DIY cantenna - part 2

My cantennas here at home are 83.5mm diameter and 186mm length, and 156mm diameter and 180mm length. The probe of both is 31mm copper wire, thick enough to fit +- snuggly into the hole in the connector, then soldered in. Both cans have ripples all down the side.

The placement of the probe varies according to the diameter of the can. Both work pretty well, (the narrower tin maybe a little better), for web browsing, but I sometimes get errors when downloading big files. For real use I have a big parabolic. The cans are more like an experiment. I read somewhere that optimum would be a 90mm diameter can with a length of 200mm or longer. Aim for 90mm. I'd love to make one of stainless steel, to the best dimensions .... mine rust. Will have to find time to go to a junkyard :P FWIW ....

sillyputty hath wroth:

One nice thing about can antennas is that the design is very forgiving. The can can be fairly sloppy in construction (ripples, edge rims, dents, dings, etc) and it will still mostly work. The only items that are critical are:

1. Grounding of the coax connector.
2. Length of the probe.
3. Position of the probe relative to the bottom of the can.

Sigh. Good luck. See suggestions below.

I've watched some of the locals build cantennas. I try to talk them into building better antennas, but the simplicity and price of a coffee can antenna is just too easy to ignore. The one's that are successful usually end up building several. How much V8 can you drink?

Yes, but it's not necessary. The ripples don't cause much of a problem with the gain pattern. The ripples at the bottom of the can might cause some tuning changes that result in the position of the probe being not exactly where the calculations suggest, but that can be compensated with a bit of tweaking (tune for max signal).

The aluminum foil covered insides will have a very minor benefit in reducing the surface conductivity (skin effect) of the antenna at RF frequencies. However, you won't notice the difference without a pile of expensive test equipment.

If you're really looking for the ultimate can antenna, then I suggest you visit the hardware store, not the grocery store. Purchase a small roll of aluminum or copper flashing and roll your own can. Copper can easily be soldering into shape. Aluminum can't be soldered and the typical "conductive" epoxy is worthless at RF frequencies. So, I use aluminum foil duct tape to finish the seams.

If you're going to roll your own, then I suggest you look into building a proper horn antenna, which is what the waveguide beyond cutoff type of can antenna is trying to simulate.

Now, if you're really cheap, as are most hams, you can build an acceptable horn antenna out of cardboard, and embalm it in aluminum foil. This is not terribly useful for small antennas, but really works well with giant horns used for long distance experiments.

Antennas have also be constructed from trash cans, step ladders, HVAC ducting, roof flashing, plywood, chicken wire, barbeque grills, snow dishes, kitchen cookware, and other abominations. The reason these work is that at low gains and wide beamwidths (such as the typical can antenna), the tolerances are VERY wide, thus making precision a non-issue. However, if you're planning on building a fairly high gain can or horn antenna, some precision will be required.

I played with many different sized cans. I decided that two "13oz" coffee cans soldered end to end were the best, based on signal testing, but in actual use, one "3lb" coffee can was less sensitive to being pointed and held exactly on target.

Can Diameter Length

What thickness of copper flashing would you recommend? Same samples:

sillyputty hath wroth:

Any thickness. It has no effect on the performance. However, with todays spectacular price increases for copper, methinks you should make your initial tests using aluminum foil and cardboard. Only the INSIDE of the horn needs to be accurate. Use aluminum foil coated duct tape to hold the seams together. Once you have something that's worth building, either build a better one out of aluminum roof flashing or dive into your pocket book and solder one using copper roof flashing.

A problem you're going to need to deal with when using copper is that it oxidizes rather badly. Gain is affected by surface conductivity and a copper horn, covered with green crud, is not going to work very well. I have my own silver plating system, but it's a dangerous mess and I'm not going recommend it. Last time I built a few prototypes out of copper, I found the local plating shop and had them silver plate the antenna. Be sure to remove or carefully mask the connectors before plating.

If you don't want to deal with silver plating, it is possible to paint the antenna. I've had some luck with clear Krylon (acrylic) but suspect that almost any other paint will work. I've done little painting because I also tend to modify the antenna after plating and need to maintain a solderable surface.

One trick you should consider when building a rectangular horn is to make the back end of the horn, near the feed, adjustable in position. Besides the length of the driven element, the exact location of this "reflector" is the only other critical part of horn construction. You could make the position of the feed adjustable, but it's much easier to make the position of the reflector adjustable. When a maximum gain location is found, tack solder it into position.

Note: I'm not an antenna expert. In general, the requirement for precision is proportional to the gain. At best, the common coffee can antenna could yield about 12dBi of gain when build perfectly. Few are, and what I've seen is more like about 8-10dBi. Good enough. However, if you're going to be building a high gain big dish, big horn, or some kind of array, dimensions become more critical roughly with increasing gain.

Also, I had original (wrongly) proclaimed that can antennas and do thyself reflectors were horrible antennas because of construction tolerances and material surface imperfections. Aparently, that's not the case as long as the gain is fairly low. I was accustomed to squeezing every last fraction of a dB of gain out the system for long range point to point, satellite, and TVRO systems, where every fraction of a dB is precious. That's not the case with Wi-Fi, where considerable sloppiness can be tolerated with low gain antennas.

My suggestion is to build something cheap and easy to start, and spend some effort setting up a method of comparing the gain to a known reference. Netstumbler is good enough. Check your gain against a commercial antenna with known gain. Tweak, tune, cut, trim, and hammer on the antenna. When it's close to the theoretical calculations, you're done.

Thanks for all the good advice! :)