cantenna experiments
in [Wireless Internet]
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From: bjs555 on 5 Oct 2005 20:07 <jeffl(a)comix.santa-cruz.ca.us> wrote: >The exposed center conductor of the coax cable is *FAR* too long. >Doing a coax to board transition is not easy, especially with the fat >coax you're using. Continue the bare copper wire shield wrap right up >to the end of the coax. Trim the center conductor to where it just >barely sticks out beyond the shield. Shave some of the insulation to >get the center conductor closer to the SMA center pin. Solder the >copper wrap to the SMA ground pins. I'll try that. >Otherwise, do it right and get a pigtail or adapter. I really should but I can't find them locally so I'll have to order them over the net. >The probe length should be 1/4 wave (31mm) from the shield on the F >connector. You might wanna try a conical probe for improved >bandwidth. I wasn't sure whether the length should include the connector shield. I guessed wrong. Also, I'll minimize the amount of connector extending into the can as Clarance Dold suggests. I'm still surprised that I don't pick up signals other than my own with the cantenna like the usb dongle does. Does the high directionality have something to do with that? But I tried pointing it every which way. Maybe things will improve after I make some of the suggested changes. Thanks again. Bruce
From: Jeff Liebermann on 6 Oct 2005 00:52 On Wed, 05 Oct 2005 20:07:09 -0400, bjs555 <aaa(a)bbb.com> wrote: >I'll minimize the amount of connector extending >into the can as Clarance Dold suggests. That's not really that critical. >I'm still surprised that I don't pick up signals other than my own >with the cantenna like the usb dongle does. Does the high >directionality have something to do with that? But I tried pointing it >every which way. Maybe things will improve after I make some of the >suggested changes. See: http://802.11junk.com/jeffl/pics/antennas/coffee2400/index.html for roughly what the pattern is suppose to look like. Yes, you have to point it at the other station. -- Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060 831.336.2558 voice http://www.LearnByDestroying.com AE6KS http://802.11junk.com Skype: JeffLiebermann jeffl(a)comix.santa-cruz.ca.us jeffl(a)cruzio.com
From: bjs555 on 6 Oct 2005 14:48 More results: I lengthened the probe in the can so that it's now 1.21 inches (yeah, 10 mil accuracy :) and reduced the amount of connector shield inside the can. I didn't see any improvement and possibly even a slightly lower signal than with the shorter probe. But my test setup leaves a lot to be desired. I'm sure that there are all kinds of reflections since I can get wildly different readings depending on where I point the can. As more experienced people have pointed out, a small room isn't the best place to run tests. One puzzling thing for me continues to be the fact that I don't pick up a lot of access points other than my own with my cantenna even if I try pointing it in many different directions. It seems to me that the cantenna works best when it's receiving a signal from another radio that also has a directional antenna attached. I say that because I see a lot better signal from the cantenna when I put a windsurfer reflector (www.freeantennas.com) on my ap. That is, without changing the distance between the cantenna and ap, and aiming the cantenna for max signal, I pick up a much stronger signal with the windsurfer attached. Didn't one guy try putting a funnel-like attachment on the end of a cantenna? I think I saw a picture like that on someone's web site. Looked goofy, and I don't see how the geometry really feeds more signal into the can, but the idea of having a larger signal collection area rings sorta true (like a 12 inch telescope vs a 6 inch one). So maybe the reason I don't see a lot of other people's access points with the cantenna is that they are using omni directional antennas. Would that explain the lack of other access points showing up in Netstumbler? I know there are at least a few around that I pick up with a usb dongle. I haven't yet shortened the exposed center conductor where I soldered the RG-6 cable onto the pci client card. I'll have to try that later. For fun, I tried disconnecting my cantenna entirely and seeing if I could get a signal with just the RG-6 cable attached. I did pick up enough signal from my own ap 20 feet away to connect even without any cantenna. Maybe the exposed center conductor was feeding enough energy into the radio. I also tried using both the cantenna and the rubber duck antenna at the same time since I didn't remove the client card connector. Kind of a stupid thing to do, I know, but surprisingly it didn't kill the connection and the signal level reported by the Linksys monitor went up by 10%. Guess you can get away with almost anything at 20 feet. Along those lines, I found this site that seems to be a quite different kind of antenna: http://www.andrew.com/products/trans_line/radiax/default.aspx Wish I could be more quantitative but I don't have the equipment or enough knowledge yet to make careful measurements. I'm having some fun trying to figure out what's going on and I'm very interested in hearing what others have done. Bruce
From: David Taylor on 6 Oct 2005 15:39 > One puzzling thing for me continues to be the fact that I don't pick > up a lot of access points other than my own with my cantenna even if I > try pointing it in many different directions. It seems to me that the Have you tried rotating the cantenna at all?
From: dold on 6 Oct 2005 15:51
bjs555 <aaa(a)bbb.com> wrote: > cantenna works best when it's receiving a signal from another radio > that also has a directional antenna attached. I say that because I see Polarity comes to mind. The can is three dimensional. You may have been rotating the aziumth and elevation, but the orientation of the antenna probe itself is also important. Try rotating the antenna on the long axis, so that the probe is vertical, horizontal, and other points in between. Especially if your small room is full of reflections, the optimum angle for the probe may not be obvious. > a lot better signal from the cantenna when I put a windsurfer > reflector (www.freeantennas.com) on my ap. That is, without changing I like the Windsurfer. I have made several. I keep forgetting to leave enough "tab", so it takes me a while to assemble. http://www.rahul.net/dold/clarence/EZ12-windsurfer.jpg > cantenna. Maybe the exposed center conductor was feeding enough energy > into the radio. Once upon a time From Don Widders: You can just make an antenna out of the end of the LMR195. Remove about 3 inches of the plastic outer 'jacket' of the coax. Then pull the copper braid back over the remaining jacket. Get a piece of brass tubing at a hobby shop that will just slide over the braid and cut a piece of the tubing to 32 mm. Trim away any excess braid. Cut the center conductor so that it extends exactly 32 mm from where it exits from the braid. -- --- Clarence A Dold - Hidden Valley (Lake County) CA USA 38.8,-122.5 |