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Wi-Fi antenna postscrip

Discussion in 'Boat Electronics' started by Bruce, Apr 18, 2010.

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  1. Bruce

    Bruce Guest

    I ended up building two wi-fi parabolic antenna. the first a 300 mm
    screen-wire dipper (used for frying bananas) measured 300 mm in
    diameter and about 90 mm deep. Later built a second dish using a 430
    mm aluminum "wok" about 100 mm deep.

    The first antenna gave a better signal then the adapter but the second
    antenna gave, using the instruments available to me, approximately 3
    times the signal that the bare wi-fi "adapter" had.

    As I previously mentioned I do not have a signal strength meter and
    used the standard Linux utility "iwconfig" to produce some sort of
    data. It gave a reading of 6 for signal strength using the bare
    adapter and as high as 23 with the wok I can only assume that whatever
    the value of the increments that the ratio is accurate.

    As luck would have it, I found an abandoned TV "cable" antenna - as
    used here "cable" is received on a 3 foot dia. parabolic antenna and I
    will probably try that at some later date.


  2. My question is what makes you think that these two "dishes" are
    Parabolas, and not just dish shaped? I would bet that the TV dish will
    work significantly better than any dish shaped reflector, simply because
    it will have the correct mathematical shape, and the Focal Point will be
    defined properly by the designers.
  3. Bruce

    Bruce Guest

    I don't particularly think that the cooking dishes have a correct
    parabolic shape. I was attempting to solve a problem with what I could
    get my hands on. The discovery of the "cable TV dish" came shortly
    after I had completed the 17" wok.


  4. As I stated Bruce, the TV antenna is a good plan. It will be a true parabola and the gain over what you have will be significant.
    Don't forget about the 30 degree included angle that is built in. You will have to move the detector up to the center for your
  5. Bruce

    Bruce Guest

    No worry about misalignment a there is no "horn" on it. Just a dish
    It is currently being used as a "roof" for a well pump and I have some
    20 ltr. pails that, with a notch cut in one side to clear the piping
    will work even better so I reckon a trade can be made... once I and
    them get back to Phuket.

  6. Larry

    Larry Guest

    We get about 1.5 miles range from my 200mw hotspot with a 6db antenna 15
    meters up in a tree. The whole hotspot is built into an inverted
    plastic bucket. The pringle's can antennas are very directional and
    great for point to point work like you're doing.....and can be built and
    rebuilt for nothing.
  7. Larry

    Larry Guest
    Kismet is the program for Linux
    Network STumbler for Windows is better and if you hook a GPS receiver to
    the PC it will even log the position of every signal it finds that
    couples directly to Google EArth! Way cool scanner for wifi at home or
    riding around in a vehicle.
  8. Bruce,
    I assume you know how to find the parabola's focal point. I won't bore you with a plan if you already know. Let me know if my
    assumption is incorrect.
  9. Bruce

    Bruce Guest

    Thanks for that. I had a couple of others but more is better.
    Net Stumbles has a problem in that it only works with certain specific
    Wireless chip sets. Unfortunately not the ones I have. Cheers,

  10. Bruce

    Bruce Guest

    Yes, have the formula and know about the reflected sunlight method,
    although that never seemed to work for me as when I put out a piece of
    paper to measure the reflection it shaded the dish and the reflected
    beam was too weak to see :) I'll have to get a special piece of
    something transparent to use.


  11. Bruce

    Bruce Guest

    I was going to built a tin can antenna but the devil is in the details
    and so far I haven't been able to locate the tiny coax connectors that
    connect to the wi-fi adapter and as I had read that at wi-fi
    frequencies the loss in the usually available cable is nearly equal to
    the gain of the antennas I have been a bit reluctant to try.

    I am making a trip down to the "electronic district" tomorrow and,
    after having written the above, will undoubtedly discover a source of
    not only the connectors but also a coil of low loss cable that I can
    get free :)


  12. Here's a tip. Construct a Tee piece say 2 inch across the top of the tee. Emanating at a 90 degree angle from the center of this 2
    inch piece is a straight edge long enough to reach and pass the theoretical focal point....make an educated guess....Mark the
    center of the parabolic dish with a magic marker. Use a tape measure or fasten a rod of any material perpendicular to the dish
    center. Place the Tee piece anywhere on the surface of the dish and where the leg of the Tee crosses the center rod is the focal
    point. Do this several times from different locations on the dish surface and average the crossing point on the perpendicular.
  13. Bruce

    Bruce Guest

    The point where a number of lines drawn at right angle to parabolic
    dish intersect a line drawn at 90 degrees from the center of the dish.

    I'm still going to get a sheet of Plexiglas, or something and try
    focusing the dish, if for no other reason then to say that I did it


  14. Bruce

    Bruce Guest

    Since I wrote the original I have done more research and it appears
    that the current favorite is the offset feed dishes. One article has a
    method of calculating the reflection angle and then cutting a string
    for the far side of the dish and a second for the near side and mount
    the feed at the point that the two strings meet.

    Also I found several articles abut the construction of double quad
    antennas and one of the articles showed the graphs of test with 12 db

    I may change to the quad if I can get that much gain from a smaller
    antenna as none of the tests I saw indicated that the dish produced
    really astonishing gain.


  15. You

    You Guest

    It is a "Good Thing" you do NOT design Antenna Systems for a living,
    because you would be out of business in a week. Don't give up your DAY
  16. Larry

    Larry Guest

    All of this discussion is MOST commendable, however it is also most
    moot. All the gain in the world isn't going to overcome the natural
    signal to noise ratio problem of inverse square law propagation from the
    router's antenna against the intense solar and thermal noise of a sunny
    day and a hot parking lot. The routers were specifically designed to
    limit range to approximately 100 meters by reducing their power output
    to a pittance, like your sellphone. Some routers only run 10-20mw into
    horrible antennas made of a piece of pc board. The best ones only run
    200mw tops into a 3db whip with space diversity receivers to hear your
    20mw powerful beast coming back to them.

    Highly directional antennas, just as with UHF analog TV and its
    "ghosting" problem, do help reduce multipath propagation IF the antenna
    is very tight patterned with very little back pattern, such as the
    Pringle's Cantenna we've been building for years.

    These waveguide antennas have no rear radiation as the back of them is
    solid metal. Antennas designed around HF, VHF even UHF are of little
    use on microwaves, however cute. Waveguide antennas are used for radar
    for a reason.

    As you can see, their signal to noise ratio is quite impressive, much
    more impressive than an open HF/VHF/UHF design.

    "The test partner (AP side) signal results were virtually the same.
    Interestingly, even at only 0.6 mile, we saw some thermal fade effect;
    as the evening turned into night, we saw about 3db gain across the board
    (it had been a particularly hot day: almost 100 degrees. I don't know
    what the relative humidity was, but it felt fairly dry.)"

    Our measurements between my hotspot 20 meters up an oak tree under an
    inverted plastic bucket and the USAF enlisted barracks (I support the
    troops) are very similar. Some days the combination of high humidity
    and high temperature obscure my 200mw into a 6db co-linear quite badly
    over the 1.2km path length to the roof of the 4 story barracks building
    where the Pingle's Cantennas are mounted on various pipes to hide them
    from paranoid schitzophrenic inspections.

    Sky News in London gave the Pringle's Cantenna a boost, recently:

    of course, blaming it for hacking, to infer it should be outlawed by the
    UK nanny state.'s+Can+antenna&aq=f
    There's lots of great videos from the "hackers". Oddly, most of them
    look like anyone in your neighborhood, not some dangerous cyber

    I'm sure the news paranoids will win, at some point, and we'll all be
    arrested by Homeland Security in the USA.
  17. Larry

    Larry Guest

    We've tried his type of UHF antennas. The signal to noise ratio in
    daylight is horrible from all the back pattern and many side lobes of these
    designs for lower frequencies. A yagi isn't a good microwave antenna when
    multipath reflections are eating up your data with odd pulsewidths and
    pulses and the sun is boiling the molecules in the parking lot. You can
    even see the S/N ratio drop by turning on a nearby incandescent light bulb!

    Microwave antennas for both 2400 and 5200 Mhz (N band) are too easy to
    build out of a Pringle's can or old juice can that can reduce background
    radiation and thermal noise by a huge margin.

    It's like looking at a distant object with your naked eye in bright
    sunlight, squinting because of the glare (noise)......then, looking at the
    same object through a paper towel cardboard tube that's been painted flat
    black inside (pringle's cantenna). Without all the noise, the object is
    much easier to look at. The same is true of these little round microwave
    antennas, with or without the internal washer-made yagis to improve the
    tuning and bandwidth.
  18. Bruce

    Bruce Guest

  19. Bruce

    Bruce Guest

    Generally I agree with you (hard to argue with facts :) but the point
    is, if with Antenna X, you have a very low signal strength and with
    Antenna Y you have a much stronger signal then antenna Y will. in
    nearly all cases, give better results.

    All of the high gain wi-fi antennas, that I have looked at, mainly the
    cans, quad, corner reflector, etc., which have high gain also have
    good signal/noise ratios and are highly directive.

    In addition, from the reading I have done, all of the high frequency
    antennas are sensitive to the accuracy with which they are built -
    tests I have seen on various cans showed a difference in gain that was
    probably caused by fractions of a millimeter in inaccuracy.

    Likely I could beg/buy/make a signal strength meter and SWR meter and
    set up a proper antenna test range and spend days building a perfect,
    antenna, but I'm not interested in that and all I want to do is log on
    the Internet and read RBE.

    The dish I built gave me a much better signal than the wifi adapter
    alone however SHMBO is not happy with a wok hung on the wall - says it
    does nothing for the decor and in Thailand a wok belongs in the
    kitchen, so I am trying to build something that gives approximately
    the same gain, or better, as the dish and is not so weird looking
    hanging on the bed-sitter wall..


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