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Unregulated frequencies

Discussion in 'Electronic Design' started by anon, Apr 17, 2006.

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  1. anon

    anon Guest



    I wonder if anyone can point me in the right direction.

    I'm a hobbyist building a timing system for use in amateur bicycle
    races in the UK. Commercially available systems use RFID tags and cost
    many thousands of pounds. We don't need something that gives
    world-record levels of accuracy so I decided to design a cheaper

    The way it works is this: each competitor carries a small circuit with
    a microcontroller and passive receiver. On each lap, they pass over an
    induction loop. This triggers the microcontroller, that has a crystal
    controlled oscillator. It saves the lap time, and at the end of the
    race they are all uploaded to a computer. It's a lot simpler than an
    RFID system that would need to read multiple tags simultaneously. And
    at the end of the race everyone has to return their expensive tags or
    else they don't get their times.

    I've managed to get prototypes working at both 8khz and 125khz (RFID

    My understanding is that frequencies below 9khz are unregulated in the
    UK, though presumably there may be other restrictions on emitting
    magnetic / electromagnetic radiation below these frequencies (EMC

    I would prefer to use 125khz as this would be less likely to interfere
    with hearing aids, pacemakers or heart-rate monitors. The power I need
    is far less than the power that an active RFID tag requires. So I feel
    my system would be unlikely to interfere with anything that a legal
    RFID transmitter would not interfere with. But I can't find anything
    that confirms this frequency is unregulated and believe me I've looked
    hard. My tags aren't technically RFID tags as they only receive, and
    don't transmit.

    So which frequencies might be legal for me to use? And are there any
    other restrictions on power output?

  2. Luhan

    Luhan Guest

    There are some principles of US law that may apply here. One is that
    you can really use any frequency you want if it does not radiate past
    your property line. Another is that infringements are mostly based on
    interference with 'other services'.

    If you are making something for sale, of course, its a whole different
    picture. You need to jump thru lots of hoops.

    Making 'one off' projects seldom gets you in trouble unless you really
    piss off somebody. I have personally 'witnessed' several over the

    Note: this is not legal advice; I am not an attorny.

  3. John - KD5YI

    John - KD5YI Guest

    Hi, Luhan -

    Can you please cite a reference regarding radiation within the confines of
    one's property line? In many passes through Title 47 of the Code of Federal
    Regulations, I have never seen any such property line statement. Most of the
    regulations of which I am aware concern not exceeding specified volts/meter
    at a specified distance (3 meters, 30 meters, etc). One exception is the
    100mW final stage input power with antenna/lead-in/ground of not more than 3
    meters on the AM broadcast band.

    If I am wrong, I would appreciate the correction.

  4. Luhan

    Luhan Guest

    John - KD5YI wrote:

    This is not taken from FCC regs, it has more to do with property law.
    I do not remember the reference. It may not in fact exist.

    Much of my comments are simply common sense. What is legal is one
    matter, what is practially enforcable is another. I've seen many
    projects on the internet using 'illegal' transmiiting devices,
    including the guy who use the innerds of his microwave oven to light up
    flourescent lamps accross his basement.

    I try to be pragmatic in my designs for clients. Many of 'one of a
    kind' industrial production and test devices have no certification with
    regard to spurious radiation, or meet with UL approval. This is true
    for companies I've worked for accross the country.

    As far as I know, the FCC does not much snoop around looking for
    problems, they simply process compaints from 'other services'. Maybe
    you can tell me if this is not so.

    Thanks for your unusually polite comments.

  5. Guest

    Run it at the horizontal scanning frequency of local TV's (would be
    around 15.75 KHz in the US).

    Then the only interference you might have to worry about would be
    TV-tax vans

    (which in the unlikely even they have actual receivers are probably
    looking for the colorburst clock ot the receiver LO, not the horizontal
  6. Luhan

    Luhan Guest

    Mental Note: Use 15,750 Khz for clandestine transmissions.

    Luhan (Spooky) Monat
  7. Guest

    Mental Note:

    Cover spy as sprinkler piping installer for golf course. You're going
    to need a big antenna to get much range (miles and miles and miles if
    the Navy's ELF submarine links are any hint)
  8. Luhan

    Luhan Guest

    For RF transmission, yes. Not for induction coupling.

  9. John - KD5YI

    John - KD5YI Guest

    Like you, I suspect that the FCC does not actively look for low-power
    unlicensed emissions unless they receive a complaint. However, I cannot tell
    you if that is their policy.

  10. Guest

    Yes, but your proposed "clandestine transmission" isn't going to go
    very far inductively coupled. Perhaps it's a digital dead drop that
    you simply bicycle over each day?

  11. Why? The horizontal sweep rate is 15.734.34 hz. Do you really think
    you can pick up a clean signal less than 16 Hz away from all the TV sets
    radiating noise for miles?

    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
  12. Christopher

    Christopher Guest


    You might also consider detection of an "overhead" continuous 40khz
    ultra-sonic blast from a tweeter speaker or two.

    Then have the listening device on each bicycle for detection and
    tripping your micro-controller.

    If you can make home brewed circuit boards for the receivers, assembly
    would be quick for each unit required.

    * * *

    Temecula CA.USA
  13. anon

    anon Guest

    It's a nice idea, but my circuit is cheaper and more energy efficient.
    It will run for over a hundred days on a CR2032 battery and has a
    quiescent power consumption that will let it stay in standby for
    several years. It has 11 components overall and fits 35x50mm.
  14. anon

    anon Guest

    I didn't know you had tv tax vans in the US. Actually that's a concern
    because I don't have a TV and the UK TV tax people are scary.

    I think I managed to answer my own question in the end. OFCOM classes
    inductively coupled equipment as 'class 1' and you don't need a
    license, or approval, or anything. There are just some power
    restrictions that one mustn't exceed.

    I buy all your comments about 'if it doesn't cause interference don't
    worry' but I'm spending other people's time, effort and money on this
    system and I want to get it right. If OFCOM stopped me from using it
    my name would be mud and I don't want to risk this happening.

    The other concern is stopping pacemakers. Whilst unlikely, this
    doesn't bear thinking about which is why I wanted to do my due

    Thanks for all your help
  15. YD

    YD Guest

    If you're low powered enough to not be "heard" beyond a few meters, at
    any frequency, who's to complain?

    - YD.
  16. Guest

    No, we don't have TV tax, and even you probably would not get their
    attention with a horizontal output inductive transmitter.

    But not having a TV - I hear that does confuse them.
  17. Luhan

    Luhan Guest

    Yes of course. I was refering back to the original post where you
    would ride over it with a bicycle.

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