I used a hacksaw, a dremel with the heavy-duty cut-off wheel and some small files
To figure out where to cut, I drew lines to cut along, on the outside of the extinguisher body. This includes a nice ink pen, an architects scale (tri-angle type ruler), and a piece of paper. The ink pen could have been a scribe, instead. A sharpened nail, or ice-pick would work for that
I went and recycled a few photos between steps, as I didn't take enough pictures during the construction
Step 1: It all started with Pachanka's Wokfi
And this, is a 24 inch wok I found at Goodwill for $7, mounted on top of a telescope tripod. Handles were removed and the thumb-fi taped and hung off of some string into the focal point
But before I could test this wokfi, I read yet another instructable for a yagi cantenna, and after following some of the links in the comments I decided to build this cantenna
Step 2: In hindsight
So this is what I started with, a Kidde fire extinguisher that was empty and had been cluttering up a shelf for several years now.
It has an aluminum body with a nice flat bottom, which I thought would work well as a waveguide.
It measured around 3-1/4 inches outside diameter. This is like the kind you might find in a car or around a kitchen. If it had a plastic body, I suppose it could be wrapped in aluminum foil, but I would've scrapped the idea and gone for some exhaust tubing or something like that.
Step 3: Deconstruction
Seeing as this extinguisher was empty and not pressurized, I didn't need to empty it first. But if I did had to do that, I would just pull the safety and shoot the thing untill no air came out
The fun part was years ago with my nephews assistance, right now all I need to do is unscrew the top and dump out the remaining powder
The top unscrewed easy, by hand. No fatal explosion. For the faint of heart, you could tie the trigger valve open so that there's not even a chance of pressure building up while it sits, but why miss out on the adventure?
After filling with some water and rinsing out the last of the powder, I needed to figure out where to cut this thing
Step 4: Cutting the head off
Using a close estimate of what the inside diameter might be, about 3-1/8, I decided to cut as close to the top as possible, but without any of the curve from the tops shoulder left behind. I could shorten the can later, if needed
This is where that piece of paper comes in. This is a standard technique that I use on so many other projects, that it probably should have it's own instructable. But it's the first time I'm bringing this up, so I'll include it here
I'm going to be using a hand-held hacksaw, and I want to make a nice and even cut so that the top is square and flat. A quick and easy way to do this is to first draw a line to use as a guide, going around the extinguisher
To do this, I wrap the paper around the can, and because it's a standard printer paper, it has a very straight edge on all sides. This means that after wrapping it tight, and after adjusting the paper so that the edges where it meets are lined up with each other, all I have to do is slide it up to where I want the cutting line, then carefully draw along the edge of the paper, all the way around the top of the extinguisher.
Once I have a nice cutting line to go by, I follow that with the hacksaw and proceed to cut the top off. The after the top is off, I went over it with a flat file to even it down and give the inside edge a nice crisp corner. I think that any imperfections here might disturb the radio wave
Step 5: Lengthy Layout For A Thumb-Hole
This gives an upper and lower frequency range that looks good. The full wave length is 11.07 inches, 3/4 wave is 8.3. I decided to leave the length alone
The 1/4 wave measure of 2.77 inches, is where I want to position the thumb-fi, and I reasoned that this is probably the center of the thumb-fi
First, another standard layout trick. Using any nice length of angle stock will allow a method to draw lines perfectly along the length of tube or pipe. In this case, I'm using that funny ruler. Laying it along the length of the extinguisher, near the rear end, it fit's automatically and I use that to draw a starting line. Doesn't matter too much where as long as it goes through the likely placement of the hole, but it will help if the line ends right at the bottom of the extinguisher, because I'm going to eyeball the next lines position based on the first one
To find where to put the second line, I rested the thumb-fi on the bottom of the extinguisher so that it lined up with the line I had drawn. Then I placed the ruler so it lined up on the other side of the thumb-fi. Carefully holding the ruler so it wouldn't budge, I just let the thumb-fi fall onto the work bench and proceeded to draw the second line along the length of the extinguisher
These two lines give two sides of the hole that will be cut. The quarter wave dimension will now be used to find the other two sides. I fudged this step, I made a reasonable guess where the bottom of the inside of the extinguisher would be. So, with the ruler laid along the length of the extinguisher, I slid it down to the bottom and lined up the zero with where I thought the inside bottom might be, then made a mark at 2.77 inches up. This is going to be between the last two sides I will be marking out, now I need to know how thick the thumb-fi is
Another bit of eye-balling here, as I just placed the thumb-fi on the ruler and slid it down untill one edge looked lined up with zero, then looked down the other edge to find it's approximate width. Slightly over 1/2 inch
Now, placing the ruler back on the extinguisher, and putting one edge of the ruler on that 2.77 inch mark, I slid the ruler along that mark untill the number one mark on the ruler lined up with the 2.77 mark on the extinguisher. Now I just go a 1/4 inch to either side and there's my last two sides, almost. One last time with that paper wrapped around the extinguisher, lined up with each of those last two marks. Now the last of the sides are drawn. I expect to have to use some filing to finish the hole, and I was right as we'll see next....
Step 6: Cutting the Thumb-Hole
After a bit of filing to clean up the hole, I found that the thumb-fi didn't want to go all the way in. So, some more filing on each side to keep it centered to the starting dimesion. I was only off about a 1/16th
Step 7: It's a Cantenna!
Then I went back to the shooting range to see how this compared to the wok-fi
I used the telescope tripod, and tied the can to it so that it could be rotated as well as aimed up and down or side to side
It didn't work! I was able to see a bunch of networks, but couldn't get any of them to lock in. Bummer...
Step 8: Fine tuning
That did it! Now I can get some signal levels!
The thumb-fi is about a half inch into the can.
Step 9: Final Conclusion
Using the Cantenna, I was able to get about 12 signals in just one direction. Though not as strong as I would like, still a bit better than the Wok-Fi did
The best signal looked like it was a mile away, going by where the Cantenna was pointed when I moved it untill the signal dropped off.
Edit: 3-26-'09 Well, seeing as I continue to get comments on this instructable I guess I could say a bit more.
The retired dish-tv experiment was an improvement on this. See The Wireless Internet Cantenna gets Dished
But my best performance is a simpler approach. Just add a cone to the end of this fire-extinguisher to get the Conetenna. I'm regularly establishing a connection about 5600 feet away from my location using the Conetenna. The connection is too slow for any kind of streaming video, but enough to check my e-mail.
At another location, using the Conetenna, I've established a 100% 8-out-of-8 signal with the maximum data rate possible over the adapter - from across the backyard and inside of a garage.
I was going to build an even better antenna, but the Conetenna is enough for now, and other projects beckon. Thanks.