Radio Inteference.

Some useful info on radio frequencies:

(From M. Grant, REP Education Associate -

I only had a bout a half hour to research this subject and here is what I
came up with:

The 900 Mhz radio modem for the robot has a frequency band of 902-928 Mhz.
The range of the signal covers 100 to 300 feet. 35 out of 40 channels are
for competition use. Control System Radio Modems operate on channel 40 and
the Program/Debug Radios operate on channel 22.

Possible Interference sources may include automated utility meter readers,
automobile and railroad traffic signal control devices, wide-area data
networking, cordless telephones, wireless headphones and speakers, retail
security scanners, home security systems, lifesaving medical devices

FYI- This particular frequency band is an unlicensed one, so anyone can use

ALso some cordless phones, radio telephones on commercial airlines.

whenever it seems like our bot is not linking up right (the radio link error LED comes on) its usually because someone has plugged in one of our other operator interfaces, and they are trying to use the same channel, or the same team number - like a student testing the edu bot from last year in another room.

if you want to use more that one robot at your site, you have to wire up a competition port adapter for at least one of them, to get it off the default channel.

We’ve found that flourescent lights near the radio can considerably interfere with the signal.

The reason for the post was we just moved our shop location, and have been experiencing unexplained severe communication problems. We found there was a 900 MHZ shop system
cordless phone station less than 100 feet away. It also seems the problem gets worse the closer we get to the cordless phone station. It was really quite dramatic - If we started the robot from one side of the field it went about 20 feet, and than as it got a certain distance closer to the cordless base station it slammed to a stop, the no signal light turning a solid red.

(We are also about 1/2 mile from an active runway, emergency medical/police stqation, etc.) TOO many possible sources of inteference. It seems that the older radios (2001 vintage) may be more sensitive.

On the Bright side, the high frequency TIG welders and regular cell phones don’t seem to be a problem :wink: Also from past experience - putting the reciever antenna closer than a foot to a sparking drill motor is probably a bad idea.

if you build the competition port adapter you are able to switch the link between (I think) four different channels (frequencies)

I would try all four, maybe one of them is off the channel your phone system is using.

If you’re really feeling adventerous, you could hook one of these up and switch to any channel, or even go to 2.4 ghz. If the pinouts match, they are a drop in replacement…You’d have to do a little testing to check, but it’d be cool to have your bot on 2.4 ghz. This also has the potential of hooking one up to a laptop to listen to the robot as it broadcasts and log data. You could also hook one up and have a wireless programming link.

Just a few thoughts.


Like the 900 MHz band, the 2.4 GHz band is an unlicensed ISM band because it is prone to interference from common electronic equipment. While 900 MHz may seem rather crowded, the 2.4 GHz band may be even worse. 802.11 wireless networks, cordless phones, microwave ovens, Bluetooth devices, and wireless video transmitters all operate in the 2.4 GHz range. It is very likely - especially in a populated area - that the interference experienced in the 2.4 GHz band would be far worse than that found in the 900 MHz range.

The 2.4 GHz wireless video works great in my home… if I unplug my 2.4 GHz telephone… and my 802.11b wireless LAN…

But then is is really great! I purchased the 2.4 GHz stereo A/V sender/receiver from:

A bit pricey, but very usefull. I use a high-res B/W camera that gives a very sharp picture.

One problem is that wireless LAN, bluetooth and cordless phones operate on 2.4 GHz. However it’s not really a problem as long as your reciever is not near one of these devices. If the robot is closer than the interfering device, then you should be fine. It is still sensitive to reflections and when the robot is driving around you will get some breakups. Maybe 1.2 or 1.7 GHz would be better?

My TX operates on 4 channels, 3 of which are in the Amateur Radio band, but with low enough power that you don’t need a license. I plan to increase my TX output in the amateur band from 100 mW to 1 W or more, but then it may knock out the wireless LANs at the competition.

For even more fun, I used two servos to control the pan/tilt of the camera with a joystick… We can look down into the robot to see mechanisms working, and then tilt up to see where we are going.

It’s a big hit with the students to have a “robot” view. At our open house this weekend we plan to project the “robot cam” up on a wall for the spectators to see… Anything to generate more interest in FIRST.