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Thread: Deep Dish Cylindrical Parabolic Template

  1. #1

    Deep Dish Cylindrical Parabolic Template

    Deep Dish Cylindrical Parabolic Template

    NOTE: Some pictures on this site are contributed by persons who have built this antenna, all pictures on this site are used with the permission of their author(s). Please send me a picture of your effort and let me know how it went.

    Advantages over other antennas such as the Pringles Can Antenna
    1. No Pigtail Required
    2. No Modification to AP (No voiding of warranty)
    3. No Matching (SWR) Problems
    4. No Purchased Parts
    5. Trivially Easy Construction
    6. Very Low Probability of Error
    7. As Good As or BETTER Performance than the Pringles Can Antenna
    8. Superior Front to Back/Front to Rear Ratio
    9. Improves Wireless LAN Privacy
    10. Reduces Interference

    Basically this antenna is so easy to make, tune, and install, and it performs so well that it is foolish not to try one before electing to purchase a commercial antenna, if for no other reason than you can check to see if you are purchasing enough commercial antenna to make the link you want to make. I needed a parabolic reflector to eliminate off property coverage. This one can eliminate signal from some areas while enhancing signal in other areas. A six or eight inch version of this reflector is just perfect for eliminating off site coverage while enhancing signal in poorly "lit" areas. You can make one from a Pringles can, there is little need to get fancy. I designed this reflector to be installed in outdoor enclosures with WAP-11 access points but it is becoming with people building indoor LAN's and people building very short point-to-point links between homes and offices because of it's performance and easy availability (scissors, tape, cardboard, tin foil and twenty minutes and you are in business). It can easily complete links up to one Km by sitting two WAP-11's in windows at each end of a link with clean line of site. The six inch version of the antenna will give you about 10 to 12 dB of gain over what you have. This equates to approximately 27 to 33 dB of EIRP. That means you wind up with an apparent power in the favored direction between 500 mw and 2 Watts. These numbers depend upon your access point, cards in use, which WiFi gear, etc... etc... That gain has to come from somewhere and it comes from the back side of the reflector, power normally transmitted in that direction is "bounced" forward. Therefore you have more control of where your signal is going when using this reflector. That feature of this antenna can be used to enhance the privacy of your wireless network and that was my reason for designing it in the first place, the rest is just gravy but it is very real and rather tasty gravy. Please also note that antenna gain is preferable to amplifier gain because it adds to both transmitted and received power. Usually amplifier gain only increases transmitted signal. It does one little good to increase transmitted signal when the access point is unable to "hear" the weak little PCMCIA card at the other end of the link.
    The drawing can be scaled upon a copy machine to make a dish of any reasonable size. The gain computations for various sizes of the dish are also provided as well as rough graphs showing beam width and gain/frequency. This reflector is frequency independent, meaning it will work with any wireless gear on any band.
    There is a square drawn upon the diagram. It will help you to ensure that your scaling does not corrupt the aspect ratio of the template. In other words, if the square is still square after you enlarge or reduce the template, you still have a good template.
    Focal length varies with the size of the dish (but proportionally) therefore the focal point is also shown on the drawings. Positioning of the feed point (focal point) is the most critical aspect of a deep dish parabolic. Errors of 1/4" are unacceptable at these frequencies. It may help to "fiddle" with the positioning as small irregularities (~> 1/4 inch) will move the focal point slightly. If the dipole is not in the focal point, you will loose gain.
    Parabolic reflectors also loose gain if your finished reflector varies much from the correct curve. This drawing should be accurate enough to be scaled to any reasonable size.
    The reflector is designed to be fed by a dipole. That is why it is not circular. A dipole is long and cylindrical, the focal point on a circular dish is circular. The focal point on this design is a cylinder. Many access points use dipole(s) as their antenna, the WAP-11 is one such AP. This reflector is the optimal shape for such an antenna. More recent units, such as the WET-11, do NOT use dipoles as their antenna. I have added a template designed with those antennas in mind also.
    The reflector is designed to be "square". Meaning you will use a piece of square material to shape the curve. If you need to reduce height for packaging reasons, a shorter antenna will work but you will lose roughly 3 dB for each halving of reflector height. It is also important to try to get the dipole lined up in the center of the reflector (see above).
    If you don't have a copier, or you want to make a very large scale version, you can make your own graph paper and transfer the curve manually. Brown wrapping paper works fine. You might scale each 1/4" block to be 1" and the final result would be a 24" reflector. Smaller reflectors around 6 or 8 inches are more practical at home. If your access point has two antennas, just make two reflectors one for each antenna.
    The template is a bit large but be patient. Right click on any image and select "Save Picture As" or "Print Picture"... Once you have a printed copy, ensure the grid on the printed copy is square. You can then use the image directly, as indicated in the lower pictures, or you may scale it on a copier, using paint, or manually. You might use the template to make a more rigid cardboard template so that you can check the entire surface of your reflector as you shape it. If you decide to scale the image in Paint be sure to check the little square drawn on the template. Scaling can change the "aspect ratio" of an image and that would create a bad template. So long as the little square on the template is still square after you re-size the template, you still have a good template. You can also decide to use only a portion of the curve from the template. If you do that you should still use the same focal point that is marked on the template.
    Front to back ratio is a measurement of how well a directional antenna rejects interference from directions other than the desired direction. This reflector has excellent front to back ratio. Front to back ratio with this antenna depen*s upon the size of the wire mesh you use to make the antenna. Finer mesh will yield only slightly better gain but will yield much better front to back ratio. Modeling shows the F/B ratio to be better than ~25 dB if you use 1/4" or smaller mesh. My calculated gain figures presume the reflector is 55% efficient. If you use a solid sheet of aluminum or copper as your reflector, your gain figures may be a little bit higher than these. The radiation pattern is narrower in the vertical plane than the horizontal plane. With larger sizes pointing the antenna can require care.
    So if you have an AP in your house, snatch one of your cookie sheets and get out the tin snips =), just don't get caught by Mom Actually you can make this reflector from just about anything. People have made good ones from Pringles cans, large tin cans, wire screen, aluminum sheet, and tin roofing material. Any flat metal surface or screen, even tinfoil taped to cardboard will work. You can build one of these in less than a half an hour using an old shoe box and a roll of tin foil.
    I would really like to hear from people who have made one of these as this helps me to improve this page.


  2. #2

    Re: Deep Dish Cylindrical Parabolic Template

    Gallery of Successful Builds:

    An eight inch version of this reflector out performs the "cantenna" in all respects and it's much easier to make a good one on the first try.

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