Build Your Own Combat Robot

Chapter 8: Remotely Controlling Your Robot 173
Some of these systems offer more control flexibility than traditional R/C systems.
The drawback of using these frequencies is that other ground-use systems also
transmit at the same frequencies. For example, cordless phones transmit at the
900-MHz and 2.4-GHz frequencies. A cordless phone near your robot could
cause radio interference with your robot. Because of this, it is recommended that
you use only radio transmitting equipment that has built-in error correction methods
that can filter out unwanted information, such as the IFI Robotics system.
Antennas and Shielding
Antennas are used in combat robots to transmit data from the hand-held transmitter
to the receiver on the robot. Without the antenna, you cannot communicate
with your robot. One of the biggest problems most robots have with reliable control
is not electrical noise but improper antenna setup.
The ideal antenna configuration would be a vertical wire of a length equal to one
wavelength of the radio wave used for communication. This works out to nearly
14 feet, which is not practical for most combat robots—or most model aircraft or
cars, for that matter. Most 72- and 75-MHz radios come with a 1/4-wave antenna
attached, with a length in the range of 37 to 42 inches. Most robots do not have
the length or convenient mounting room to carry an external antenna of this size,
so the usual antenna length and placement are far from optimal.
A 1/4-wave antenna means 1/4 of the wavelength of the transmitter/receiver
system’s operating frequency. It’s a unique characteristic of the physics of antenna
design. The higher the frequency, the shorter the wavelength and, of course, the
shorter length a 1/4-wave antenna will be. Light and radio waves travel at 300 million
meters per second, so a 75-MHz signal will have a wavelength of
300,000,000 meters per second divided by 75 million cycles per second, resulting
in a 4-meter-long wavelength—or about 157 inches. A 1/4-wave antenna should
be 1/4 this wavelength, or about 39 inches.
A very important fact about antennas is that they should be mounted vertically.
This not only applies to the receiver’s antenna on the robot but also to the
hand-held transmitter’s antenna. These types of antennas emit their energy in a
pattern much like a flattened doughnut, with the antenna passing through the
doughnut hole. The greatest thickness of the doughnut, as well as the most significant
signal from the antenna, is at the sides. Conversely, the “thinnest” part of the
doughnut is the hole, which is what you see when you look straight down on it.
And the thinnest signal comes straight out the end of the antenna.
If the transmitter’s and the receiver’s antennas were placed in space where there
are no reflections, no signal would be created if they were pointed at each other.
The greatest signal would be created when they were parallel to each other. In situations
on Earth, especially in a room with a metal floor, the signals bounce around
and reception can be accomplished with almost any orientation. You should always
keep in mind that these reflections are far weaker than a direct signal,