2.4GHz is an internationally regulated microwave frequency band for industrial, scientific and medical equipment. Equipment using this band must be tolerant of and avoid causing undue interference. Typically equipment using this band use digital coded transmissions similar to PCM, and must negotiate a frequency automatically (negating the need for crystals and to a large degree any frequency control system.
A number of manufacturers have produced radio equipment that uses the 2.4GHz band for radio controlled model use.
- Futaba has FASST.
- Spektrum has DSM (now superseded by DSM2).
- JR usually use Spektrum's DSM2 system, but they have their own DSMJ system for use only in Japan.
- Multiplex has M-Link, a bi-directional FHSS system.
- Walkera has a protocol that is used for its ready to fly electric helicopters.
- Esky has a protocol that is used for its ready to fly electric helicopters.
- Corona manufacture 2.4GHz receivers and modules, not believed to be compatible with any other system.
- Xtreme Power Systems manufacture a range of 2.4GHz radio equipment using their own XPS protocol.
- Weatronic produce a range of technically advanced transmitter modules and redundant receivers, featuring bi-directional FHSS technology.
All 2.4GHz equipment uses a binding procedure to allow the receiver to identify the transmitter (which contains a unique identifying code). This process may need to be carried out every time the receiver and transmitter is switched on, or just when the equipment is first bought, or the transmitter is changed.
Broadly, 2.4GHz equipment works in one of two ways.
- The transmitter can find one or more 'empty' channels and broadcast on those until the transmitter is reset. The receiver will scan all frequencies at startup for its paired transmitter, and then listen on only those broadcast frequencies.
- The transmitter can skip 'randomly' from one channel to another, only remaining on each channel for a fraction of a second. Once the receiver detects a transmission from it's paired transmitter, it can guess which channel the transmitter will jump to next and so maintain the radio link. This is known as 'Frequency Hopping Spread Spectrum' (FHSS).
The allowed power output for the first method is often much lower than the power output for the second method. While Futaba's FASST is a true FHSS system, Spektrum's DSM is a 'Direct Sequence Spread Spectrum' (DSSS) system, which switches rapidly between only two frequencies; in Japan this was not considered sufficiently 'spread' to qualify for the higher (100mW) power output and would have been limited to only 10mW. This prompted JR to develop the FHSS DSMJ protocol. Some rumors suggest there may be some discussion within the EU about whether any new licenses for DSSS systems will be granted for the higher output power (currently granted licenses should continue to be valid, however).
France has additional restrictions on the power and frequency ranges that 2.4GHz systems can use; on Futaba transmitters (and receivers?) there is a switch or menu option to restrict the transmitter to the frequency range where 100mW can be legally used outdoors.
2.4GHz systems are less susceptible to interference than traditional VHF systems for a number of reasons:
- Automatic frequency selection reduces the possibility of being shot down to essentially zero;
- Digitally coded transmissions allow any corrupted data to be rejected;
- There is far less natural 'electrical noise' at microwave frequencies.
However, 2.4GHz systems are potentially more liable to signal fading due to how the receiver is installed in the model.
- Multiplex M-Link factsheet.
- UK microwave bandplan (old).
- More information on 2.4GHz systems for RC use.
- Discussion of DSSS vs FHSS on RC Model Flyers.
- JR vs. Futaba vs. VHF 100mW range tests on Helifreak.
- EN 300 328 v1.7.1 European standard for 2.4GHz ISM band.
(WikiNote: Don't try to create a page called "2.4GHz". The wiki software doesn't seem to like full stops in the name).
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