Short Range Device

Devices With Low Radio Output Power, Working On 315MHz, 433MHz, 868MHz, And 915MHz

Short Range Devices (SRD, formerly known as Low Powered Devices, LPD) transmit radio frequencies with very low output power.

Examples for SRD are wireless sensors, microphones, garage door opener, home automation, etc.

Designated Frequencies

SRD operate on distinct frequencies:

Frequency Region max. Output Power
315MHz US 1W
433MHz Europe/Asia 10mW
868MHz Europe/Asia 25mW/1% duty cycle
915MHz US 1W/400ms dwell time

Antenna Lengths

Antennas are very important for safe and efficient usage.

Never operate a sender without a proper antenna or else you might damage it. Operating a receiver without antenna is technically benign but severely impacts receiver sensitivity.

If you don’t have an antenna at hand, you can easily build one yourself: a simple copper wire is sufficient.

What matters is its length that must match the frequency you want to use.

Calculating Antenna Length

The proper antenna length is calculated by the formula 299792458 / frequency (in Hertz). The former is the speed of light.

For a typical garage door opener operating on 433.95MHz, the formula would be 299792458 / 433950000. The result is 0.6908456227676: the full wave length for this frequency is 69cm.

Typical antennas use a quarter of the full wavelength (lambda/4 or 1/4). The proper antenna length for 433.95MHz devices would therefore be 17.3cm.

Get-AntennaLength Command

To calculate antenna lengths more easily, you can use PowerShell. It is preinstalled on any Windows PC and can be downloaded for most other operating systems for free.

First, install the DoneLandTools extension:

Install-Module -Name DoneLandTools -Scope CurrentUser -Force -AllowClobber

You may omit -Force -AllowClobber if you prefer to see confirmation dialogs.

Next, it’s simple to calculate antenna lengths:

Get-AntennaLength 433.95

The result lists various resonant antenna lengths, and you can pick the length that works best for you.

Antennas work better the longer they are - as long as the picked length is resonant. So pick any length listed, and choose the longest one you can accommodate. If you wind up the antenna wire to a coil, the length is not as much important, and you should go with the 1/4 length.

Frequency (MHz) : 433.95
Lambda          : 69.1
Lambda 7/8 (cm) : 60.4
Lambda 5/8 (cm) : 43.2
Lambda/2 (cm)   : 34.5
Lambda/4 (cm)   : 17.3
Lambda/8 (cm)   : 8.6


The modulation and the data format control how information is sent and received over the air. There are two fundamental signal transmission concepts:

  • AM (Amplitude Modulation): the information is added to the radio wave amplitude. The frequency stays fixed.
  • FM (Frequency Modulation): the information is modulated onto the carrier frequency. The amplitude stays fixed. The frequency varies (within the defined bandwidth).

There is also phase-modulation which is a third way of modulating information onto a radio wave. Digital communications uses PSK (Phase-shift keying) and combinations with other encodings.

To actually transmit data using one of these ways, additional information encoding is required.

Encoding schemes start with simple On-Off schemes similar to morse code, and can become extremely sophisticated with chirp symbols and signal spreading. More sophisticated encoding schemes generally are able to transmit more information with higher speed and/or extend the distance that radio waves can travel.

AM (Amplitude Modulation)

For digital transmissions on AM, ASK (Amplitude-Shift keying) is used: in its simplest form (OOK, On-Off keying), bit 1 is represented by full amplitude, and bit 0 is represented by absence of signal.

EV1527 - Remote Controls

One coding format used on ASK is EV1527, a 24bit code with roughly a million different ID codes and four bits of payload. EV1527 is often used with simple remote controls, especially those with learning codes.


WiFi also uses AM. To encode the information and achieve high transmission speeds, much more sophisticated encodings are used (i.e. QAM, Quadrature Amplitude Modulation).

QAD is a combination of PSK (Phase-shift keying) and ASK (Amplitude-shift keying). Using both phase and amplitude shifts can encode more information and is one of the reasons why modern WiFi standards were able to increase the maximum data transmission speed.

Standard Year Speed Encoding
802.11 1997 1MBit DBPSK (Differential-binary Phase-shift keying)
802.11 1997 2MBit DQPSK (Differential-quadrature Phase-shift keying)
802.11b 1999 5.5/11MBit CCK (Complementary-code keying)

FM (Frequency Modulation)

For digital transmissions on FM, FSK (Frequency-Shift keying) is used: bits (or symbols) are encoded in shifts of frequency.

There are plenty of FSK transceiver breakout boards available.

LoRa - Chirp Spread Spectrum

LoRa (Long Range) is also using FM. It specializes in transmitting signals over great distances with very low-powered radio signals (10mW LoRa radio signals can travel 2km within cities and up to 40km in rural areas). To achieve this, it is using the proprietary, patented LoRa (LongRange) wireless protocol based on the Chirp-Spread-Spectrum modulation technique.

Spread Spectrum

Aside from modulation (like AM and FM), another technology is used in modern digital data transmissions: spread spectrum.

With spread spectrum, the original focused radio signal with its small bandwidth is deliberately spread out over a much larger frequency spectrum.

Spreading out radio signals yields a number of benefits:

  • Distance: strong interference and noise signals on one particular frequency cannot distort the signal as only a very small fraction of it is affected.
  • Security: the spread-out signal is almost impossible to detect and intercept without knowing the parameters in which it was spread out
  • Field Strength: the field strength for a particular frequency is lower

A number of modern transmission techniques use one form of spread spectrum or another, including LoRa and WiFi (DSSS, Direct Sequence Spread Spectrum).


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(content created Apr 01, 2024)