Maxrad Collinear Omni


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.

Parts List

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).

Construction


measurements

Below is my assembly procedure:

NOTE: Try to be as accurate as possible in cutting the parts and spacing them during assembly. .5mm accuracy is difficult but at least try to get within 1mm of what's specified. Otherwise, you may be sorely disappointed with the performance of your finished product. I used a pair of vernier calipers extensively during the construction of this antenna.
  1. Hold a piece of 3/64" brass wire against 9/64" drill bit and wind 4 turns, each turn separated by about 3.5cm. Leave about 10mm of straight wire before the turns and 24mm after the turns. Adjust the spacing of the turns with needle nose pliers while the coil is still on the drill bit. Bend the ends neatly with the needle nose pliers. Repeat until you have 4 coils.
  2. Cut four 57mm long pieces of 3/32" tubing and one 91.5mm long piece. I used the cutter on a pair of needle-nosed pliers to cut them; this flattened them, which is not bad because the hole in the tubing is actually a loose fit. Next, I used the needle-nosed pliers to squeeze the tubing until the hole opened up large enough to pass the wire into it. Finally, I filed the tubing smooth, which took off about .5mm off its length.
  3. Cut a 30mm piece of 11/32" brass tubing. I used a hacksaw and mitre box.
  4. Solder the 91.5mm long tube to the center pin of the N-jack. This is the feedline
  5. Slip the 30mm tube over the 91.5mm tube, and solder to the N-jack. This tube is the decoupler. The feedline must be supported in order to keep it from shorting on the decoupler; I slid a piece of rubber hose over the feedline.
  6. Solder a coil to the feedline, leaving about 3mm of wire between the feedline and the start of the coil.
  7. Solder a piece of 3/32" tubing to the other end of the coil, leaving 22mm of straight wire between the last turn and the tubing.
  8. Repeat until you get to the top of the antenna.
  9. Cut the PVC pipe to the proper length to enclose the antenna and glue on the end cap.
  10. Cut pieces of foam to support the antenna inside the PVC pipe - I stuffed the foam into the coils - and carefully slide the antenna into the pipe.
  11. Attach the N-jack to the other end of the pipe. You can screw it onto a flat end cap if you can find that style. I chose to just tack the N-jack directly to the pipe with hot glue. Then, I used epoxy over the hot glue for strength and heat resistance.

decoupler


nude antenna


completed antenna in radome

Performance

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.









1