understand the frequencies that are legal to use, the schemes in which data can be sent and received over the air, plus the specific physical pros and cons attached to given frequency ranges.

Here is a quick guideline:

Use Case Recommended Frequency
Indoor 2.4GHz
Garden 433MHz
Long Range 868MHz

Maximizing Range

Two rules describe the effect of frequency on the range that can be reached:


Rule #1: The higher a frequency, the shorter its antenna (higher frequencies have shorter wave lengths).

Antennas operating at higher frequencies can be considerably smaller.:

  • 2.4GHz WiFi/Bluetooth/ESPnow/nRF24 antennas measure around 3.06cm for a lamda/4 monopole
  • 868MHz SRD/LoRa antennas measure around 8.63cm for the same Lambda/4 monopole
  • 433MHz SRD/LoRa antennas measure around 17.27cm for the same Lambda/4 monopole

Smaller antenna size can be advantageous where space is limited or where smaller devices are needed.


Rule #2: Use higher frequencies in urban environments and inside buildings. Use lower frequencies for rural areas and long range connections.

While it is true that higher frequencies are more sensitive to a free line of sight with no obstacles in the way (like mountain ranges), this applies to long range communications. It does not so much apply to the kind of short range communications typically needed in DIY projects.

Obviously, a metal-enforced concrete wall will attenuate or block a radio signal, and the higher a frequency is, the more it is susceptible to this kind of obstacle.

However, most concrete walls and other obstacles do have holes, i.e. windows, doors, or hallways (normal window glass does not interfere with radio waves). These holes do not block radio waves at all provided the holes are bigger than the radio wave length that wants to pass it.

Higher Frequency = Smaller Wavelength

The wave length for 2.4GHz (as used in WiFi/Bluetooth/ESPnow/nRF24) is just 12.5cm, and most openings in urban obstacles are much larger.

The wave length for 433.92MHz (as used in SRD/LoRa) is 69cm, and such a radio wave would have to pass a door stock at pretty much the right phase angle to pass it unattenuated.

Smaller Wavelength = Better For Urban Environments

That’s why for indoor use and in urban environments, higher frequencies perform generally better even though they are attenuated more when hitting obstacles.

Another important factor in choosing a frequency is radio interference from other users. SRD is especially susceptible as it does not use frequency spread. In Europe, the license-free 433MHz band is narrow with traffic focusing on a few frequencies only. The 868MHz band is typically less limited.

Frequency Allocations

Radio waves are emissions to the public: anyone in the vicinity can detect and read them. So anyone can also interfere with anyone else in a radio spectrum.

Radio waves can also harm people, at least when sent with sufficient transmit power. This is why both frequency and maximum emission power is strictly regulated in all countries of the world.

At the end of the day, national governments allocate frequencies and determine the conditions of use.

In a globalized world, harmonizations take place, though, and supra-national organizations negotiate regional and global frequency allocations.

The ITU (International Telecommunications Union) has devided the world into three regions with geographical definitions.

ISM Radio Band

Almost all frequency ranges are licensed: you cannot use them without a proper license.

ISM Radio Bands are frequency ranges within the radio spectrum reserved internationally for industrial, scientific, and medical (ISM) purposes. These bands are unlicensed, making them attractive for DIY Projects and short range devices.


ISM bands are unlicensed: users do not need to acquire a license to use them.

Unlicensed means that you do not need a specific license (i.e. to operate a microwave oven which emits radio frequencies in the ISM frequency range).

You still have to adhere to regulations. So while you can operate a microwave oven, you cannot operate your own public broadcast radio station on the same frequency. Emissions must match approved use-cases, and your emitted radio power must meet technical criteria.

So unlicensed does not at all mean unregulated.

Here are the key aspects of ISM radio bands:

  • No License: Users need no specific license when they operate senders in accordance with the rules and technical specifications for a given ISM band.
  • Interference: Users must tolerate even strong interference from other users.


Most other frequency bands in the radio spectrum are licensed: a licensed band is assigned strictly to certain users. Using a licensed band requires a permission from the national government.

Typically you need a license (i.e. GSM, Radio Amateur, Public Broadcast Station) or be member of a licensed group (i.e. police, ambulance service, aviation, military).

It’s a bit like in the physical world: ISM bands are public parks: anyone can go there but must live with interference caused by others (i.e. music, ball games, etc.). Licensed bands are private property, and when you illegally sneak in and make yourself at home, you are trespassing and will be prosecuted.

Licensed bands grant exclusive use and guarantee operation without interference from other services or users. ISM bands being unlicensed grant no protection from interference.

History and Evolution

The initial industrial and medical use of ISM bands focused on strong senders such as microwave ovens or diathermy devices. While these devices can emit very strong radio signals, they are no communications device and have no receivers. Interference from other users and devices therefore was no problem.

For the same reasons, ISM bands were initially not used for communications.

Only in recent decades, the ever increasing demand for free wireless communications with increasingly congested radio spectrums moved ISM bands to the fore again. After all, ISM bands are attractive for free public communication as they do not require any license to use.

Resiliency To Interference

Technological advances in microelectronics and subsequently in modulations and encodings eventually enabled reliable communications on ISM bands despite strong interference:

  • Spread Spectrum: Communications systems with focus on reliability and/or long distance transmissions use digital spread spectrum techniques that spread out the RF signal onto a much wider frequency range. This makes them resilient against strong interferences from industrial applications which are typically blocking out only a distinct frequency. WiFi and Bluetooth are examples.
  • Low Range Devices: Communications systems with must bridge a very short distance can typically be used safely even in the presence of strong interfering signals as their signal field strengths are strong enough in close vicinity. Examples include garage door openers and wireless sensors, wireless door bells, baby phones and alike.

Since public communications is neither industrial, nor scientific or medical, these use-cases are often referred to as Non-ISM.

ISM Frequencies

ISM is comprised of a number of frequency ranges (“bands”) defined by the ITU Radio Regulations:

Frequency Range Bandwidth Example
6.765-6.795 MHz 30kHz SRD
13.553-13.567MHz 14kHz NFR (Near-field communication), RFID, SRD, Heating
26.957-27.283MHz 326kHz NFR, CB Radio
40.66-40.7MHz 40kHz SRD, earth exploration satellites
433.05-434.79MHz 1.74MHz SRD, just Region 1
902-928MHz 26MHz SRD, just Region 2
2.4-2.5GHz 100MHz WiFi (802.11), Bluetooth, Plasma Lamps, ANT, Zigbee, Microwaves
5.725-5.875GHz 150MHz WiFi (802.11)
24-24.25GHz 250MHz Radar
61 GHz-61.5GHz 500MHz car distance sensors, WiFi
122-123GHz 1GHz local approval required
244-246GHz 2GHz local approval required

Non-ISM Frequencies

The frequency range 863-870MHz is often confused with ISM when in reality it is a licensed band.

In Europe, this frequency range is allocated by CEPT for short range devices (SRD) as part of ERC Recommendation 70-03: the EU863-870 band can be typically used wherever the ISM radio spectrum is defined by the ETSI 307 standard.

Materials: ETSI Short Range Devices

National Exceptions

Many national exceptions exist as the national governments have the final say.


For example, in Germany the frequency ranges 9-10kHz and 149.995-150.005 (FreeNet voice communications) are allocated for unlicensed use.


The US FCC on the other hand allows “periodic operation” of control signals anywhere above 70MHz as long as they meet Section 15.231. This is the legal foundation for garage door openers and other digital senders operating at 315MHz. More info on the US-specific 315MHz band.

Band Plans

Based on region, band plans are created that define in detail which frequency is reserved for a particular purpose and usage.

Below are regional band plans that allocate frequency ranges for license-free radio communications in different regions of the world:

Area Band Plan
Europe EU433, EU863-870, WiFi, BlueTooth
South America AU915-928
North America US902-928
India IN865-867
Indonesia AS923-925
Malaysia AS920-923

The two most influential agencies - based on size of market they impact - are the FCC in the US and ETSI in Europe. Among others, these agencies work on band plans and their future development, and also enforce rules.

Still, at the end each national government has the final say over the frequency allocations for their territory. They can adopt standardized regional band plans, add national exceptions and make adjustments.

The Asian region frequently uses band plan AS923-925 but there are exceptions.

Likewise, South America typically uses AU915-928. Countries like Mexico adopted the North American standard US902-928.

There may also be additional national opportunities. For example, in the US, short range devices can legally use frequencies as low as 300-390MHz (i.e. garage door openers). In most other areas of the world, this band is exclusively reserved for military.

Allocating Frequencies To Services

How complex the actual frequency allocation within a band plan for a given country can become illustrates the picture below (for the US):

To review the individual allocations, a detailed list exists. Similar lists exists for Europe and other regions of the world.

Freely Usable Radio Frequencies

Fortunately, picking a legal frequency for DIY data transmission is not hard:

Only a few frequency bands exist that are free to use and do not require a license to transmit digital data:

Type Frequency Remarks
Short Range Device (SRD) in Europe/Asia 433MHz and 868MHz 10mW at 433MHz/25mW at 868MHz with <1% duty cycle
Short Range Device (SRD) in the US 310MHz and 915MHz up to 1W
Bluetooth 2.4GHz internationally harmonized 2-way radio, up to 100mW (depending on Bluetooth standard)
WiFi 2.4GHz 802.11b/g/n/ax, most often used in WiFi-enabled microcontrollers
WiFi 5GHz 802.11a/h/ac/ax, very infrequently used in DIY data transmission due to its expense and limited distance

Harmonized vs Individual Rules

If you plan to use WiFi or Bluetooth then you are in luck: both technologies are largely harmonized throughout the world, and only very few exceptions exist for the common operating modes.

If you in contrast plan to transmit radio waves as a Short Range Device, substantial regional differences exist, both in legal frequencies and maximum allowable radio power.

Just how complex it can get when you want to stay within legal bounds illustrates this document.

A very good application note exists, explaining the FCC requirements for SRDs in various frequency ranges, including the underdocumented 315MHz range.

Once you make sure you are using a legal frequency (not using 315MHz transmitters in Europe, and not using 433MHz transmitters in the US), and when your transmitter does not exceed 10mW, you are most likely fully compliant, and more importantly: not interfering with anyone else.

Use Cases

To find out the right frequency and technology for your project, compare it to the typical use cases below, and pick the same frequency and transmission rules.

Remote Controls, i.e. Garage Door Openers, Homematic IP, Sensor Transmission, IoT

These are the most typical short range devices. They predominantly use ASK modulation (Amplitude-Shift-Keyed), a digital modulation based on traditional AM (Amplitude Modulation).

  • In Europe, 433.92MHz and 868.35MHz are used. The maximum transmit power is limited to 10mW on 433MHz (with no duty cycle), and 25mW for 868MHz (with a duty cycle of below 1%).

  • In the US, 315MHz is used.

LoRa - LongRange Transmission

Typically, short range devices have a short range (which is why they are called that way). However, the regulatory limitation is low transmit power.

Low transmit power does not necessarily translate into low transmission range, though:

LoRa is a proprietary radio transmission protocol specifically designed to provide a high transmission range despite using a low transmission energy. It is based on FSK (Frequency-shift keying), a digital encoding on top of FM (Frequency Modulation).

In fact, LoRa is an excellent example for all the sophisticated new modulations evolving, as it uses Chirp (increasing and decreasing frequency shifts) and spread-spectrum (spreading the carrier signal to be more resilient towards noises).

To achieve its exceptional long range capabilities, LoRa can also be used in mesh topologies where other independently operated LoRa devices can pick up and forward the signal, and by reducing the data transmission rate to make it more fault tolerant.

  • In Europe, LoRa typically uses 863-870MHz, less frequenly 433MHz. The maximum transmission power is 40mW at a 1% duty cycle (devices can transmit only 1% of the time).

  • In the US, LoRa uses 902-928MHz. The maximum transmission power is 1W with a 400ms dwell time (a maximum of 400ms transmission time).

This is a rough advisory, there are additional rules and requirements. The legal frequency range is for example organized in channels with designated band widths, and the use of channels may be restricted to specific tasks, i.e. upload or download, among other requirements.


WiFi is typically used to create a computer network and enable multiple devices to talk to each other.

Since WiFi supports mesh technology, it is simple to cover a large area by using multiple access points.

Modern microcontroller boards like ESP8266 and ESP32 come with WiFi-functionality built-in. They can act as access point (setting up a new wireless network), station mode (joining an existing wireless network), and both modes combined.

By using WiFi to communicate, you are benefitting from a number of advantages:

  • Reach: within the covered area of your WiFi network, reliable high speed data transmission is possible. The covered area can easily be extended by adding more meshed access points.
  • Legal: WiFi is using an internationally harmonized frequency range. The typical WiFi standards are supported and legal to operate in most parts of the world.
  • Transparent Transmission: you don’t actually need to care much about implementing the data transport layer. Instead, your firmware can communicate with other devices inside your own WiFi or anywhere else on the world (provided your WiFi is connected to the Internet) simply by using http requests.

WiFi is not a premier option if you:

  • want to use microcontroller boards that do not have WiFi built-in (i.e. Arduino)
  • want to bridge areas not covered by your WiFi (i.e. sending data from your house to your neighbors house)


Bluetooth is a two-way transmission standard commonly used to exchange data between two devices. For this, the devices are coupled before they can communicate with each other.

Many modern microcontroller boards come with Bluetooth transceivers built-in. This enables you to create your own Bluetooth controllers or controlled devices.

In fact, you can use Bluetooth also on both ends, i.e. use a Bluetooth-enabled microcontroller board on both ends of your communication.

Starting with Bluetooth 5.0, the emission power is up to 100mW, which can gap distances of up to 40m indoors and 200m outdoors.

Bluetooth is using the same internationally harmonized ISM frequency band as WiFi (2.4GHz).

Choosing Frequency

Picking an appropriate frequency is the initial and fundamental step in designing a radio project:

  • Legal: you must pick a frequency that is legal to use in your country. Else, you may be interfering with other services, and may become liable for damages you cause. In addition, (severe) legal punishment is possible even if the risk of ham and detection may be low with low emission devices.
  • Technical: when you have the choice of multiple legal frequencies, your use case determines which one to choose. The lower a radio frequency the better can it pass buildings (walls, doors). The higher a frequency the more does it need a free line of sight. For example, if you live in a densly populated city in Europe, the 433MHz band can pass buildings better than the 815MHz band. If you’d like to set up data transmission with free line of sight between sender and receiver, the 815MHz band works better (because of lower interference from other services in the same band).

WiFi frequencies are largely harmonized across the world. They are the only wireless technology that has no fundamental regional regulations to watch out for (even though the use of selected WiFi channels may be limited to some regions).

Maximum Emissions

The more radio power a transmitter emits, the greater is the distance it can gap, but also the greater is the area in which it can cause interference.

With DIY projects, most breakout transmitter boards use very low transmission power (typically <10mW). At this level, interference and bodily harm are very unlikely.

When picking hardware, transmission power can be defined in a number of units:

Watt and +dBm

The maximum legal emission power can for example be quantified in raw emission power expressed either in Watt or in dBm (which is essentially the same, just logarithmic), i.e. 10mW/+10dBm, 100mW/+20dBm or 10W/+40dBm.

Both Watts and dBm are focusing on the properties of the sender.

Effective Radiated Emission (ERP)

ERP (Effective Radiated Power) is a more meaningful parameter: it views the effective radiated power at the receivers side.

This also takes into account the kind of antenna the sender is using, and its directivity: When you connect a highly directional antenna, the total radio power is radiated in one narrow direction, just like light in a highly focused flash light or a laser beam.

If you are restricted by a certain emission threshold but free to use any antenna you like, then using a highly directional antenna can multiply the distance you can gap by multitudes.

Security By Obscurity

While extending the reach through more radio power sounds great at first (“the more the better”), it not only extends the area in which you can interfere with others. It also extends the area in which others can listen in and spy on your transmissions.

A low ERP and a low maximum radio distance does not only limit interference with others. It also shields you from others and often is much more effective than encrypting data transmissions.

For most use cases, a high radio power is not needed and even counter-productive: your garage door opener in Germany typically does not need to be received in Argentina. Especially with DIY projects, a tiny ERP may be all you need to gap the few meters or 100m you need for your project.

Free Voice Radio Frequencies

There are additional free-to-use radio frequencies designated for analog and/or digital voice communications (i.e. walky talkys):

Name Frequency Wave Length
CB (Citizen Band) 27MHz 11m
FreeNet 149MHz 2m
MURS (Multi-Use Radio Service) 151-154MHz 2m
LPD (Low Power Device Voice) 433MHz 70cm
PMR (Personal Mobile Radio) 446MHz 70cm
FRS (Family Radio Service) 462-467MHz 70cm
GMRS (General Mobile Radio Services) 462-467MHz 70cm

The table presents an overview only. Additional rules exist. For example, FreeNet is available in Germany only. PMR is available in the EU. FRS, GMRS, and MURS are available in the US. GMRS requires an (easy to obtain) license. For MURS, additional regulations apply.

These frequencies are not usable for DIY projects as they are highly regulated, require certified devices and prohibt the transmission of radio data other than voice.

Amateur Radio (HAM)

Licensed radio amateurs are private persons that went through classes, took a test and received a certification ensuring they technically know precisely how radios work and how to not interfere with other radio services.

Radio amateurs typically are enthusiasts with a strong focus on radio technology.

Radio amateurs can use a large number of additional frequency bands, and emit radio power up to many kilowatts, but solely for experimental purposes related to radio communications.

This includes DIY projects, however strictly related to radio amateur purposes. A licensed radio amateur cannot for example use his or her special privileges to transmit personal weather station data on a privileged frequency - except if this weather data is used in close relation to experiments testing radio emissions under different weather conditions.


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(content created Apr 15, 2024 - last updated May 05, 2024)