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.
Read more about this topic: Radio-controlled Aircraft, Frequencies and Sub-channels
Famous quotes containing the word channels:
“The enthusiastic uprising of the people in our cause, is our great reliance; and we can not safely give it any check, even though it overflows, and runs in channels not laid down in any chart.”
—Abraham Lincoln (1809–1865)
“Not too many years ago, a child’s experience was limited by how far he or she could ride a bicycle or by the physical boundaries that parents set. Today ... the real boundaries of a child’s life are set more by the number of available cable channels and videotapes, by the simulated reality of videogames, by the number of megabytes of memory in the home computer. Now kids can go anywhere, as long as they stay inside the electronic bubble.”
—Richard Louv (20th century)
“People between twenty and forty are not sympathetic. The child has the capacity to do but it can’t know. It only knows when it is no longer able to do—after forty. Between twenty and forty the will of the child to do gets stronger, more dangerous, but it has not begun to learn to know yet. Since his capacity to do is forced into channels of evil through environment and pressures, man is strong before he is moral. The world’s anguish is caused by people between twenty and forty.”
—William Faulkner (1897–1962)