The Cantenna, Part 2

[This will be one of three or four entries I hope to complete tonight. I haven't posted in a few days, but I have been doing stuff -- if not necessarily the most productive stuff.]

So I built a tin can waveguide antenna (or “the Cantenna” as it shall be called hereafter). I’ve got pictures of it up at http://www.codefu.org/people/darkness/cantenna/ in various stages of design and at various angles. The pictures aren’t the best; they were done with my Intel web cam and Windows 2000 Imaging. Also, if you notice, I’ve blurred out certain unimportant details (like my face) from the pictures. Pay no attention to the man behind the can. If there are problems with the pictures that you think were a result of conversion from TIF to JPEG, please let me know; I did it real quick with ACDSee, which has a decent interface for converting a bunch of files. BTW, ACDSee will also apparently handle multi-page TIF files and splitting them up in to separate files. If only I had known this, I would have used the faster method of snapping a bunch of pictures in the Intel web cam tools, then saving it to a multi-page TIF with Windows Imaging, then just split it out to separate files with ACDSee. Sigh.

Where to start? This was over the past weekend, so I might screw up some details or leave some steps out. For starters I removed the label from the can, opened it with *gasp* a can opener, emptied the contents into the Memory Hole, and then washed it with Dawn and water. This is not precisely rocket science, of course. I measured the displacement of the element from the back of the can using the calculator given on the Cantenna how-to page. For the record again, the can’s diameter was 8.5725cm, giving us a 1/4 waveguide length of 5.6412cm. This is actually calculated by me using a set of RPN scripts I wrote; the Cantenna how-to’s calculator disagrees with this value by something like 0.0508cm I believe. (BTW, RPN, despite having what I would categorize as a weird scripting language, is quite nice. I now long for an HP 48GX. I also want dc for the Palm Vx, or an HP 48 emulator. Someone please let me know if either of these exist.)

I borrowed darkho’s drill for this activity. It was a Black & Decker something-or-another with a set of DeWalt drill bits that worked swimmingly. I used a 3/32″ bit to drill the hole for the element originally, but I had to work it bigger and may have eventually resorted to the 7/64″ bit I used for the #4 (7/64″) screws. (Er, I think I used the 7/64″ bit. Maybe I used the 3/32″ bit for all of them. I started with the 5/64″ bit, which was way to small. To duplicate my experience exactly you could start with the 5/64″ and work your way up.) The can was relatively easy to drill: it didn’t bend or collapse like I suspected, though I had to put what I felt was a lot of pressure on it to actually get the bit through. (I most of this on the kitchen table, BTW.)

After drilling the hole for the little brass extrusion on the N connector, I inserted it and proceeded to try and mark the holes on the can. My fat sharpie wouldn’t fit through the holes to touch the actual can, so I tried a Pilot P-500 roller ball pen — which actually marked on the can, to my surprise. I then removed the N connector and remarked the pen spots with the Sharpie. Drilling the holes for the screws was similar to drilling the holes for the element, except the holes for the screws fell on the can’s grooves in such a way that it was difficult to keep the bit on the spot to be drilled. I suppose I should have taken Nightwolf up on his offer of a drill press here. No matter; as a result of the position of the holes with relation to the grooves on the can the connector ended up being mounted sort of twisted. The element remained in the right place it seems, but the connector had to be twisted a little crooked. You can easily see this in the pictures. After I drilled the first hole and managed a screw in it, I attached the connector with the one screw and started drilling through it, both to make sure the holes were in the right place and to try and steady the bit in the grooves again.

So the connector is crooked, the screws are at weird angles, but it all seems to be OK WRT the actual element inside the can, which I presume is the important part. Once I read the suggestion of putting the heads of the screws inside the can and the nuts on the outside, I made the holes as bit bigger so I could easily slip the screws in from the inside with my hands. Now it was time to solder some wire into the extrusion from the N connector. I happened upon a 50′ spool of Red 12 gauge copper wire at Lowe’s for something like $3; quite a bargain. I took off a five inch or so piece, stripped it down, and stuffed it into the extrusion on the connector. It was a pretty good fit: not tight, but not real loose. I measured the height up to 1.21″, or 3.07cm, and tried to cut it off carefully at that height. (Incidentally, while I know nothing about the calculations used in the design of a waveguide antenna, I suspect this is 1/4″ wavelength of the 2.4GHz band used by 802.11b.) I actually took a rubber-covered hammer to the piece of wire at some point to try and get it as straight as possible. I then heated up ye ole’ soldering iron and put the wire in place. I was pleased with the straightness of the end result, after some very small adjustments by bending the wire a bit. Make sure to wait for the solder to dry.

Almost done now. The hole I made for the element wasn’t quite enough to accommodate the bit of solder I had globbed on to it, so I had to use the drill again to widen the hole a bit. Holding the N connector in place with the element inside the can I inserted the screws from the inside of the can and attached the bolts. Voila, Cantenna complete. Now for testing.

At my disposal I had an older Orinoco Silver card and a brand new Cisco Aironet 350 LMC card. I have pigtails for both (from FA&B of course). I was running the tests on my Powerbook, running Yellow Dog Linux 2.3, their kernel 2.4.19-4a, orinoco_cs 0.13a, and either the version of airo_cs that came with that kernel or the latest one from CVS (I can’t remember which). I also used wavemon to monitor the card. Oddly my Orinoco card/drivers weren’t reporting signal strength. So I ended up using the Cisco 350 card for the remainder of the tests. Note that I did have both antennas on (it has two MMCX connectors) enabled, and in retrospect maybe that’s a bad thing. I always figured that “two antennas are better than one”, but I know in some cases pointing the actual antenna connector at the AP did seem to increase the signal strength noticeably. I was testing to an Orinoco RG-1000 AP — which seems kind of shitty to me, at least WRT features and interface, and might be my biggest problem in the first place. The AP was on channel 11.

First I just tested in my room about 3′ from the AP to see if I could notice a difference in wavemon when I pointed the antenna at the AP. I could. I remember the jump being significant, probably between +10dBm and +15dBm or something. (BTW, decibels still confuse me, so sorry if my terminology is all off here. It might be just “a gain of +15dB”, but the signal is reported to me in dBm, so I’ll keep it that way.) Then I went and sat the AP on top of the couch in the living room, about 2.5′ off the ground I’d estimate, with the lights (i.e., the front) facing the wall in anticipation of my coming tests. Testing in the living room (probably like 8′-10′ from the AP) I observed basically the same results. Then I took the antenna outside. I could still get signal, but it was faint. I also knew there was an AP somewhere else in the complex. I was able to pick up the signal, and I think I pinpointed it to the building across from the back of our building. I had some trouble pinpointing this signal from our third story apartment, though. It would seem to lose signal at random. I eventually think I found one position where the antenna was sitting on a railing that kept the beacon packets coming.

BTW, the Cisco card seems to “lose” the signal from time to time. I can detect this time because of a lack of change in the “link quality” metric in wavemon (and from the wireless extensions in general, I presume). This figure is usually jumping all around. When my signal level was low and I stop seeing a lot of changes in the numbers for a second or two, I presume this to mean a loss of signal. BTW, the link quality seems to be kind of not what I expected with the Cisco card/drivers: it seems that the lower the number, the better the link.

I then tried to talk to the AP from ardent’s apartment, which is the final goal of all this. I estimate his apartment to be no more than 250′ from mine in a straight line (he’s first floor, I’m third). No luck. No signal at all, in fact. I thought for sure this antenna would be able to power through the three or four apartments or whatever to get to the AP which was sitting on the wall closest his apartment, but no. I couldn’t even get it from the hall outside his apartment. I had to go out to the free space behind his apartment to get even a meager -89dBm signal.

I was quite disappointed. We tried a few different orientations, but no luck with them. I decided to test the antenna a bit like the guys tested their original designs. We went out to the parking lot in front of our apartment where we had a clear shot all the way to the other side of the apartment complex, by the apartment office. I ran an extension cord from the alternator in my car to the middle of the street where we hooked up the AP. Then I took my GPS to measure distance, my laptop, and the Cantenna. I walked to the end of the complex and the GPS told me it was about 550′. (It was reporting accuracy to 10′ or 20′ or something like that.) When I first pointed the antenna to the AP, I couldn’t get any signal at all to it! Then I radioed ardent (we were using my walkie-talkies again; me == dork) and asked him to pick the AP up off the ground. Boom, I had a signal. It was hover around -85dBm IIRC — though it may have been -75dBm. I wish I had taken better notes. Of the notes I did scribble down long after the experiment I wrote -85dBm first, so that’s the number I’ll stick with.

Needless to say, at half the distance from the AP the guys in the home-brew wireless antenna shoot-out were using, I was getting less signal than they were. I even had a better can, IIRC (their diameter was supposed to be too big for an optimal design). I tried to follow all instructions and was very careful about it, but the antenna wasn’t good enough. I think I need to e-mail the Cantenna guy(s) for some advice on what I did wrong.

Feeling a bit puzzled I decided to whip out my FA&B 5dBi omni-directional mobile antenna and see what it could do. Quite very surprisingly, it allowed us to get a signal to my AP — which was unmoved — from inside ardent’s apartment! We found that holding it in some odd places and odd angles helped. For example, we got a good result holding it near the bottom of his porch door and tilted back about 45 degrees. The tilting back makes sense if you look at the radiation pattern of the antenna, I think. Also we found that blinds made a great big difference. I suspect window screens might also.

So in the end we might just end up with some omni-directional antennas. Or maybe ardent will get an omni-directional antenna and I’ll get one of FA&B’s cheap (but +15dBi or something like that) parabolic antennas, which I believe are pretty damn directional. I could point the antenna at his omni and hopefully get good signal. We’ll have to look in to this more, and think about the investment (probably ~$60).

This concludes my adventure in Cantenna land. If you have questions on what I did, or comments to make it work better, please e-mail me.