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Requirements / Gereklilikler: Windows 2000 / XP

Like many would-be 802.11b hackers, I'm increasingly obsessed with pushing more bits further and faster for less cost (I believe the unofficial goal of our community wireless project is to provide infinite bandwidth everywhere for free. Of course, there are problems with approaching infinity, but it's still fun to try!)

The work that Andrew Clapp and others have done is helping to demystify the ancient black magick of Resonance (i.e. antenna building). And so, over last weekend, some friends and I decided to give it a go for ourselves.

(standard disclaimer): Anything you do with your gear is YOUR RESPONSIBILITY. This is a stupid idea that will probably ruin your radio, set your house on fire, bring the FCC to your door, ruin your crops, and send famine and pestilence across the land. And as the operator, it is YOUR RESPONSIBILITY to not take the word of some raving lunatic on the web with funny colored hair, and find things out for yourself. Your mileage will vary. I'm probably lying. You have been warned.

Anyway, our first run was a direct rip-off of Andrew Clapp's terrific original design (knowing next to nothing about antenna construction, it's helpful to start off with a working known good.) By using PVC, all-thread, washers, some cheap copper tubing, a Pringles can, and some scrap cardboard, we were able to make a prototype shotgun yagi in a matter of hours. Having a couple of other excited alpha geeks around can help move construction projects along very quickly.

Once this was up and running, we looked at the design, and of course speculated about ways to optimize it. While a directional antenna showing between 12 and 15db gain is impressive, it's also pretty large, physically. We realized that, if we were careful, we could fit a full wavelength inside the Pringles can itself (at a reduced total gain), but make the entire antenna much more compact.

In about 45 minutes, we had the collector rod built, the locknuts on, and the whole thing in place. The result: A Pringles can that pulls about 12db!

Parts list :

Of course, buying in bulk helps alot. You probably won't be able to find a 6" piece of all-thread; buy the standard size (usually one or two feet) and a 10-pack of washers and nuts while you're at it. Then, you'll have enough for two, for about $10.

Tools required:

Ruler
Scissors
Pipe cutter (or hacksaw or dremel tool, in a pinch)
Heavy duty cutters (or dremel again, to cut the all-thread)
Something sharp to pierce the plastic (like an awl or a drill bit)
Hot glue gun
Soldering Iron

Construction time: about an hour

Front collector construction:

Mark and cut four pieces of tubing, about 1.2" (1 15/64"). Where did I get this number? First figure out the wavelength at the bottom of the frequency range we're using (2.412 GHz, or channel 1). This will be the longest that the pipe should be:

 W = 3.0 * 10^8 * (1 / 2.412) * 10^-9
 W = (3.0 / 2.412) * 10^-1
 W = 0.124 Meters
 W = 4.88 inches 

We'll be cutting the pipe to quarter wavelength, so:

 1/4 W = 4.88 / 4
 1/4 W = 1.22" 

Now figure out what the shortest we'll ever use is (2.462 Ghz, or channel 11 in the US):

 W = 3.0 * 10^8 * (1 / 2.462) * 10^-9
 W = (3.0 / 2.462) * 10^-1
 W = 0.122 Meters
 W = 4.80 inches 
 1/4 W = 1.20" 

Practically speaking, what's the difference between the shortest pipe and the longest pipe length? about 0.02", or less than 1/32". That's probably about the size of the pipe cutter blade you're using. So, just shoot for 1.2", and you'll get it close enough.

Cut the all-thread to exactly 5 5/8". The washers we used are about 1/16" thick, so that should leave just enough room for the pipe, washers, and nuts.

Pierce a hole in the center of the Pringles can lid big enough for the all-thread to pass through. Now is probably a good time to start eating Pringles (we found it better for all concerned to just toss the things; Salt & Vinegar Pringles get to be almost caustic after the first fifteen or so.)

Cut a 3" plastic disc, just big enough to fit snugly inside the can. We found another Pringles lid, with the outer ridge trimmed off, to be ideal. Poke a hole in the center of it, and slip it over one of the lengths of pipe.

Now, assemble the pipe. You might have to use a file or dremel tool to shave the tips of the thread, if you have trouble getting the nuts on. The pipe is a sandwich that goes on the all-thread like this:

Nut Lid Washer Pipe Washer Pipe Washer Pipe-with-Plastic Washer Pipe Washer Nut

Tighten down the nuts to be snug, but don't overtighten (I bent the tubing on our first try; aluminum bends VERY easily.) Just get it snug. Congratulations, you now you have the front collector.

Now for the can:

By now you should have eaten (or tossed) the actual chips. Wipe out the can, and measure 3 3/8" up from the bottom of the can. Cut a hole just big enough for the connector to pass through. We found through trial and error that this seems to be the "sweet spot" of the can.

On the Pringles Salt & Vinegar can, the N connector was directly between Sodium and Protein.

Element construction:

Straighten the heavy copper wire, and solder it to the connector. When inside the can, the wire should be just below the midpoint of the can (ours turned out to be about 1 1/16"). You lose a few db by going longer, so cut it just shy of the middle of the can

We were in a hurry, so we used hot glue to hold the connector in place. If you have a connector that uses a compression nut and washer, and you're really careful about cutting the hole, you could use that instead.

Now, insert the collector assembly into the can, and close the lid. The inside end of the pipe should NOT touch the copper element; it should be just forward of it. If it touches, your all-thread is probably too long.

Now, just read FCC Part 15.247, connect your pigtail, aim carefully, and have fun!

Unfortunately, I don't have access to any of the necessary equipment (spectrum analyzer, power meter, or even an SWR meter) to properly evaluate the characteristics of the antenna. SWR in particular would be a really good idea to measure, as we're not sure how much power is feeding back into the circuit (too much and you can easily blow your transmitter.) With the extremely low power output (15dbm) of the Orinoco cards, I don't think this is too much of a danger, but remember, anything you do with your equipment is your responsibility, and at your own risk!

Without the proper (multi-thousand dollar) tools, how were we able to estimate antenna performance?

Using the Link Test software that comes with the Orinoco silver cards, you can see the signal and noise readings (in db) of a received signal, and your test partner's reception of your signal. As I happen to be 0.6 mile LOS from ORA headquarters, with very little noise on the channel between, we had a fairly controlled testbed to experiment with. We shot at the omni on the roof, and used the access point at ORA as our link test partner.

To estimate antenna performance, we started by connecting commercial antennas of known gain, and taking readings. Then, we connected our test antennas and compared the results. We had the following at our disposal:

two 10db, 180 degree sector panel antennas
one 11db, 120 degree sector panel antenna
one 24db parabolic dish
a couple of Pringles cans and some tin foil

Here were the average received signal and noise readings from each, in roughly the same position:

The test partner (AP side) signal results were virtually the same. Interestingly, even at only 0.6 mile, we saw some thermal fade effect; as the evening turned into night, we saw about 3db gain across the board (it had been a particularly hot day: almost 100 degrees. I don't know what the relative humidity was, but it felt fairly dry.)

Yagis and dishes are much more directional than sectors and omnis. This bore out in the numbers, as the perceived noise level was consistently lower with the more directional antennas. This can help alot on long distance shots, as not only will your perceived signal be greater, the competing noise will seem to be less. More directional antennas also help keep noise down for your neighbors trying to share the spectrum as well. Be a good neighbor and use the most directional antennas that will work for your application (yes, noise is everybody's problem.)

When trying to aim a yagi (like our little can), keep in mind that they have large side lobes that extend up to 45 degrees from the center of the can. Don't point directly at where you're trying to go, aim slightly to the left or the right. We also found that elevating the antenna helped a bit as well. When aiming the antenna, hold it behind the connector, and SLOWLY sweep from left to right, with the Link Test program running. When you get the maximum signal, slowly raise the end of the can to see if it makes a difference. Go slowly, changing only one variable at a time.

Remember that the can is polarized, so match the phase of the antenna you're talking to (for example, if shooting at an omni, be sure the element is on the bottom or the top of the can, or you won't be able to see it!) You can use this to your advantage to try to eliminate some noise on a long distance link: slowly turn both ends of the link from vertical through horizontal, and stop at the point that you see the most gain (and lowest noise.)

We haven't looked into weatherproof housing for the can; sinking the whole thing into some 3" PVC should do the trick. Of course, at $10 for two, it might just be more economical to replace them when they fail.

Apparently, antennas of comparable gain cost upwards of $150. Over a clear line of sight, with short antenna cable runs, a 12db to 12db can-to-can shot should be able to carry an 11Mbps link well over ten miles.

Have fun!

Tthe dishes are being trashed, and are available for other uses such as the one I describe here. It is easy to make a surplus Primestar dish into a highly directional antenna for the very popular IEEE 802.11 wireless networking. The resulting antenna has about 22 db of gain, and is fed with 50 ohm coaxial cable. Usually LMR400 or 9913 low loss cable is used if the source is more than a few feet from the antenna. The range using two of these antennas with a line of sight path is around 10 miles at full bandwidth. I must stress the line of sight part though. Leaves really attenuate the signal.

Things You Will Need:

1. A Primestar dish.  (You may use any old dish, but if it is bigger than the Primestar the gain will be higher, and it may not be within the Federal Communications Commission rules for use within the United States.  In fact I have come to find out that there seem to be several different dishes that Primestar used, and I am only sure that the one I used, pictured above, used with the ordinary Wavelan or Airport transceiver card is within the effective radiated power limits given by the FCC.)
2. A juice can (about 4 inches in diameter and at least 8 inches long).
3. A chassis mount N connector.
4. You will also need a "pigtail" connector which has the proprietary Lucent connector (for the PCMCIA card) on one end and an N connector on the other. The pigtail can be obtained from a number of online stores for $35 to $40.

Construction Steps:

1. After deciding on a place to mount your antenna (which hopefully has a line of sight path to the access point or other IEEE 802.11 site), remove the apparatus at the feed position of the dish, saving the mounting hardware.
2. Using a can opener, cut one end of the juice can out, drink the juice and wash it out
3. Solder a quarter wavelength (1.15 inches) of wire onto the center conductor of the chassis mount N connector.
4. Using a punch or whatever other tools you deem necessary, mount the N connector so that it is about 1.2 inches from the closed end of the juice can as shown below. It is also a good idea to put a drip hole at the lowest point of the can to insure that water doesn't build up inside. After having this up a for a few months, I think it would be nice to put a plastic lid on the open end of the can so that the inside doesn't rust. During the time mine has been up, it has rusted and I have lost a couple of db on the signal strength. These two things may be correlated.
5. If you are certain of the polarization you will need, mount the juice can so that that polarization is achieved. (You want the antenna you are communicating with to be lined up with yours.) If you don't know the polarization you can set everything up and before mounting the juice can, experiment to get the maximum signal strength by rotating the juice can around its axis. Most commercial antennas I've seen are using vertical polarization, so that the picture below shows you the proper orientation. You want to mount the juice can so that the opening is just at the focus of the dish. In my installation I didn't quite achieve this, but I only lost a db or two by taking the easy route. I still have about 25 db signal to noise ratio, so this wasn't important to me. The easy route is to mount the can as far back as you can along the mount, by punching two holes through the can and bolting it in. The perfectionist's method would be to find the best feed place (which I found to be just a little farther back) and use some PVC tubing or something to extend the mount so the feed is in the perfect position. In some installations, every decibel will count and this should be considered.

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Some Considerations:

This antenna is very directional. You must have it aligned very carefully, or you will lose a lot of signal. It also needs to be mounted securely, so the wind won't be able to rotate it even a few degrees.

This antenna is an offset fed dish, which means that the feed horn (our juice can) is not positioned as much in the way of an incoming signal, so it doesn't shadow the dish. This makes the aiming a bit tricky, because it actually looks like it is aimed down when it is aimed for the horizon. See the photo below of it aimed actually a few degrees above the horizon. You can use the scale on the dish mount to determine the elevation it is aimed at. The dish isn't as directional in the up/down directions as it is side to side. This is fortunate, because without turning the mounting upside down we can only get it set so it is a few degrees above the horizon. I sacrificed a db of gain here by not turning it over, mostly because I'm mounting it on a vent pipe, and didn't want to put that kind of wind load on it. As mentioned above I don't really need the extra signal either.

I believe this feed system could be improved by linking two cans together (or using a section of pipe) to give the effect of lengthening the length of the circular wave guide feed. This would further attenuate undesired modes (other than the TE11 mode). The optimum length would be between 14 and 21 inches. The opening would still need to be at the focus of the dish.

Use the Feed Can By Itself

You can use the feed can by itself as a cheap antenna. It works as well as the commercially available "range extender" antenna, but only in one direction, and it is so easy to construct!

Some More Photos