Page 183 - Build Your Own Combat Robot

Build Your Own Combat Robot
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and shape, swapping the crystals between the two radio frequency bands will not
work. Switching from one band to another requires retuning the radio, which
should be done only by an FCC licensed technician.
If your robot is going to use a traditional R/C system, the frequency bands that
you are allowed to use by law are 75 MHz, 27 MHz, and 50 MHz. The dilemma in
this scenario is the fact that R/C systems that are meant for ground applications
usually have only a few channels available for driving two or three servos. The
high-quality, multi-channel radios are almost exclusively made for aircraft use. In
the early days of robot competition, many robot builders used aircraft frequency
(72 MHz) radios exclusively, because good-quality ground frequency (75 MHz)
radios were not available. In recent years, however, competition organizers have
begun enforcing FCC regulations about channel number and frequency band use,
forcing robot builders to switch to non-aircraft frequencies.
Most 72-MHz R/C systems can be converted to operate on 75 MHz, but only
after an extensive retuning process. Legally, retuning for 75 MHz has to be done
by an FCC licensed technician. In most cases, this is most easily done by the radio’s
original manufacturer—although some third-party shops, such as Vantec, can do
the conversion process. For a nominal fee, some radio manufacturers will retune a
radio for the 75-MHz ground frequency band when the radio is sold.

United Kingdom Radio Frequency Bands

Radio control systems in the United Kingdom are similar to those in the United
States, but the particular radio frequencies used are different. The UK hobby radio
control system runs on the 35-MHz and 40-MHz bands. The 35-MHz frequency
band is reserved for aircraft use, and the 40-MHz band is reserved for ground ap-
plications such as combat robots. The 40-MHz band is separated into radio con-
trol channels every .010 MHz, from 40.665 to 40.995 MHz. As with those in the
United States, robot builders in the U.K. must either purchase a 40-MHz ground
radio or have a 35-MHz aircraft radio set converted into a 40-MHz system for
ground channel use.

Radio Frequency Crystals

Within the frequency bands is a set of individual channel numbers that can be
used for R/C applications. For example, 30 different radio channel numbers can
be used in the 75-MHz frequency band. The specific channel number frequency is
controlled by an oscillator called a frequency crystal, which is shown in Figure 8-4.
The frequency crystals come in pairs: one for the transmitter and one for the receiver.
To change the channel number on your radio, you simply replace the frequency
crystals. Both the transmitter and receiver must use the same channel number, or
the system will not work. The 72-MHz and 75-MHz crystals look identical, but
the crystals are not interchangeable between frequency bands. In other words,
putting a 75-MHz crystal in a 72-MHz radio will not work.