Radio controller or Rx/Tx

Hobbyists use a shorthand for receiver and transmitter in radio control: "Rx" for receiver, "Tx" for the other, and "Rx/Tx" as a short of shorthand for the process of doing radio control. And then there is just "RC", as in an "RC car".

Back in the day, radio transmitters were used to send analog pulses which were interpreted by receivers and turned into movements of servos. These servos were then used to manipulate the controls of the car, boat, airplane or whatever.

Nowadays, transmitters are computerized packages that bundle several signals together (channels) into one multiplexed signal. The receiver then takes this signal and breaks out the individual channels into separate PWM (pulse width modulation). Then the individual signals can be turned into meaningful commands.

Photo by Tony Podlaski

So what decisions go into selecting a radio control system?

Channels
The number of channels that your receiver can receive is the number of different controls you can operate on the RC device. For an RC car, you need steering and throttle, both of which can be controlled through a range of motion. For a plane you need at least three channels (aileron, elevator and throttle).

Multicopters with just about any number of motors have a computer (the flight controller) that manipulates the speed of each motor to produce movement along various degrees of motion. Run all of the motors faster and the airframe goes up in the sky, slow them all and it descends. Multicopters can also tip in any direction, causing forward movement in that direction. And spinning clockwise moving motors faster than the counterclockwise motors (or vice versa) causes the vehicle to rotate.

So since all of these movements are controlled by the multicopter, the radio controller just needs to give movement directions to the flight controller, rather than having a human make individual control inputs for each motor and function, as one would do for a single rotor helicopter.

With three directions of movement, three channels are needed as a minimum. Additional channels are used for other commands. In fact, the Arduino-based flight controller I settled on is built around receiving from an eight-channel receiver. (Details on that board here.) This leaves plenty of control channels for later operating a camera mount.

Frequency
There are a few choices you could make on frequency, such as Rx/Tx that broadcast in the range set aside for only model aircraft (72Mhz), but the "new" technology is the 2.7Ghz Spread Spectrum. It's one drawback is that smaller radio waves--gigahertz are smaller than megahertz--do not bounce off of objects and terrain very well. This means you have to keep line-of-sight with your model.

But the benefits are many: Tx and Rx pairs are assigned unique numbers so you will never conflict with another operator. In fact, the Spread Spectrum technology means that you are bouncing from frequency to frequency every few milliseconds, so it is hard for any radio interference to send your aircraft into the ground.

Decision
With all of the bells and whistles that transmitters can come with, it's hard to pick one without experience. In the end I'll just go with the Turnigy 9ch model recommended by Frank in his Instructable (see links on right). It comes with an 8ch receiver and can be programmed in various ways, should I ever advance to that stage. Most importantly, it's only $55 at HobbyKing.