A Bridge Too Far?

Bad Ideas and Further Research

I've tried a number of things in terms of cheap waveguide antennas. Nothing beats the tin can antenna so far-in fact, even improving on the tin can antenna has been difficult. Theoretically, putting a 'horn' on the front of the can will improve its operation. I found a 4" to 6" stove pipe reducer at Home Depot that fit snugly over the outside of a coffee can. It wasn't of quite the right dimensions to be a horn, but it was certainly cheap and easy. See Figure 15.11.

Figure 15.11: The Duntemann, Mark I with optional Stove-Pipe Reducer Horn: Not Recommended.

Unfortunately, the horn didn't improve things much, and I don't consider it worthwhile. Attaching a correctly dimensioned horn would doubtless work better, but would take some significant metalwork. If you're good with a tinsnips and pop riveter, and don't mind pushing some numbers, it would be worth a try. Here's a document that will explain how to calculate a proper horn for waveguide antennas:

http://www.qsl.net/n1bwt/chap2.pdf

Tetra Brik Antennas

Not all waveguides are round in cross section. Rectangular waveguides are very common too, and if there were a metallic container with the right dimensions and proportions, you could build an antenna from it. Such a container exists, and it's called a 'Tetra Brik' sanitary liquid container. A Tetra Brik is a European-designed layered rectangular package, 60 mm by 90 mm by 200 mm long. It has cardboard on the outside, aluminum foil in the middle, and a thin plastic sheet on the inside to keep the soup from attacking the foil.The dimensions are about right for a rectangular waveguide, and the material can be easily cut with a sharp knife.

After seeing people on the Web create waveguide antennas from Tetra Briks, I tried it myself, using a Tetra Brik that had contained beef broth. Mine came together in about twenty minutes without any drilling-using only a Swiss Army knife and a soldering iron. The antenna is small, light, and not easily damaged. (See Figure 15.12.) It's not quite as effective as a tin can antenna, especially for long distances, but it definitely works in finessing weak spots and dead spots. Here are some tips:

Figure 15.12: A Tetra Brik Antenna.

• As with circular waveguide antennas, one end is open, the other end is closed.

• You can use the same probe as with a tin can antenna. The probe's length depends solely on the frequency, not the shape or dimensions of the waveguide.

• Mount the probe centered on one of the 90 mm faces, 42 mm (1 11/16") from the closed end.

• This is the trick: Make sure that there is electrical contact between the probe N-connector's body and the foil lining of the Tetra Brik. The plastic film covers the foil and will normally insulate the probe assembly from the foil. However, if you place 'spiny' lock washers under the screw heads holding the probe assembly to the Brik and tighten the screws very tightly, the lock washers' spines will pierce the inner plastic film covering the foil lining, allowing the screws to make contact with the foil.

As Figure 15.12 shows, you don't necessarily need a gooseneck base. Once I worked out the correct angle for aiming the antenna from my problematic coffee table, I just bent a scrap of aluminum to that angle and made it the base. (The scrap heat sink at the bottom is just ballast to keep it upright.)

I was actually unaware of Tetra Brik containers before seeing the following Web sites. They are much more common in Europe than here. If you can't find them at your mainstream supermarket, try an import market. Note that some of these sites are not in English-open a browser window to Babelfish! ( http://babel.altavista.com/)

http://users.skynet.be/chricat/horn/horn-javascript.html

http://reseaucitoyen.be/?BoiteDeLait (French)

http://reseaucitoyen.be/?BoiteDeLait2 (French)

For further research, calculate and attach a foil-lined horn to the opening of the Tetra Brik, and see how much that improves its performance.

PrimeStar TV Dish Antennas

Several people have used tin can antennas as feed points for surplus PrimeStar satellite TV parabolic dish antennas. Such a rig could net you tremendous gain over a 'naked' tin can, but even without trying it myself I see some challenges:

• The PrimeStar dishes are offset antennas, meaning that microwave signals strike the dish from an angle and exit at an angle. The can antenna feeder thus does not cast a microwave shadow on the disk (which is good) but this makes aiming the antenna, which has a very narrow beam width, difficult and touchy.

• The PrimeStar dishes are 'solid' antennas rather than grid or mesh. In other words, they are made of unperforated metal. This gives them a slightly higher gain, but also makes them wind scoops, and heavy: Mounting one will be tricky and must be done in such a way that the wind will not knock it out of its (touchy) alignment.

If you have such an antenna lying around (or know where you can find one) this might be worth some research. The following Web site will get you started:

http://www.wwc.edu/~frohro/Airport/Primestar/Primestar.html

As always, I'm interested in hearing from people who have put Wi-Fi antennas together from unconventional materials.