While the operation of radio control models as a hobby is a relatively recent event, the technology behind the hobby dates back to the nineteenth century. During the course of this blog, we’ll trace the evolution of rc model transmitters and their applications.
The first use of radio control technology dates back to 1898, when Nikola Tesla built a pair of radio controlled boats, demonstrating them to a shocked crowd at Madison Square Garden. Though only able to cruise a short distance, the boats showed the potential of radio control. During World War I, Archibald Low designs an aerial drone plane for the Royal Flying Corps for use as a radio-controlled guided bomb. By the 1930s practical rc planes were available to hobbyists, with Walter and William Good building and flying the first fully-functional rc aircraft in 1937. As a result of progress in radio control technology during World War II, the use of rc models increased dramatically during the 1950s, though the battery capacities were limited and needed frequent rechargings until transistors became available.
Transistors also reduced the voltage requirements of a battery, virtually eliminating the older high voltage batteries. One channel rc radio kits were introduced in the 1950s with preassembled units offered later. In both tube and transistor radios of the era, the rc planes control functions were operated by an electromagnetic escapement utilizing a rubber- band loop system from which the rc pilot could control rudder and speed functions. By the 1960s, crystal-controlled superheterodyne receivers became available. These offered a true three-dimensional control of rc aircraft (yaw, pitch and motor speed). Heterodyne receivers also provided a more sensitive signal selection coupled with a more stable rc model control. After World War II commercial rc planes control signals were limited to two or three channels using amplitude modulation. More channel choices were added in the 1960s using frequency modulation, which offered twenty or more operating channels.
The next generation of rc model transmitters was developed in the mid-1970s. A Pulse Width Modulation (PWM) Signal is a method by which an analog signal is produced using a digital source. PWM signals consist of two primary components that define its behavior: a duty cycle and a frequency. The duty cycle is the amount of time the signal is in a high (on) state as a percentage of the total time it takes to complete one cycle. The frequency determines how fast the PWM completes a cycle, and consequently how fast it switches between high and low states. By cycling a digital signal on and off at a fast enough rate, and with a certain duty cycle, the output will appear to behave like a constant voltage analog signal when providing power to devices. These signals transmitted in a rapid succession could control multiple functions on the rc model.
On the heels of PWM signal technology Pulse Code Modulation (PCM) was developed. While analog technology uses continuous signals, digital technology encodes the information into discrete signal states. With two states assigned per digital signal, they are called binary signals, while a single binary digit is termed a bit. While PCM is an efficient means of signal transmission, it is by no means foolproof due to the proliferation of radio control devices in both the hobby and industrial markets. To overcome this problem, some late model FM receivers which still use PWM coding can be modified by the use of advanced computer chips to detect the individual signal characteristics of a particular Pulse Width Modulation without needing a designated code, as required with PCM coding.
By the early 2000s, Spread Spectrum rc contol systems came into use. A Spread Spectrum rc control system uses a variable frequency of operation, usually in the 2.4 gigahertz band, in which the rc transmitter stays on a given frequency for a minimal amount of time. With the enhanced security offered by Spread Spectrum systems, an increasing number of radio manufacturers are offering the units to hobbyists at a price from $3,000 to as low as $30. A number of manufacturers are now selling conversion kits for older digital 72 MHz radios and receivers, providing even more options for the rc model operator.