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Thread: How much power do I need for ~1mile omni in a field?

  1. #1
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    How much power do I need for ~1mile omni in a field?

    Hello,

    I'm trying to transmit telemetry from a race car to a base station via
    802.11. It's mostly wide open terrain, but the range need is ~0.5
    mile, if I could get 1 mile I would be completely set for all tracks. Is it possible to achieve this via 802.11? I've seen 27dBm transmitters (http://www.quickertek.com/Quicky.php), 12-15 dB omnidirectional antennas, and amplifiers, but very scant info regarding the actual range achieved (because of variable conditions, etc). I can put a laptop (or even PC) in the race car, and put an omni antenna +/- amplifier if need be at both locations... but would it be enough? What do I really need?
    Also, how close would I be to FCC limits? Does it matter if I use an amplifier vs an antenna for the limits? What ARE the limits? I hear 500mW, 100mW, 1W, 4W.....

    Thanks!

    Alex

  2. #2
    Maximum Bitrate BarryWoods's Avatar
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    I wouldn't use wifi for telemetry. You really won't need that kind of bandwidth. Look into something in the 900Mhz range for this. You won't need near the power, and it will probably come out cheaper/easier in the end.
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  3. #3
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    Quote Originally Posted by agarbino View Post
    Hello,

    I'm trying to transmit telemetry from a race car to a base station via
    802.11. It's mostly wide open terrain, but the range need is ~0.5
    mile, if I could get 1 mile I would be completely set for all tracks. Is it possible to achieve this via 802.11? I've seen 27dBm transmitters (http://www.quickertek.com/Quicky.php), 12-15 dB omnidirectional antennas, and amplifiers, but very scant info regarding the actual range achieved (because of variable conditions, etc). I can put a laptop (or even PC) in the race car, and put an omni antenna +/- amplifier if need be at both locations... but would it be enough? What do I really need?
    Also, how close would I be to FCC limits? Does it matter if I use an amplifier vs an antenna for the limits? What ARE the limits? I hear 500mW, 100mW, 1W, 4W.....

    Thanks!

    Alex
    I have used 802.11 for telemetry in my race car. I used externbal antennas on both units, and it worked flawlessly at Streets of Willow and Buttonwillow raceway. Never lost signal, though it did drop sometimes, depending on the traffic and elevation changes. If you have a toy hauler, you could mount the "base" antenna on top, and probably get even better range.

    HOWEVER, your question is like asking "how strong do I have to yell for someone to hear me 100 yards away" There is really no magic formula, though there are programs that will theoretiucally tell you the range coverage of a given tranmsitter, but they are expensive and difficult to use.

    Michael
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  4. #4
    Constant Bitrate
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    That's not true. We do it here at work all the time. It is called a link budget. The problem is you need to know all the limits of your equipment. You have a good start, here. It is done by converting everything to dB and simply adding. That is why RF engineers use dB's.

    Here is an example:

    power out = +27 dBm
    cable loss from Tx to antenna (I'm guessing) = -3dB
    Gain of Tx antenna = +15 dB
    Path loss from antenna to Rx (1 mile) = -104.33 dB
    Gain of Rx antenna (assuming identical antennas) = +15 dB
    Cable loss from Rx antenna to Rx (estimate) = -3 dB

    The rough total is -53.33 dB. This is the sensitivity required by the Rx. Actually -63.33 dB sensitivity would be preferred.

    This is however an ideal situation. The higher the gain of your antennas the narrower the beamwidth, so it would be easier to lose signal with motions of either antenna. Also, there might be more losses due to polarization mismatch between the two antennas and any terrain or foliage, other cars or rain between the two antennas. The -104.33 value is for what they call free space, meaning what you would see in a vacuum or just air.

    Make sure your antennas are oriented the same way, (i.e. vertical).

    This is just a brief overview but good for an example. But you need to know the sensitivity of your Rx. I believe -53 dB is well within the sensitivity of most 2.4 systems, but so are lots of hills and trees on a racetrack.

    Good luck.

  5. #5
    Newbie ncervantez's Avatar
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    Here is a site I have been looking into for the same reason. I want to be able to send location and status messages to and from a desert race truck. The 1 watt 900mhz has a max range of 40 miles line of sight at a max 115.2Kbps

    http://www.maxstream.net/products/xt...-modem-usb.php

  6. #6
    Constant Bitrate KB5WID's Avatar
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    Quote Originally Posted by agarbino View Post
    Hello,

    I'm trying to transmit telemetry from a race car to a base station via
    802.11. It's mostly wide open terrain, but the range need is ~0.5
    mile, if I could get 1 mile I would be completely set for all tracks. Is it possible to achieve this via 802.11? I've seen 27dBm transmitters (http://www.quickertek.com/Quicky.php), 12-15 dB omnidirectional antennas, and amplifiers, but very scant info regarding the actual range achieved (because of variable conditions, etc). I can put a laptop (or even PC) in the race car, and put an omni antenna +/- amplifier if need be at both locations... but would it be enough? What do I really need?
    Also, how close would I be to FCC limits? Does it matter if I use an amplifier vs an antenna for the limits? What ARE the limits? I hear 500mW, 100mW, 1W, 4W.....

    Thanks!

    Alex

    Here is a real good resource on 802.11. http://www.oreilly.com/catalog/802do...pter/ch15.html

    Also If you are a HAM radio operator, channels 1-3 are within the ham radio band. You can effectively use higher power( $$$$$$ at 1500 watts ) as long as you follow the ham radio rules on content and identification. http://www.ssbusa.com/m13amp.html
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