frequencies and sub channels

 Frequency

Frequency determines the line of communication between a receiver and transmitter. The transmitter and receiver must both be on the same frequency so the plane can be controlled.

 Reserved frequencies

Many countries reserve specific frequency bands (ranges) for radio control use. Due to the longer range and potentially worse consequences of radio interference, model aircraft have exclusive use of their own frequency allocation in some countries.
USA and Canada reserved frequency bands

• 72 MHz: aircraft only (France also uses US/Canada channels 21 through 35).
• 75 MHz: surface vehicles.
• 53 MHz: all vehicles, only for older equipment on 100 kHz spacing, with the operator holding a valid amateur radio (FCC in the USA) license. The 53 MHz band began to become vulnerable to amateur radio repeater stations operating on the 53 MHz area of the 6-meter band during the early 1980s. The 53 MHz bands can still be used with relative safety for ground-based (cars, boats/ships) powered modeling activities.
• 50.8 to 51 MHz: on the 6-meter band for all vehicles at 20 kHz spacing, with the operator holding a valid amateur radio (FCC in the USA) license. Added in the 1980s as the amateur radio repeater interference problem on the earlier 53 MHz bands in the United States began to manifest itself.
• 27 MHz: general use, toys.
• 2.400-2.485 GHz: Spread Spectrum band for general use (amateur radio license holders have 2.39-2.45 GHz licensed for their general use in the USA) and using both frequency-hopping spread spectrum and direct-sequence spread spectrum RF technology to maximize the number of available frequencies on this band, especially at organized events in North America.

European reserved frequency bands

• 35 MHz: aircraft only.
• 40 MHz: surface vehicles or aircraft.
• 27 MHz: general use, toys, citizens band radio.
• 2.4 GHz spread spectrum: surface vehicles, boats and aircraft.

Within the 35 MHz range, there are designated A and B bands. Some European countries allow use only in the A band, whereas others allow use in both bands.
Singapore reserved frequency bands

• 29 MHz: aircraft only

Australian reserved frequency bands

• 36 MHz: aircraft and water-craft (odd channels for aircraft only)
• 29 MHz: general use
• 27 MHz: light electric aircraft, general use
• 2.400-2.485 GHz: Spread Spectrum band for general use

New Zealand reserved frequency bands

• 35 MHz: aircraft only
• 40 MHz: aircraft only
• 27 MHz: general use
• 29 MHz: general use
• 36 MHz: general use
• 72 MHz: general use (US 72 MHz "even-numbered" channels 12 through 56, at 40 kHz spacing)
• 2.400-2.4835 GHz: general use

The frequencies are permitted under legislation, provided equipment meets the appropriate standards, bears the New Zealand supplier's Supplier Code Number and has the correct compliance documentation (Radio Spectrum Management information available on the RSM website)
Detailed information, including cautions for transmitting on some of the 'general use' frequencies, can be found on the NZMAA website.
Amateur radio license reserved frequency bands

• 50 and 53 MHz in the USA and Canada
• 433–434 MHz in Germany (some of these German "ham RC" UHF band channels are also usable by "hams" in Switzerland)

 Channels

Traditionally most RC aircraft in the USA utilized a 72 MHz frequency band for communication. The transmitter radio broadcasts using AM or FM using PPM or PCM. Each aircraft needs a way to determine which transmitter to receive communications from, so a specific channel within the frequency band is used for each aircraft (except for 2.4 GHz systems which use spread spectrum modulation, described below).
Most systems use crystals to set the operating channel in the receiver and transmitter. It is important that each aircraft uses a different channel, otherwise interference could result. For example, if a person is flying an aircraft on channel 35, and someone else turns their radio on the same channel, the aircraft's control will be compromised and the result is almost always a crash. For this reason, when flying at RC airfields, there is normally a board where hobbyists can post their channel flag (or "frequency pin", based on a spring-loaded clothespin with the channel marked upon it) so everyone knows what channel they are using, avoiding such incidents.
A modern computer radio transmitter and receiver can be equipped with synthesizer technology, using a phase-locked loop (PLL), with the advantage of giving the pilot the opportunity to select any of the available channels with no need of changing a crystal. This is very popular in flying clubs where a lot of pilots have to share a limited number of channels. Latest receivers now available use synthesiser technology and are 'locked' to the transmitter being used. Double conversion radio reception is normal and can offer the advantage of a built-in 'failsafe' mode too. Using sythesised receivers saves on crystal costs and enables full use of the bandwidth available, for example the 35 MHz band.
Newer Transmitters use spread spectrum technology in the 2.4 GHz frequency for communication. Spread spectrum technology allows many pilots to transmit in the same band (2.4 GHz) in close proximity to each other with little fear of conflicts. Receivers in this band are virtually immune to most sources of electrical interference. Amateur radio licensees in the United States also have general use of an overlapping band in this same area, which exists from 2.39 to 2.45 GHz