External antennae for notebooks and others...

Discussion in 'Wireless Internet' started by John Beeston, Jul 2, 2004.

  1. John Beeston

    John Beeston Guest

    Hi,

    I have seen a lot of discussion about this, and have seen a number of
    offerings, all of which seem to involve lots of "bodges" ... with pigtails,
    and replacement connectors.... etc etc ...

    Given that the radio signal appears to be the critical part, and the more
    connectors / adapters you add the greater the signal loss, is it possible to
    use a USB wifi "dongle" with a metal mesh reflector, and rely on the USB
    cable to carry the digital signal ...

    Would there be any snags to this approach?

    What would be the maximum length of USB cable? And could a Powered USB Hub
    increase this ?

    Regards

    John
     
    John Beeston, Jul 2, 2004
    #1
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  2. John Beeston

    David Taylor Guest

    Given that the radio signal appears to be the critical part, and the more
    You mean something like this?

    http://www.nodomainname.co.uk/cantenna/IMG_0384s.jpg

    http://www.nodomainname.co.uk/cantenna3/cantenna3.htm

    :)
    Not really
    USB cable length is 5m max, you can string 5 active cables together so
    lets call that 25m.

    David.
     
    David Taylor, Jul 2, 2004
    #2
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  3. John Beeston

    Mike E Guest

    would this be any good?

    http://www.linksys.com/products/product.asp?grid=33&scid=36&prid=612
     
    Mike E, Jul 2, 2004
    #3
  4. John Beeston

    John Beeston Guest

    Very impressive, but I was thinking of something far simpler ... this
    approach still has mentions of brass bits, solder, and pigtails...

    But there again, perhaps I do not understand the full complexities...

    I was thinking more of a small self contained USB device such as the
    linksys WUSB12 or D-Link DWL122 being placed at the focus of a suitable
    dish and connected back to the PC using a standard USB extension cable (or
    cables)

    For the reflector I had something like a SKY satellite dish in mind, as
    these are cheap, and easy to install.

    John
     
    John Beeston, Jul 2, 2004
    #4
  5. John Beeston

    dold Guest

    The mini-USB won't properly paint a satellite dish. It might work in a
    small can, like the one on David's page.
    I used a DWL-122 witha 9" pie plate pretty effectively. I'm going to mount
    it in the bottom of a coffee can next.

    You need two coffee cans soldered together and cut off to about 6 inches,
    and then mount the USB mini about half inch from the bottom end.

    http://www.turnpoint.net/wireless/cantennahowto.html provides the
    calculator and ideas. I was going to put the miniUSB in there, although
    some experimentation to find the right focus and orientation.
     
    dold, Jul 2, 2004
    #5
  6. http://www.freeantennas.com
    http://www.freeantennas.com/projects/Ez-10/

    The nice part is that it doesn't require an pigtails. A USB radio
    should work as well. I'll leave it to your ingenuity to figure out
    how to do it with a PCMCIA card in a laptop. (ribbon cable
    extension?)
    That depends on your construction abilities. There have been
    aluminium foil and cardboard corner reflectors built, that I consider
    to be marginal and unstable, but it can't be any worse than a
    cardboard pringles can antenna.
    16ft for USB.
     
    Jeff Liebermann, Jul 2, 2004
    #6
  7. John Beeston

    John Beeston Guest

    Clarence, I am interested in your pie plate ... how / where did you mount
    the DWL-122?

    John
     
    John Beeston, Jul 2, 2004
    #7
  8. John Beeston

    David Taylor Guest

    Very impressive, but I was thinking of something far simpler ... this
    Not in the first picture I linked you to. All that's going on there is
    a the antenna from a Netgear USB NIC poked up into the cardboard tube
    (foil lined).
    It has been done before but it's just as easy to stuff the antenna into
    a can and use the can in the place of the LNB.

    David.
     
    David Taylor, Jul 2, 2004
    #8
  9. John Beeston

    David Taylor Guest

    16ft for USB.

    or 5 active cables strung together. :)
     
    David Taylor, Jul 2, 2004
    #9
  10. John Beeston

    dold Guest

    I used a reflector with an Orinoco PCMCIA card.
    I was holding the reflector with one hand, and the laptop with the other,
    so it wasn't very useful, but NetStumbler definitely showed the effects of
    the reflector being in place.
    "I'll leave it to your ingenuity to figure out how to do it" in some
    worthwhile fashion ;-)
     
    dold, Jul 2, 2004
    #10
  11. John Beeston

    dold Guest

    I used a Marie Calendar's pie tin, which is just a touch too small of a
    surface. I used double back tape to affix the mini-USB to a wall, a
    window, a cookie sheet, and the pie tin. In each case, I also used
    varying thicknesses of material behind the mini-USB, and settled on about
    3/4" standoff from the surface.

    I have some NetStumbler charts that made sense at the time, but I can't
    correlate the charts to the different applications now. azimuth,
    elevation, and orientation were all important. Without learnbydestorying's
    help, I don't know how the antenna element in the dongle is oriented.

    The pie plate was directional enough that I could see that my strongest
    signal was coming in through a window, rather than a straight line through
    the wall.

    The coffee can is the next step, although I haven't decided how to route
    the wire. The easiest would be to just run it down the side, inside the
    can. Better would be poking a hole in some lined cardboard, like David
    Taylor, but Pringles is way too small, so I'm going to play with the coffee
    can. I only have one can, and I need two, so I'm a little slow getting
    started.

    Although Jeff might point out that all of my attempts are so haphazard,
    a few inches of length might not be important.
     
    dold, Jul 2, 2004
    #11
  12. Careful. Tuning by signal strength alone is not sufficient. You need
    to keep an eye on the S/N (signal to noise) ratio. You can have a
    very strong signal, but if multipath, reflections, or interference get
    in the way, your S/N ratio and thus your thruput will suffer.
    The Orinoco/Proxim/Wavelan/Agere/Avaya/Whatever card has a connector
    for a pigtail to an external antenna. Methinks that will work better
    although it is admittedly more expensive.
     
    Jeff Liebermann, Jul 3, 2004
    #12
  13. Yep. Let's do the math the long way (to illustrate the problem).

    The 2.4GHz band is 83MHz wide.
    One wavelength at the bottom of the band is:
    3x10^8 meters/sec / 2.400x10^9 cycles/sec = 12.50 cm/wavelenth.
    and at the top of the band:
    3x10^8 meters/sec / 2.483x10^9 cycles/sec = 12.08 cm/wavelenth.
    For a half wave dipole, the difference between the top and bottom of
    the band is:
    6.25 cm - 6.04 cm = 0.21 cm = 2.1 mm.
    So, your basic tolerance for a simple (62mm) half wave dipole is plus
    or minus 1.0 mm or your antenna falls out of the ISM band. Actually,
    it's somewhat tighter than that as being 1.0mm off is sufficient to
    screw up the VSWR and induce some entertaining group delay across the
    operating bandwidth.

    To be fair, the reflector does not need to be that accurate. Rule of
    thumb for big dishes is flat within 1/10 wavelength or within 12.5mm
    of flat at 2.4GHz. Well, ok... I guess your wrinkled aluminium foil
    reflector is close enough.
     
    Jeff Liebermann, Jul 3, 2004
    #13
  14. John Beeston

    dold Guest

    The NetStumbler screen combines strength and SNR in one graph. There are
    occasions where the "red" portion gets a little larger, but I live in the
    sticks with no alternate sources of noise. The noise is generally -100dB,
    so the SNR is pretty good. I do see it come up, but the best signal
    strength is the lowest SNR in coincidence. I haven't tested throughput as
    a tuning aid. I think that might be valuable with finely focused antennas
    at long distance. Maybe I'll do a little checking with iperf.
    I have a few of those. A mag mount for the car, a Conifer that provides a
    claimed 11dBi, and a spare "N" adapter that's not being used currently.

    Flat pieces of metal are flat pieces of metal. I don't see the difference
    in a commercially built antenna and a similar design built at home out of
    Radio Shack PC board material. The commercial antenna might be built of
    better material for outdoor use, or have plastic housings to make it look
    pretty, but the dimensions are all that matter.

    My wrinkled aluminum foil works as well as the Conifer, except that the
    beam is narrower. I haven't looked at the specs to see if that is supposed
    to be true. I started to learnbydestroying to measure the dimensions of
    the Conifer, but I couldn't get myself to break the waterproof seal.

    The only specs I can find for the Conifer are in Russian.

    < http://www.rahul.net/dold/clarence/SMC/EZ10-strength.htm >
    < http://www.is.net.ua/wireless/prod/dl2410.html >
    < http://dast.nlanr.net/Projects/Iperf/ >
     
    dold, Jul 3, 2004
    #14
  15. John Beeston

    Barry Jones Guest

    Check these links:

    http://www.usbwifi.orcon.net.nz/
    http://www.manuka.orcon.net.nz/usbscoop.jpg
    http://www.manuka.orcon.net.nz/roofscan.jpg

    HTH
     
    Barry Jones, Jul 3, 2004
    #15
  16. John Beeston

    dold Guest

    I'm not building a dipole. I'm using the prebuilt Mini-USB for that.
    I was referring to the length of the coffee can. The "2 lb" can is 4" in
    diameter, and 4" long (IIRC. I don't have the can here). According to
    http://www.turnpoint.net/wireless/cantennahowto.html
    The can is only 1/2 of a wavelength, and should be at least 3/4.

    The wrinkled aluminum foil might be more of a problem with off angle
    reflections, but there's also been talk of the ripples in the metal cans
    being a bad thing, as opposed to a Pringles can, which is smooth but too
    small. The wrinkled foil reference is to my reflector at
    < http://www.rahul.net/dold/clarence/SMC/EZ10-strength.htm >

    I hadn't thought about using that reflector for a mini-USB. The reflector
    could be smaller, since the dipole is smaller than a WAP.
    I still don't know how the antenna is oriented, or even what type of
    antenna it is. I don't think it is a patch, because it doesn't seem to
    have a front-to-back ratio.

    Smooth lined cardboard cans could be better for cantennas. I see that
    Country Time Lemonade is 5" diameter and is 9" long. That's plenty long,
    but I don't know if it's the proper material. Can I test with an ohmmeter
    for conductivity? Is that what I want to measure? That wouldn't be good
    for outdoor use, as David Taylor notes after leaving his outside.

    http://www.nodomainname.co.uk/cantenna/cantenna.htm
    "Unfortunately, I left the whole lot outside one night and it rained
    causing the milk tub to get a bit soggy - RIP trust tub!"
     
    dold, Jul 4, 2004
    #16
  17. John Beeston

    dold Guest

    That site is what caused me to buy a mini-USB. Using a mini-USB dongle
    allows you to remove the dongle and use it normally in a higher-strength
    environment. It seems so much easier than building the can as an antenna,
    which requires relatively more expensive cables and an internal card.

    I'm going to have to buy another to play with, though, because I gave my
    other one away to someone who is just not trying hard enough ;-).


    Belkin is pretty cheap at CompUSA this weekend, but I never seem to find
    the sale items available at CompUSA.
    Standard sized USB $9.99
    < http://www.compusa.com/products/product_info.asp?product_code=287283&pfp=hpf3&tabtype=rb >

    Mini-USB $29.99
    < http://www.compusa.com/products/product_info.asp?product_code=313104&pfp=BROWSE >
     
    dold, Jul 4, 2004
    #17
  18. Oh. I was just trying to point out how critical the actual dimensions
    of the driven elements can become.
    The USB radio does not use a dipole. A proper 1/2 wave dipole would
    be 6.25cm long. That's longer than the USB radio. Placed on a
    ceramic substrate (e=10), the dipole shrinks to about 2.0cm long.
    It's a ceramic patch antenna which is about 7mm x 7mm. See:
    http://www.component.tdk.com/2.4GHAnntena.pdf
    The pattern is...well...ugly and simulates something resembling a
    warped cardioids. It won't illuminate a reflector very well, with
    lots of signal lost by spraying RF over the edges of the reflector.
    However, it's better than the USB radio by itself. Note the antenna
    "gain" of -1dBi. Yeah, that's about 3dB worse than an ordinary
    dipole.

    This is a photo of an older Linksys WUSB11 radio.
    http://jeffl.ihwy.com/linksys/wusb11/wusb11-3.jpg
    The ceramic patch antenna is the round white thing in the lower left.
    As I recall, it had an even worse -4dBi gain...err..loss.
    At RF frequencies, ALL the RF conductivity is on the surface of the
    conductor. You could make it out of a silver plated plastic insulator
    and it will work better than a crappy zinc plated magnetic steel
    candidate for the recycle bin. Literally every material that
    tarnishes (reacts with oxides or sulfates/sulfides) has crappy skin
    resistance. Same with all magnetic materials. The one exception is
    silver. It retains its low conductivity even when oxidized. If you
    want a cheap but decent antenna, think PVC or ABS pipe that has been
    silver plated on the inside. I can supply instructions if anyone
    feels ambitious. Otherwise, aluminium and non-magnetic stainless will
    work.

    I'm also disgusted at the prevalence of using cylindrical horns
    instead of a much better and higher gain horn antennas. Basically,
    what these antennas are trying to accomplish is match the driven
    impedance (50 ohms) with the impedance of free space (377 ohms). To
    do so with a cylinder results in a rather abrupt transition and a
    small aperture area. To do so with a horn results in a more efficient
    gradual transition with the added bonus of a larger aperture area.
    http://www.seattlewireless.net/index.cgi/CardboardHorn
    I don't suggest building one out of carboard and aluminium foil, but
    it does illustrate the increase in relative gain over a similar size
    can. Ideally, it would be a section of WR-340 or WG-9A waveguide with
    the horn mounted on the flange. The RF source is mounted on the back
    end of the waveguide, with a piston reflector plugging the back end of
    the waveguide for tuning. It's also easier to mount the N connector
    on a flat surface or to shove the USB radio inside.
    No. DC conductivity does NOT equate to RF conductivity. RF
    conductivity is all skin effect and is on the surface of the
    conductor. Some skin effect references:
    http://www.geocities.com/CapeCanaveral/Hangar/6160/skineffect.html
    http://www.microwaves101.com/encyclopedia/skindepth.cfm
    Ummm... Perhaps a stainless can antenna?
    http://www.cantenna.com/catalogue/SC12.html
     
    Jeff Liebermann, Jul 5, 2004
    #18
  19. John Beeston

    dold Guest

    There was a picture on one of the .nz web pages, but I wasn't sure if it
    was the Dick Smith or the DWL mini-USB. Probably doesn't matter.
    http://www.usbwifi.orcon.net.nz/dsewifi.jpg
    The top portion of the jpeg-as-web-page shows the naked dongle on a
    broomstick above the roof.
    I recall skin effect from my days in heavy radar, but I don't know if this
    is the proper skin. Silver plated anything is a little on the pricey side
    for the homebrew antenna market.
    David Taylor speaks of putting a horn on the end of a can.
    http://www.nodomainname.co.uk/Antennas/antenna.htm#Can with conical horn
    That shows another 6dB or so.
    That's in a material that is conductive. I don't know that
    the liner in these cardboard tins is conductive, or all the same.
    I really am interested in the USB dongle. It's so much cheaper than the
    802.11b card+pigtail+cantenna. The guys in nz suggest pulling the ceramic
    patch off and soldering on a different antenna, but then you've destroyed
    the dongle, and need to adhere to stricter design for radiator sizing.
     
    dold, Jul 5, 2004
    #19
  20. I think you mean this one.
    http://www.usbwifi.orcon.net.nz/usbscoop.jpg
    A little reminder... the efficiency of a parbolic reflector is at best
    50% assuming a decent feed that doesn't over or under illuminate the
    dish. My guess(tm) is that the efficiency of a ceramic patch antenna,
    illuminating a chrome plated steel vegetable scoop is close to 10%.
    Chrome also makes a lousy RF conductor.
    Swell. Tear apart the USB radio, remove the ceramic patch antenna,
    and replace it with a coax connector or solder on pigtail. Attach a
    real antenna. See:
    http://jeffl.ihwy.com/linksys/wusb11/wusb11-2.jpg
    The pads in the upper right are where the ceramic patch antenna was
    connected. The pattern is perfect for a PC mount SMA connector.
    However, I got lazy and soldered some semi-rigid coax to the pads,
    with an SMA at the end.
    Not really. I make my antenna parts for copper water pipe, copper
    flashing, and sheet brass. Mostly, I use electroless silver solution
    (silver cyanide) to plate the copper and brass. The problem with
    electroless silver is that it's not really thick enough at the lower
    ham frequencies. I have the calcs if you want them. However, for
    2.4Ghz, it's not problem. One skin depth is:

    The skin depth is:
    depth = 2.6 * K1 / sqrt(freq)
    where:
    Depth = inches
    Freq = Hz
    K1 = 0.94 for silver

    depth = 2.6 * 0.94 / 49000 = 0.000026 in = 26 micro inches.
    (or about 0.3 microns).

    Electroless silver is self limiting at 2-3 microns, so this should be
    no problem at 2.4GHz where anything over 0.3 microns will suffice. If
    you don't mind making a mess, silver plating is fairly easy. The
    biggest headache was getting the chemicals shipped to me without a
    hazardous substance waver.

    Some useful instructions on plating:
    http://www.LearnByDestroying.com/crud/silver-plating.txt
    http://www.qsl.net/n9zia/why_silver_plate.html
    More on skin effect:
    http://www.geocities.com/CapeCanaveral/Hangar/6160/skineffect.html
    Bah humbug. I was thinking of something more substantial.
    http://www.setileague.org/photos/wghorn/ewen2001.jpg (1420MHz)
    http://www.setileague.org/photos/wghorn.htm
    http://www.barc.org/ao40_antennas/rxantenna.html
    Neither do I. However, even if they are conductive at DC (as measured
    by an ohms-guesser), it's still the surface conductivity and skin
    effect that determines the performance at 2.4GHz. One of my better
    screwups was instead of painting an aluminium antenna, I cleverly had
    it black anodized. Performance stunk until I scraped off the plating
    with sandpaper. The same problem occurrs with chrome, and zinc
    plating.
    Sacrifices must be made. Use the force (or a suitable disassembly
    tool).
     
    Jeff Liebermann, Jul 5, 2004
    #20
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