Choosing a Radio - Novice
- Adam Wrigley
Last updated - July 2006

DISCLAIMER: Combat robotics is a dangerous sport by it's nature. Extreme caution should be taken when performing any operation explained on this site. Any injury or death resulting from the use of these pages is the sole responsibility of the user and not totalinsanity.net. By undertaking the construction of a combat robot you assume all responsibility for your actions. When building a combat robot, always make sure you are, or are with, a responsible adult.

Build Responsibly.

Radios are a fundamental component in all combat robots. They are what link you, to your robot. There are several fundamental types of radios, as well as many variations. This tutorial will go over the most common radio systems in use today, and help you decide which system is best for you.

The radio transmitter is one part of your robot which you can reuse over and over through all your designs. Buying a good transmitter will save you money in the long run, since you'll never need to buy another. Maybe you only want to build 150g robots now, but if you ever decide you want to build 12lbers or 120lbers, you may won't need to upgrade your radio system if you purchase correctly from the start.

There are several parts of a radio that you must understand before purchasing:

Transmitters/Receivers
Frequencies
Bands
Failsafes

Transmitters/Receivers

The Transmitter is what you use to control the robot. There are two main types of transmitters available; trigger style, and twin stick style.

Trigger

Trigger style radios look like the above photo. They have a trigger, as the name suggests, and a wheel on one side. The trigger controls speed, while the wheel controls steering. These radios were originally intended for R/C cars and tend to lack in the feature department. They also tend to lack extra channels and are not generally used in robotic combat. Trigger are always offered in ground frequencies however, which is a plus. Newer trigger radios are looking promising. If you have a trigger style system already, it may work for a wedge type robot, but not much else.

Twin Stick

Twin stick radios are what everyone uses. These were originally created to be used with R/C planes and helicopters. The "aileron" mixers available with these radios are great for "mixing" the drive on a tank steered robot. Some people like to use one stick for each side of their robot, so you would push up on both to go forwards, or push up on just the left stick to go right(this format requires no mixing). However aileron mixing allows 1 stick to control all movement. Push the stick left, the bot goes left, push it up, the bot goes straight, etc. Besides the sticks, the radios also offer various other knobs, dials and switches to control other features on your robot.

It is always recommended to get a twin stick radio if at all possible. If you have a trigger system already, it can be used, but will most likely require an external mixer (see the bottom of this page).

Receivers are usually included with a transmitter in a package (generally along with servos). The most important aspect of receivers is the failsafe feature (see below) and second to that, the number of channels. Each open channel on a receiver can control one aspect of your robot (the left drive, the right drive, the spinner, lifter, etc). Make sure your receiver has enough channels for what you want to do. It isn't uncommon for a transmitter to come with a receiver that has fewer open channels. If you are building in the lower classes, it may also be important to look at the size of the receiver. The insect classes tend to use special micro receivers which are purchased separately from the transmitter in most cases.

One major point about receivers is that they generally come with very long antenna wires. NEVER under ANY circumstance should you cut this wire. They are made to exact lengths so that they work without interference. Cutting them any shorter can severely limit your maximum range and permanently damage your receiver. If you must have a shorter antenna, we recommend purchasing a specially designed whip antenna, available at the robot marketplace.

Frequencies

Most radios come in a variety of frequencies. The FCC regulates which frequencies are allowed for ground use, and these are the only frequencies you'll be allowed to use on your robot:

27 MHz
75 MHz
2.4 GHz

You may see a lot of 72 MHz radios while searching online, do NOT purchase a 72 MHz radio for use with a ground controlled robot. 72 MHz is a frequency regulated for air use only, and it is illegal to use 72 MHz on a ground robot.

Aside from frequencies, there are also different "channels" within each frequency. Although this is the same word as used in the receiver section above, it refers to something completely different. For example, the 75 MHz frequency contains channels 61 through 90. Each channel represents a small range of the frequency. Channel 61 is actually 75.410 MHz. All radios will operate on a specific channel. The important thing to know is that you cannot run your robot while another robot is running on the same channel. This would cause interference. Before a match, you must always make sure you are running on a different channel than your opponent. This can be controlled one of several ways. Most radios use what are called "crystals" to change the frequency. This means you actually need to take out and replace a crystal in your receiver, and your transmitter, to change your frequency. A newer method is to use a frequency synthesizer. This would require you simply changing a setting on your receiver and transmitter, no actual crystals need to be changed. The third method is the simplest, and this is just having your transmitter/receiver do it for you. Newer digital radio systems can actually search and find an open channel, and change channels automatically if interference becomes a problem.

Bands

There are several different bands of radios in use today, as listed below:

AM - Amplitude Modulation
FM - Frequency Modulation
PCM - Pulse Code Modulation
2.4 GHz - Newer, Digital Radios

What's the difference? It really isn't important to get into the technical details of the radio control systems, what is important to know is which one will get you the least interference with your robot. You don't want your robot going crazy and killing everyone within a 5 mile radius of your house because someone down the street turned on their R/C car.

AM

AM should NEVER be used in a combat robot. AM is the cheapest, and worst, of the radio technologies available today. AM radios are generally found in the cheap R/C cars you find at toy stores. Their range is normally extremely limited and AM systems generally don't have all the features a builder would want. AM radios are usually against the rules of all competitions, but may be allowed in some insect events.

FM

FM radios are still cheap, but better than AM. They can get a significant amount of interference but not as much as AM. FM has the advantage of being universal, in that any FM receiver should work with any FM transmitter on the same channel. FM radios are normally only legal up to the 60 lb weightclass because of their interference problems.

PCM

PCM is actually a variation of FM. PCM radios still use FM channels and crystals, but add in an extra layer of safety. PCM radios can actually tell the difference between "good" and "bad" radio transmissions. This means that if there is interference, it won't cause your robot to go crazy, it will just cause your robot to stop. Interference on AM or FM systems could cause your robot to become erratic. PCM radios also tend to have an extremely large amount of features, and can even be programmed to behave how you want. PCM radios and transmitters of different brands will not work together, your transmitter and receiver must be from the same company. PCM radios are legal for all weightclasses.

2.4 GHz

Newer digital radios are taking advantage of the 2.4 GHz band commonly found on portable phones and wireless networking systems. These radios are the easiest to operate, don't require crystals, and have the least amount of interference. They work similar to xbox 360 controllers. You can use your transmitter with different receivers, or you receiver with different transmitters, just by "syncing" them prior to use. 2.4 GHz radios and transmitters of different brands will not work together, your transmitter and receiver must be from the same company. 2.4 GHz radios are legal for all weightclasses. 2.4 GHz receivers are also naturally small, and sturdier than standard receivers. Perhaps the most surprising thing about 2.4 GHz radios is that they aren't always the most expensive.

Failsafes

Failsafes are arguably the most important part of your radio system. Failsafes will turn your robot off in the event of signal loss, or extreme signal interference. Before purchasing a radio it should be made certain that there is a programmable failsafe built into the receiver. Every event will have you run a test to make sure your failsafe is working. The test involves running your robot, and then turning off your transmitter. If the robot stops, you pass, if it keeps moving, you fail.

Choosing

If you're sure you only ever want to build in the smaller classes, you may be able to get away with an FM radio. However, building is contagious. You may want to build a larger robot later and need to upgrade down the line. PCM radios currently have the largest feature set, while 2.4 GHz systems are catching up. 2.4 GHz have the least amount of interference and are the easiest system to use for both the builder, and the event operator. When using 2.4 GHZ there is no need to worry about confusing channels, since the radio does it for you.

We recommend the Spektrum DX6. The Spektrum is a 6 channel 2.4 GHz system that comes with a specially modified "bot receiver" that conforms to all failsafe requirements. At $200 it just can't be beat. It also comes with 4 micro servos you can use to build a cheap and easy 1lber. The receiver is small, and also has a small antenna which makes it great for the insect classes. Even the interior construction of the receiver is sturdier than PCM and FM systems. The ease of use just can't be beat.

If you are looking for THE top of the line system, the Futaba 14Z is probably your best bet. However, it is a synthesized FM/PCM system and does not offer some of the features that a 2.4 GHz system can offer in usability, but makes up for it with advanced programming. It is uncertain whether this radio will be offered in 75 MHz however.

If you want the cheapest possible system, the GWS GWT 4A will work, but it has no advanced features. We don't recommend using this on a robot with a dangerous active weapon (this includes insect robots as well). Even if the rules don't require it, we recommend PCM or 2.4 GHz systems for any active weapon robot. You won't regret getting a more expensive radio system. However, if you are just looking for the cheapest way to get into the sport, then this is your best bet.

Other Radio Components:

Mixers

As mentioned before, most radios have mixers built in which allow you to map all movement of your robot onto one stick. However, not all radios have this. If you have a trigger style R/C system, or a cheaper twin stick, you won't have a mixer built in. What you'll need to use is an external mixer, which are available at most hobby stores and online. Trigger systems need this, since they cannot be operated without a mixer. In order to use the wheel to turn the robot, you'll need a mixer. External mixers are placed inside the robot along with the receiver.

Gyros

Originally used to keep planes and helicopters steady in flight, these are also helpful to keep your robot moving in a straight line. These are small components that are placed inside your robot along with the receiver.

 

© 2006 by T.i. Combat Robotics unless otherwise noted.