A radio transmitter can control 1 function per channel of control. More than one servo may be required for a single function, such as planes that require dual aileron servos. A typical sport plane may require a 4-channel radio with 5 servos, for example. When using a four channel radio with five servos, the two aileron servos would simply be connected with a Y-connector so both can be operated from a single receiver port.

Typical functions include:

Throttle
Elevator
Rudder
Aileron
Flaps
Retracts

When a radio system is purchased, it may be sold as transmitter only, transmitter and receiver only, or a full radio system with transmitter, receiver, servos, batteries, charger, and power switch. Some radio systems are sold with standard-sized receivers and servos, while others are sold with smaller "micro" gear for park flyers and small electric aircraft.

Some radios are sold as FM/PPM radio systems and include an FM receiver. Others are sold as PCM digital radio systems, and include a more expensive PCM class of receiver. PCM receivers are considered to be less succeptible to stray signals and interference compared to standard FM systems. PCM systems can also be set with failsafe programming so the vehicle will automatically assume a particular profile of settings if the signal from the transmitter is lost. Almost all PCM transmitters can also be operated as FM/PPM transmitters as well.

PCM radio receivers are somewhat proprietary in nature. They are not cross-compatible with different brands of equipment. You will need to use only specific recommended models of PCM receiver as listed by your transmitter's manufacturer. It is possible that a PCM receiver may not be compatible with your transmitter, even if they're both the same brand.

Some radio receivers are built around single conversion signal filtering technology. Other radio receivers are built around dual conversion signal filtering technology. Most single conversion receivers are designed to be inexpensive receivers for park flyers while most dual conversion receivers are better at blocking out signal interference and can operate at much longer ranges.

Transmitters in North America are normally designed to operate at 72Mhz and will either use positive or negative shift for carrying additional signal information. A transmitter must be matched to a receiver of like shift to operate properly, so a negative shift transmitter will require a negative shift receiver. Some transmitters are shift selectable, and can operate in either negative or positive shift mode. Some receivers are shift selectable, and will either auto-detect the shift of the transmitter or can be jumpered to a particular shift setting.

Within the 72Mhz frequency range, there are 50 seperate channels available for aircraft radio systems, numbered 11 through 60. Most transmitters and most receivers will require a frequency crystal so each operates on a specific frequency and both use the same frequency so they can communicate. Transmitter crystals are not user changable in the North American market, if you wish to change the channel of a transmitter, you're legal required to send it in for factory service so it can be recalibrated at the new frequency. Receivers can have their frequency changed by the user. You will need the specific model of crystal made by the receiver's manufacturer for that specific model of receiver.

Some transmitters and some receivers do not require crystals and their channels can be altered by the user simply by turning a dial or selecting a different channel on the radio menu. These are called synthesized radio systems and they lock into a specific frequency with a channel sythesizer system rather than with a fixed crystal.

In additon to 72Mhz FM (PPM)/PCM radio systems, there are now 2.4Ghz spread spectrum radio systems available. These radio systems are PCM only, and the specific transmitter only works with receivers designed for that particular model and/or brand of spread spectrum radio system.

In summation, you need to evaluate radio equipment by the following characteristics:

Transmitters
(72Mhz or 2.4Ghz)
(FM only or FM-PPM/PCM)
(mechanical or computer)
(fixed frequency or synthesized frequency)
(positive or negative shift or shift selectable)

Receivers
(positive or negative shift or shift selectable)
(single or dual conversion)
(fm/ppm or pcm)
(fixed frequency or synthesized frequency)

The list of variables involved in selected a transmitter and receiver is quite extensive. I haven't even begun considering standard versus digital servos or whether one should choose NiCd, NiMH, or LiPo batteries for one's receiver. If one studies these various terms and matches a transmitter to receiver based on these major characteristics, one should end up with a transmitter and a receiver that can talk to each other.