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Help with Wifi antenna

Discussion in 'Electronic Design' started by amdx, Jul 25, 2007.

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  1. amdx

    amdx Guest

    Please see subject line-- Help, Wifi Antenna-- on
    alt.binaires.schematics.electronic
    for a picture of my concept.
    Looking to combine Helical antenna and Wifi adapter card in one unit.
    My experience is limited to MW bcb. I know there are many pitfalls at
    2.4Ghz, so I'm looking for feedback on how to do this properly.
    I'm using the following page as my guide.
    http://www.wlan.org.uk/jhecker.html

    This is to extend the range of my laptop computer.

    Mike
     
  2. Note that in almost all places there are legal limitations on EIRP
    (Effective Incident Radiated Power). In plain English, the more
    you narrow a signal, the stronger it becomes.

    Since you did not say where you are, I'll mention the two places I
    know for sure. In the U.S. WiFi EIRP is limited to 1 watt for
    mobile/portable use (e.g. laptops) and 4 watts for fixed links.

    With a 100mW source and a reasonablyshort cable the famous Pringles can
    antenna would be illegal for mobile/portable use and probably illegal
    for fixed links. It happened to be developed by an FBI agent in the
    process of an investigation, so he was covered, you may not be.

    Here in Israel it is limited to 100mw EIRP, so unless you have
    a very long cable, any gain antenna would be illegal. Someone wrote
    up (in Hebrew) and posted on a web site his use of a similar
    antenna. It's not obvious to the casual reader that he did it
    in an area under the jursidiction of the IDF (Israeli Army) and
    got permission from them.

    Geoff.
     
  3. Jerry Martes

    Jerry Martes Guest

    Hi Mike

    You sure find some great information on the Web.

    I consider this article Jason Hecker publishes
    http://www.wlan.org.uk/jhecker.html to be ALL the instructions anyone
    would need for constructing a 20 dB directivity WiFi antenna. I am looking
    for some feedback from you on its performance.

    If you plan to investigate basic effects of changing size and shape of
    Helix antennas, EZNEC sure makes Helix antenna investigation easy.

    I am just curious, what kind of coax and connectors are you using, and,
    ?how much coax?.

    Jerry
     
  4. Jeff

    Jeff Guest

    Bear in mind that 2.4GHz is also an amateur band where no erp limits exist!!

    Jeff
     
  5. Oh , really ?! cite!
     
  6. gwatts

    gwatts Guest

    Only 802.11b/g channels 1-6 fall in the amateur allocation of 2390-2450 MHz.

    If operating under amateur regulations you must identify by CW, phone,
    RTTY or TV image every 10 minutes or less (see 47CFR97.119), your
    transmissions must be intended for reception by another licensed amateur
    station or station authorized to communicate with amateur stations (see
    47CFR97.111), that has to be the only reasonable way to effectively
    communicate, no other radio service available that accomplishes the same
    communications (see 47CFR97.113) and you have to use the lowest power
    level capable of accomplishing the communications (see 47CFR97.313).

    See http://www.access.gpo.gov/nara/cfr/waisidx_01/47cfr97_01.html for
    specific regulations.
     
  7. gwatts

    gwatts Guest


    I forgot to mention amateur communications cannot be encrypted, so no
    https, see 47CFR97.113 again.
     
  8. That may be true but to use it that way legally you would need an
    amateur license, and this is not just a paperwork exercise, you have to
    pass FCC exams covering electronics and radio theory (plus laws and
    regulations) to get one. For most people, it's not an option.
     
  9. Joel Kolstad

    Joel Kolstad Guest

    Most amateurs set the SSID of the access point to their call sign, since this
    is beaconed numerous times per minute. Apparently that's close enough to RTTY
    to keep most people happy.
    This particular regulation creates plenty of argument, since realistically the
    vast majority of activity on amateur radio frequencies these days could be
    just as readily accomodated by either cell phones or the Internet; I doubt the
    FCC has cited anyone for violating this rule for decades now.
    Indeed, although this too is subject to debate because people will argue that
    while, e.g., 1W will get them a bit error rate of 10%, they "need" a near-zero
    bit errorr rate and therefore transmit at 100W.

    You point about not being able to use HTTPS or other encrypted protocols is
    probably the most significant change in moving from using WiFi in an
    unlicensed mode to using it under the amateur radio service's rules. However,
    note that it is perfectly OK to obfuscate *authentication* data such as
    passwords -- packet BBSes have done this for decades.

    ---Joel
     
  10. Joel Kolstad

    Joel Kolstad Guest

    Very few people would have significnat difficulty passing the technician class
    license exam that's needed to operate on 2.4GHz. Indeed, there are many
    month-long (meet a couple times a week) classes and even weekend "cram"
    classes that have near 100% success rates in getting people their tickets. A
    passing score is 80%!
     
  11. mike

    mike Guest

    Hi Jerry, Please review the picture I posted on
    alt.binaires.schematics.electronic
    in it you will see I'm trying to avoid connectors and cable carrying 2.4Ghz
    R.F.
    Thanks for being the only poster to have anything near a response to my
    post.
    Mike
     
  12. Gary Tait

    Gary Tait Guest

    Not in most civilized countries.

    2.4 Ghz is unlicensed, not unregulated.
     
  13. Don Bowey

    Don Bowey Guest

    Hams have a segment of it, but not 2.4 to 2.5.

    However, if the wifi ends up in the ham band in a commercial purpose there
    will be hell to pay when they get caught, ham or not.
     
  14. Jim Lux

    Jim Lux Guest

    Well.. not all the 802.11b/g channels are in the amateur band, and even
    there, there is a power limit (transmitter output power, though, not
    EIRP), so you could conceivably fire up your 1500 Watt transmitter into
    a 20dBi antenna and blast away.

    There is the other rule about minimum power needed for communication,
    though.
     
  15. Do note that many people's Usenet servers do not carry any binaries
    groups, so they will be unable to look at your picture. If you were to
    place the photo on a server where it can be accessed by HTTP or FTP,
    these people would be able to get it. Many ISPs provide some web space
    served by their own web server, and there are free photo hosting sites
    like flickr.

    Dave
     
  16. Chuck Olson

    Chuck Olson Guest

    The Wifi Helical antenna is an awful lot of work, and the performance of
    even long structures on PVC tubing is vastly disappointing. The easiest 15
    dBi (my measurement) gain antenna is the Biquad. In fact, if you make the
    biquad with circular instead of square loops, the construction is even
    easier, and there's no problem measuring with all those bends - - just one
    wavelength of straight wire in a circular loop for each section - -
    http://www.wikarekare.org/Antenna/bicircle.html But try to keep a 50-ohm
    coaxial configuration all the way to the feed points as in
    http://homepages.ihug.co.nz/~redwood4/ It isn't necessary to keep the
    polyethylene insulation - - for a short length of air insulated coax, the
    tubing ID should be 2.25 times the center conductor OD for 50 ohms
    impedance.

    If you are like me, you will probably want to build the Helical anyway - -
    just to see, and perhaps to compare against the two easiest wifi antennas
    with decent performance - Biquad and Waveguide
    http://www.saunalahti.fi/elepal/antenna2.html The easiest waveguide can is
    the 83mm ID one you get with the 28oz size of Bush's Baked Beans or any of a
    number of other products like canned spaghetti sauce or family size
    Spaghettis.

    See if you can get a USB Wi-Spy Spectrum Analyzer module, too - - try to
    find one of the original (no external antenna - - cheaper) versions, and
    just put it in your own shielding enclosure and make your own modification
    to cut the path to the built-in antenna so that you can run a small coax to
    a connector on the box for your own external antenna connection. This kind
    of modification has been made by others - -
    http://www.metageek.net/default.aspx?tabid=463&forumid=11&postid=1395&view=topic
    scroll down to the post by pe2er on 9/9/06 showing how to connect a coax to
    the board. I used a type N connector on my enclosure because it's universal
    and strong, and filtered the three USB supply and signal wires with
    feed-thru capacitors so no RF can enter the enclosure through these other
    paths. Use Metageek.'s Chanalyzer software to run the Wi-Spy module - -
    preferably version 2.0 before the current 2.1.4 came out, since the need for
    compatibility with both the $199 Wi-Spy and the $399 Wi-Spy made operation
    with the $199 Wi-Spy somewhat unsatisfactory. Maybe you can ask Metageek to
    allow access to previous Chanalyzer 2.0 for owners of the older units. Why
    do you want all this? So you can make accurate measurement of the
    differences between antennas, using a reference 1/2-wave dipole, or the
    standard RPSMA antenna you find on most Wifi Routers. The dB calibration of
    the Chanalyzer display is very accurate.

    Chuck W6PKP
     
  17. Guest

    FYI, alt.internet.wireless discusses this topic often.

    I prefer the biquad antenna, which you can augment with a dish. I have
    a short-cut method to build this antenna. With a combination of these
    photographs and this link, you should be able to figure it out.

    http://martybugs.net/wireless/biquad/
    http://www.lazygranch.com/images/wifi/wifi_bq_1.jpg
    http://www.lazygranch.com/images/wifi/wifi_bq_2.jpg

    Note you don't need to make the loop a square. Use a circle of the
    same circumference.

    There is a disadvantage to using the helix. It will receive both
    horizontal and vertical polarization. Most sites just send in one
    polarization. In busy areas, the same channel will be used in
    different sites with different polarity. Now if you use the helix to
    illuminate a dish, then the circular polarization is fine and perhaps
    desirable. That is, you could sniff out signals without the
    attenuation associated with having the wrong polarization.
     
  18. Jerry Martes

    Jerry Martes Guest


    Hi Mike

    My computer skills are really lacking. I dont know how to view your
    images. I do spend alot of time learning about antennas. One of my
    buddies tells me that he had poor results with the helix he built from the
    instructions you cited. This site seems to have been better for my buddy.
    http://www.pa0hoo.tk/.
    Is it practical for you to use a Bridge at the input terminals of your
    gain antenna so that the coax loss is minimized? That way, the antenna
    (+Bridge) is connected to the computer with CAT 5 cable.
    If it isnt too complicated, tell me how to view your images.

    Thanks
    Jerry
     
  19. But, I suppose that believing, as did an earlier poster:
    "Bear in mind that 2.4GHz is also an amateur band where no erp limits exist!!"
    is acceptable for these No-Code, No-Klew Licensees?
     
  20. Chuck Olson

    Chuck Olson Guest

    Spaghettis should read Spaghettios


     
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