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Radio frequency selection problem

  1. Jan 7, 2008 #1
    Hello everybody. As I'm new here, maybe someone can help or direct me to other resources that will assist in my investigation. I am looking for information that can help me select the proper radio frequency for use in a new project. The following is an overview of the project and the inherent needs.

    I am tasked with developing a system to very accurately track (+/- 1" resolution) a relatively slow moving object 100ft/sec or less) in a crowded human environment from a distance of approximately 500ft. The object is small, requiring a miniature transmitter and thus the battery power source must be very small as well. The power source needs to be rechargeable through magnetic induction as it is embedded in the object. Battery life needs to be 10 minutes. The system needs to be robust and highly resistant to impacts.

    It would be greatly appreciated if anyone can provide some suggestions on possible radio wavelengths suitable for use in this environment with the limiting physical parameters of the object.

    Thanks for your thoughts on the topic.
  2. jcsd
  3. Jan 11, 2008 #2


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    Staff: Mentor

    There are several issues involved, including physics issues, and regulatory issues.

    First the regulatory one -- the radio spectrum is regulated by the FCC in the United States (and other government agencies in other countries), so you can't just select a frequency based on the optimized physics of the situation. If you did, and started transmitting on that frequency, you could easily cause harmful interference with other radio receivers, including emergency vehicle communications, etc. Not good. For more ranting about being careful about transmitting in the radio spectrum, please see this thread where I just posted similar cautions....


    There are some licensed bands that the FCC has set aside for things like RFID tags, etc., and one of those might be appropriate for your application. Or if this ended up being for a commercial application, there is a process where you can petition the FCC to purchase the rights to a particular frequency (in some band that is appropriate) for a new use.

    Okay, so just keep that in mind -- part of your selection of the best frequencies to use will hinge on regulatory issues. Hey, maybe you'll get extra credit for bringing that up, eh? Also, keep in mind that when you design a radio transmitter, you are limited in what your sidebands look like -- you cannot transmit a dirty signal with lots of harmonics and such. You have to stay within your alloted frequency, and use certain modulation techniques to avoid interfering with adjacent bands.

    One last thing about regulatory issues -- there are some bands where you are allowed to transmit a moderate amount of power without a license, as long as you limit your harmonics and sidebands, etc. One worldwide open band is the ISM band (Industrial, Scientific, Medical, also known as the microwave oven band) at 2.4GHz.

    On the physics side, you will need some sort of triangulation techniqe, or something like a GPS trick to track the moving device. Having humans around the moving object complicates things a bit, since at least in the ISM band, humans absorb some of the radio signal. Also, multi-path is an issue that can cause all kinds of problems, especially as you go to higher frequencies (where you are using the shorter wavelengths to help you locate the position and movement of something).

    There might be some better and creative solutions to the problem, though. For example, have the object keep track of its movements itself, and just broadcast its heading and velocity out to a monitor node on the ISM band. Or use a small GPS receiver on your moving object, and transmit that information.

    BTW, did you really mean up to 100 feet per second as the max velocity? That's pretty quick for something to be moving among humans...
  4. Jan 11, 2008 #3


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    Staff: Mentor

    What will the moving transceiver be mounted on? A human? Or will it be on its own wheels or something? How high off the ground among the humans will it be? What are the surroundings like?
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