cantenna experiments

Trying to use what I had at hand, I put together a quick cantenna and ran some tests. I'm describing the results here hoping that I can get ideas for improvement. I built the cantenna following the instructions at:

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My client card is a Linksys WMP11v4. My router is a Netgear MR814v2. I built a cantenna as follows: diameter = 3.25 inches length = 6 inches (one can) or 12 inches (two cans) probe 2.5 inches from end of can probe length = 1.2 inches (seemed short compared to the pictures but that's what the instructions gave) coax cable = 1 foot of RG-6 quad shield or 5 feet of same (bad, I know, but that's what I had) connectors = one or three F type connectors (again, what I had)

I attached the cantenna to the WMP11 and used Netstumbler to make some measurements. The overall results were disappointing. The cantenna didn't give me any improvement over the original rubber ducky antenna that came with the card. I was able to see drops in the signal level of up to about 20 dB depending on where I pointed the cantenna, but even when pointed in the optimum direction the level was about equal to level with the card's original omni antenna.

Thinking that I might be losing a lot of signal in the coax and connectors, I shortened the cable from 5 feet to 1 foot and eliminated

2 of the connectors. But that made no difference. That surprised me. Doesn't that imply that the problem is elsewhere? I know I should get some LMR cable and the right connectors, but I wonder if it will make a difference since shortening the lossy cable and removing connectors didn't help.

I also tried changing the length of the cantenna from 6 inches to 12 inches by adding a second can but that made very little difference (about 1 or 2 dB).

Another thing I noticed is that, without any modifications, a usb dongle I had on hand (Zonet ZEW2501) worked much better than the Linksys WMP11 card. The maximum signal level was about 12 dB higher for the ZEW2501 than the WMP11. The ZEW2501 is tiny. The WMP11 is a more serious looking piece of hardware, but it performs much worse than the tiny dongle. Is there a lot of difference in the quality of different manufacturer's radios?

Many thanks for help.

Confused, Bruce

Reply to
bjs555
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What you haven't said is how far away from the AP you were when testing. If you're in the same room and already have a good signal, the result will be masked by reflections and the generally good signal that you already have.

Try it once you've gone somewhere where the signal is really weak and then see what difference you have. Or find a neighbours signal with the cantenna and then take it off and see if it still works.

Yes.

David.

Reply to
David Taylor

Thank you both for your replies.

Wow, was I wrong! Now I see a 30 dB difference in Netstumbler when I aim the cantenna for minimum and maximum signal. I get about 10 dB better than the maximum signal with the original rubber duck antenna.

Heres what happened: I decided to put up some pictures to ask if anyone saw anything I missed. After taking the pictures, it was clear that that the hot coax lead had lifted off the point where I soldered it to the client card. It must have broken off due to moving the cable around after I put the card into the computer since I checked it just before. Slap me with a fish.

I'll put up the Netstumbler results and some pictures of the cantenna in a little while. I'll post the web address in this post.

I am still puzzled by some things. I get a very strong signal from my own two APs (in the same room about 20 feet away) but none from other nearby APs. There are about three others in houses close to me that I can pick up easily with my Zonet ZEW2501 usb dongle. But they don't show up when I use the cantenna even if I point it in all kinds of different directions and turn off my own routers to prevent interference. Any ideas on why this is happening?

Thanks, Bruce

Reply to
bjs555

Bruce, Get a Hawking Tech Omni Directional 6DB Antenna, I have one on the outside of the house sitting on the window sill of the 2nd floor, it has about a 26 inch cable feeding a NeatGear WG311 card, it works great for me, I picked up 9 neighborhood AP't this morning. I don't know if you live near a Micro Center, that are not too many, but they carry Hawking Products. The Tech Omni Directional 6Db antenna is only 5 bucks after a rebate, Hawking Tech Wireless G USB Network Adapter

6DB Antenna Combo 802.11G is the same deal, 5 bucks with a rebate, or was when I bought mine. I have that adapter, and it works great. Go to
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and check out these two devices, I didn't read all the details, since There is a Micro Center store in my area located in Fairfax Virginia. You might be able to get these product thru an on-line order, but check on the rebate details for on-line orders. FM
Reply to
FM

Thanks FM! I've heard good things about Hawking and that's a great deal. The cable and plug alone are worth more than that. An omni isn't the best for long distances, I'm told, but I can't argue with success. I'm definitely getting one of those for experiments.

Bruce

Reply to
bjs555

Ok, I put up pictures at:

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I'd appreciate any comments or suggestions.

Bruce

Reply to
bjs555

Haha. Didn't have the connectors and none in local shops. Didn't want to wait for delivery. Also, I was trying to minimize the number of reflections by using as few connectors as possible. But with using the wrong impedance cable and all, the connectors might have helped as much as hurt. Anyway, the board had nice big solder pins and I figured "What could go wrong?". My bad.

I've done some more testing. I don't see any difference by using one can instead of two. That is, shortening the length to 6 inches. Easier to aim. Maybe there are differences on the order of a couple of dB that I don't see in my ramshackle setup.

Reply to
bjs555

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I agree with David. When I first set up for testing, I was too close, and I saw very little difference in the NetStumbler signal. It's almost as if the device doesn't report signal above a certain strength. If you move to where the signal is in the 70-80 range in NetStumbler, you can see some antenna improvements. If it's in the 40-50 range, you aren't going to see much change at all.

It's also helpful to be outside, at least a few feet off the ground, and have a clean line of sight to the AP.

A single 3.25" can should give you 8-10 dB improvement in NetStumbler. The 20dB drop that you see by pointing it the wrong way is impressive. Or maybe your cable shorted out when you twisted it that way ;-)

That is 1.21" of _exposed_ probe, including the exposed center conductor of the connector. The wire also goes down into the connector a bit.

Reply to
dold

The wire you see twisted around the shield extends out behind the cable and is soldered to two ground contacts. The shield must be aluminum or something - I couldn't solder to it so I twisted some bare wire around it tightly. That's probably bad for long term use as the connection could oxidize but ok for a quick test. And thanks to you and Jeff I now know that I should make the exposed center conductor as short as possible. I'll try that tomorrow.

Bruce

Reply to
bjs555

I'll try that.

I really should but I can't find them locally so I'll have to order them over the net.

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

Reply to
bjs555

Yeah, it's the wrong colour! :)

Why did you fart around soldering to the back of the board when you could have just made up a short pigtail to convert from the SMA to N type?

David.

Reply to
David Taylor

Rearrange the F connector in the can so that the shielded portion is barely exposed, just enough for the attaching nut. The exposed inner conductor of the probe should be 1.21".

Reply to
dold

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.

Otherwise, do it right and get a pigtail or adapter.

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.

Reply to
Jeff Liebermann

That sounds about right.

;-)

opps.jpg (I assume "oops") shows a lifted center conductor, but I don't see the shield attachment point.

It should be coax as much as possible, with the shield and center conductor separated as little as possible, soldered to appropriate contact points.

Reply to
dold

That seems to be true for the USB dongle in a can as well. Two cans gives marginally better gain, but it is also much more directional. I thought that was good, but in use, the friend that I gave it to found it too hard to keep pointed at the WAP (using a photo tripod), and preferred a single larger diameter can.

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Reply to
dold

It looks like the total lenght of your F-Connector and probe is 1.21 inches, so the exposed conductor is substantailly too short.

Reply to
dold

That's not really that critical.

See:

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roughly what the pattern is suppose to look like. Yes, you have to point it at the other station.

Reply to
Jeff Liebermann

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

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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:

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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

Reply to
bjs555

Yes. Through 360 degrees. No luck.

Hey David, just saw on one of your web pages that you tried a horn on the end of a cantenna. Sorry I called it goofy but it is funny looking. But then I read some posts by Jeff saying that a horn makes a smoother transition from the 50 Ohm radio source impedance to the 377 Ohm impedance of free space. Foot in mouth today. Everybody's a critic, huh?

Reply to
bjs555

Tried that. Rotated slowly through the long axis 360 degrees. Picked up one ap but just barely.

Yeah, they seem to work great.

Very interesting. Thanks.

I'm starting to think that the reason I don't see many aps is that the receive sensitivity of my Linksys WMP11 card isn't nearly as good as my usb dongle. It's an older card that I just happened to have lying around. But today I ordered more junk from Compusa and MicroCenter so I can LearnByDestroying (tm). Compusa has WUSB11s for $4.99 after rebate and MicroCenter has a usb dongle for $9.99, an SMC router for $7.99, and a Hawking antenna for $4.99 all after rebates.

Reply to
bjs555

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