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Wi-Fi - Wi Fi - Antenne
Tratto dal sito:http://www.geocities.com/lincomatic/collinear.html
Although I have been aware of the guerilla.net low power collinear omni for some time, I have hesitated to build it due to its complexity, and the fact that I didn't fully understand the design. To complicate matters, the website has been down for several weeks, making the design inaccessible. I was finally able to piece it together by using a copy from google's cache, and archived JPEG's from this site: http://www.tux.org/~bball/antenna/. My google searches also yielded an attempt to explain the theory behind the antenna.
Although the guerilla.net folks claim that the antenna is their own original design, one day while surfing the FCC site for information about Lucent's ORiNOCO cards, I happened upon an interesting document. On page 13 of this document was a photo of antenna labeled Maxrad which looked almost identical to the gnet collinear omni. sparafina of the NetStumbler Forums did some initial analysis of this antenna, and found its dimensions to be very similar to the gnet design. Later, I found a test report which refers to the antenna as external antenna AUO24-OD-10, and lists its gain as 10dBi. Once I sat down to build the antenna from gnet's design, I found their description of the coil dimensions to be confusing - maybe I am just too dense. My initial attempt was a complete failure with very little gain, probably due to my confusion over the exact dimensions. Therefore, I decided to try again, but this time building it based on measurements taken from the FCC photo. Below is an internal photo from the FCC archive which I've modified to ease the estimation of the measurements:
click on photo for larger version
The photo is rather blurry and pixellated, but I've sharpened it a bit to increase clarity. If you'd like to check my measurements, an easy way to do it is use Paint Shop Pro on the larger version of the photo. Simply use the selection tool to cut out a piece of the ruler. Then you can use the selection as a ruler by dragging it around the photo. For vertical measurements, rotate the selection by 90 degrees. Very handy indeed. Not only is the photo blurry, but the quality control on the antenna appears to be deficient; from segment to segment the measurements are not identical. Here are the measurements I estimated from the photo:
The main difference between the gnet collinear and the Maxrad is just the wire diameter. The gnet design uses 3/64" wire (which is alot easier to bend) and 3/32" tubing. Also, the gnet design specifies to leave a 2cm tail on the coils, which I found to be a little too short. Otherwise, the two designs are pretty much identical. Since guerilla.net seems to be down, I hope they don't mind that I've reconstructed their page as best I could for your convenience.
Since I already had the tubing and wire from the gnet parts list, I used 3/64" wire and 3/32" tubing instead. I think the 2:1 ratio of tube to coil diameter is more critical than the actual diameters anyway. Since the Maxrad decoupler is hidden in the plastic, I used the gnet dimensions for the decoupler, but substituted a N-female jack for the SMA jack.
For those who don't feel comfortable cutting and bending the parts, aerialix sells the antenna in kit or assembled form. Their prices are very reasonable.
Below is the complete parts list with prices I paid:
Grand Total: $11.50 including tax. Everything except the N-jack was bought at Osh Hardware. It's a good idea to buy 1 extra piece each of tubing and wire in case you mess up. I used Schedule 50 pipe because it's the thinnest (I'm paranoid about losses through the radome).
I tested the collinear down the street from my AP using MiniStumbler. Below are the results:
Antenna | SNR |
ORiNOCO built-in: | 10 dB |
collinear: | 20 dB |
For some reason, the signal strength was fluctuating wildly, so this is the best estimate I got. Overall, I'm pretty happy with it, which is good because this antenna cost more and took by far more time to build than anything else I've made to date