Transmitters - Technical
- from Hobby and Engineering Supplies
There are currently two different band of frequencies used in radio controlled models. The older style AM/FM band uses frequencies in the Megahertz range whereas the newer style uses 2.4 Gigahertz.
The older style AM/FM band (still used today) uses 27 MHz the common r/c toy frequency, 29 MHz for primarily surface vehicles and 36 MHz & 40 MHz for general aircraft use. Transmitters and receivers using 36/40 MHz as their frequency are further divided into two modulation types; PPM – Pulse Position Modulated being the standard FM modulation and PCM – Pulse Code Modulation. PCM transmitters help reduce the risk of interference by converting the radio signal into a code. As interference caused by external sources is not converted, they are simply eliminated within the receiver.
The frequencies in the AM/FM band are divided into channels. Each channel has a unique frequency assigned. The transmitter and receiver must be paired using the same modulation type (PPM or PCM) and channel (frequency).
The AM/FM band is prone to interference. The user of these systems must take extra care to ensure they are operating at location away from interference sources (power lines, metal structures and etc...). It is wise to always perform a range check on your system before use.
2.4 GHz systems are now the 1st choice of the modern hobbyist. They offer a solid system less prone to interference. Unlike the AM/FM band, 2.4 GHz systems don’t use a channel system. This eliminates mishaps caused by others using the same frequency. Many 2.4 GHz transmitters may be used simultaneously but most manufacturers suggest a maximum of 15 at a time.
As opposed to long antennas on AM/FM transmitters, 2.4 GHz systems use a short antenna due to their shorter wave length. Most 2.4 GHz receivers have 2 antennas to receive radio signals in two dimensions. Care must be taken to place the receiver away from such materials as metals and carbon fibre as they will prevent the signal from going through.
Most 2.4 GHz systems use a modulation type know as “Frequency Hopping”. Unlike the earlier 2.4 GHz systems which locked on a single or double frequency, this modulation type constantly changes the frequency that it is using. This enables simultaneous use of many transmitters without any interference with each other.
The 2.4 GHz system has also enabled the transfer of data from onboard your R/C vehicle via the use of a telemetry system. Most modern transmitters have telemetry systems which enable the user to view in real time various information like rpm, temperature, speed and altitude from the screen of their transmitter.
Users must be aware that not all radio control systems are approved by the Australian R/C governing body; the MAAA. Caution must be taken when buying online from overseas; it may be illegal to operate some systems in Australia. For a list of approved systems, please see the MAAA website or ask your local hobby store.
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