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RF range calculation

Discussion in 'Electronic Design' started by [email protected], Mar 26, 2006.

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  1. Guest

    Hello,

    Can anybody tell me the real-world usable range or how to calculate it
    for a transceiver? (Not looking for answers like "infinity" or "to the
    end of the known universe")

    I'm trying to build an RF link at the board level to replace a serial
    connection to a computer.

    I've been looking at Linx and Amtel RF components on digikey.com to
    use.
    e.g.
    http://www.atmel.com/dyn/products/product_card.asp?family_id=651&family_name=Smart+RF&part_id=3413

    For instance, one of the transceivers I looked at has a dBm of +4 at
    2.4GHz. Basically I just want to make a serial RF connection to send
    96 byte data packets every few minutes. I'm looking at transceivers
    because the "field unit" will be listening for a request to send data.

    Most of the stuff I see has a range of about 300-500 feet. But some of
    the components I've looked at don't specify the range. I've found TX
    and RX separate with 3000 foot range.

    If I haven't provided enough information plug in the basics and let me
    know where to start. I think a whip antenna would be 234/Frequency so
    anywhere from 1 inch to 2 inches depending on if I stick with 2.4GHz or
    not.

    I need to keep the TX power consumption under 40mA as well.

    Thanks

    Scott
     
  2. Mark

    Mark Guest

    Most calculations are based on free space losses.
    sounds like your application is not free space

    I think your best bet is to take a system you have as a comparison and
    work from there.
    Increasing the Tx power by 10x (+10dB) will increase the range by
    about 3x.
    Ideally the power spreads by the inverse square law.

    Depend a lot on the antennas.
    Using outdoor elevated antennas (if connected with low loss feedline)
    can make a very large improvement.

    Mark
     
  3. RF attenuation in free space between isotropic antennas is given as: { 36.6
    + 20 log frequency + 20 log distance } Where frequency is in megacycles and
    distance is in miles. Calculate the free space loss and subtract that
    amount from the TX output. The result is the amount of signal appearing at
    the receiver.

    For example: At 2.4 GHz and distance of 500 feet there would be free space
    loss of 84 dB. From TX power out of +4 dbm the signal at 500 feet would be
    (+4 - 84 = -80 dbm) at the receiver. Minus 80 dBm should be well within the
    receiver sensitivity spec of the cheapest 2.4 Ghz receiver. There would be
    some gain in the 2 antennas above isotropic, minimum of 2 db each if they
    are dipoles, adding at least another 4 db of signal strength over the path.
    Be aware, you probably don't have free space conditions, so make tests in
    the working environment.

    Bob Swinney
    > wrote in message
    news:...
     
  4. Guest

    Good responses, thanks a lot.

    Any good books on RF communication, logic, theory, etc? I wouldn't
    mind learning a little more than I need to know for what I'm trying to
    do.

    I may need to look into a small RF amp as well in order to increase the
    dBm.
     
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