The radio is your connection to your robot. So, you want to get the best fit for your bot. Sometimes, you make do with what you have. But if you're getting a radio for the first time, it's best to start with a radio that will best fit your bot, as well as any future bot you may build, but also fits in your budget. So, how do you tell which radio is best for you? There's lots of variables. Let's start with the most important one: band.
Band is the type of communication your radio uses. All radios use radio waves, obviously, but there are different ways to transmit information. A few of these will sound familiar, since they are the same as with a radio you listen to. First is amplitude modulation, or AM. This band transmits signals based on amplitude, or the height of the radio-wave's disturbance from equilibrium. The transmitter outputs varying amplitudes based on how far you push one of the transmitter's joysticks. This is then interpreted by the receiver, and the signal is sent on to the ESCs, etc. See the How Bots Work section for more of that information. AM radios are the cheapest: about $30 for a new set, minus servos (which I will talk about later in this section). But you get what you pay for: they're highly sensitive to interference. Interference (sometimes called RFI: radio frequency interference) is something you really want to avoid, since it makes you lose control of your robot. Since this creates a dangerous situation, AM radios are banned in any class above antweight, and I believe they are on the way out in the antweight class.
FM, or frequency modulation, works by varying the frequency of the radio waves, or how close together the waves are. It's a step up from AM radios, as they are much less susceptible to RFI. But, they aren't immune from it. They are banned from 60-lb weight classes and up, since the larger the robot, the larger the risk when interference is a possibility. FM radios are also a bit more expensive than AM radios, averaging $100 for a decent transmitter and receiver set. They are not wallet-busters, though, so they are good for 12-lbers and below, usually. At the time of this writing, I own a Hitec brand FM radio, and it's never failed me in competition.. at least due to interference.
So, what is used in bots weighing 60 pounds and up? PCM, or pulse code modulation, is a computer-controlled band which send varying pulses - rather than waves, as I understand it - through the air: varying in length. They also have built-in fail-safes, which disable the robot if reception is lost. (Failsafe is a required safety measure in 12-lbers and up, as we learned last year: FM users must add a special attachment to handle reception loss.) PCM radios, though, tend to be more expensive still, averaging $240 or so. But they are allowed in every weight class, and programmable for ease of control, even though the actual programming itself isn't always easy.
Another popular way of controlling robots is using the IFI Isaac radio modems, which are very customizable and very secure. They are also very expensive, though ($1000 or so), and since I don't know much about them to divulge anything accurate, I will only mention them here. If you are further interested in them, check the IFI website (http://www.innovationfirst.com/) and the BattleBots Forum for more information.
So, now the bands are finished with. What else do you have to worry about? Well, so as to insure few radio-crossovers, the FCC has set several frequencies aside, which you must consider. Here in the U.S. of A., we use mainly 4 radio band frequencies in our hobby radios: 27MHz, 50MHz, 72MHz and 75MHz. Each of these is further broken into "channels" so as to even further prevent cross-overs. 27MHz is for hobby airplane/helicopter ("air"), as well as R/C car and boat ("land") use. Bots fit under "land" use. It's an all-purpose frequency, but there are only 6 channels on it, leaving little selection. 50MHz has 10 channels, and is also for air and land use, but it requires an FCC Amateur license to use. If you're up for the trouble, go for it, but these two frequencies aren't incredibly common among transmitters. 72MHz, on the other hand, is exceedingly popular among transmitters and has 60 channels! But, it is reserved for aircraft only. Why? If an RC car (or BattleBot) driver is nearby and airplane driving their mobile object, they can crash the airplane without realizing it. And hobby R/C planes can be expensive. But never fear: 75 MHz is for ground use only and has 30 channels.
Speaking of channels, another way of determining the ideal radio is number of channels. What do I mean? There are two types of "channels": those within a frequency, and those within a receiver. If my transmitter has two channels, it means I can control two things connected to my receiver separately: two different drive motors or a motor for throttle and a motor for steering, for example. With three channels, I can control my bot and add a weapon switch. With four channels, all of the above and a light bulb that blinks. See where I'm going? You need enough channels to activate the number of things you need to control separately. One problem, though, is that when you get into higher number of channels, you usually get into more and more transmitters that are set to 72MHz. Luckily, you can have a transmitter from 72 to 75MHz.. For a fee, of course. Just ask at the place where you get the transmitter. I've heard $40 for a conversion is common. You must have a 75MHz transmitter to compete, though!
And finally, sometimes radios come with little motors called servos. These are little motors which only turn part of the way, roughly 60 degrees usually. What are they for? These little boogers can be controlled precisely just by plugging into the receiver. They aren't particularly strong, and not particularly fast. They are handy for flipping mechanical switches, if you have a use for that, though. They also are a good way to test a radio: to use a computer term, they're plug-and-play. And, when properly modified, they can be used for drive in an antweight (1-lb) or lower robot. If you don't want them, you can sell them for a little money back on your radio purchase.
Anything else you need to know? First: don't cut your receiver antenna unless you know what you're doing (chances are, you don't). It's optimized for most uses. If you need a short antenna, look into little "whip" antennas: Team Delta (http://www.teamdelta.com/) sells them, and they're little 6" antennas which many use. But this isn't necessary; I've never used this before myself, and have yet to have any problems. Second: be careful how you hold your transmitter. Try to keep it pointed straight up: that's how it's meant to be held. And NEVER touch it to anything metal. It won't shock you or anything, but it may affect reception. Third: make sure you keep an eye on your receiver and transmitter power levels and power switches. If you leave the power switches on and forget about it, you will at least drain your batteries for no reason, and possibly have battery leakage which could corrode your power terminals (it's happened to me). Fourth, you can change what frequency channel you broadcast on by changing "crystals" in your transmitter and receiver. Exactly which crystals to get and where to install them varies from transmitting set to transmitting set. Fifth: You may see transmitter abbreviated as Tx and receiver abbreviated Rx. Don't let this throw you! Sixth: if doing this in school, don't get a pistol-type radio. Use your brain: if it looks like a gun, it could get you into trouble (especially in this day and age). Opt for a single stick or double stick radio, which you probably are more familiar with anyway. And last: protect your transmitter and receiver as if they were your only children. You will hopefully use them long after you finish using any of your other components, and when you upgrade you can always sell them to someone, often a newcomer who will then value it for a long time him or herself.
So, you now know enough about radios to handle most of what you are thrown in this sport at the beginning. Be sure to check the glossary [edit: coming in a long time] for radio-related terms to finish your "training" in this field.
Links:
Ebay (http://www.ebay.com/)
Yes, Ebay. This auctioning company is a very good resource for picking up used radios for cheap.. But at the same time there is a small gamble that they won't work, or that they will break sooner because they are more used.. Still, a good deal.
Tower Hobbies (http://www.towerhobbies.com/)
Tower Hobbies: the best place for radios, in my opinion. They sell radios of all kinds, plus other accessories for them (Tx batteries, Tx crystals, etc). Just make sure that your future radio is set to 75Mhz, or you will have to have it modified to compete (about $40)!
Horizon Hobby (http://www.horizonhobby.com/)
Seller of JR brand radios, as well as other hobby stuff (though for boats, cars and planes).
Team Delta (http://www.teamdelta.com/)
Team Delta is a bot team running a store with all sorts of useful things in it, including radio accessories. Take a look around.
SozBots (http://www.sozbots.com/)
SozBots (Sixteen OZ. BOTS) sells a radio system and some radio accessories under Electronics.
Robot Marketplace (http://www.robotmarketplace.com/)
A store which combines lots of other stores to get a one-stop shop. Robot Marketplace is also run by a builder, and contains plenty of radios and radio accessories.
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