Circular Waveguide Antenna for 2.45 GHz / 802.11bg

We have been experimenting with waveguide antenna, made from old food cans, to massively extend the range of 802.11b wireless networks. All that was required was fitting, in the correct place, a driven element consisting of a short piece of copper wire soldered into the centre of an N-type connector.

This was evolved primarily since the Pringle's can antenna. The Pringle's can, being cardboard, does not last long in a storm, and it is very hard to affix connectors securely. The dipole-less "yagi" bit inside is fiddly to make, and initial tests show the waveguide cans to work better.

From studies of waveguide theory, which gets complicated, it seems that a waveguide antenna or "can-tenna" should have parallel sides, be a good conductor, preferably shiny, and the end needs to be be perpendicular to the sides. For 2.4 GHz the calculations indicate that the can should have a diameter between 70 mm (millimetres) and 100 mm. These are not a "brickwall" limits, but rather roll-off points. i.e. performance will diminish increasingly beyond these sizes.

From practical use we have found that strength is a good virtue, and a fitting plastic lid is almost a must for waterproofing. See appendix for a list of cans found suitable so far.

The ARRL (Amateur Radio Relay League) say that the required waveguide length is at least two guide wavelengths - The guide wavelength is the value of Lg (in the javascript calculator or tables of values below) and is dependent upon the diameter of the can. The smaller the diameter, the longer the guide wavelength. This suggests the larger acceptable diameters should be used so the can may be shorter. Also the larger the area of the mouth of the can, the more energy can be tranferred, so the greater the received and transmitted signal.
Construction

First we selected a can with a diameter of 96 mm. We calculated from this the value for 1/4Lg (a quarter of the standing wavelength inside the can), measured this far up from the bottom of the can and drilled a small pilot hole, then drilled the hole out large enough for a chassis mount N-type connector. It is not easy to find 16 mm drill bits in the UK, so we bought a 20 mm cone cutter. To the pin of the chassis mount N-type connector we soldered about 50 mm of 1.5 mm diameter stiff copper wire. This wire was then carefully cut to the value calculated for 1/4Lo. The edges of the N-type connector and the can around the hole were then abraded heavily with glass paper. The N-type connector assembly was then soldered in place to the can, on all four sides. It is important to get a good electrical connection between the N-type connector and the can. Also we have now sourced round N-type connectors which screw into the can (from rswww.com stock no. 112-0773) , and just requires a 16 mm hole drilling.

he cone cutter will cut a perfect hole if the 16 mm washer from the connector is placed over the tip before cutting. With the tools to hand, the whole process took ten minutes.

After a couple of years of experience and learning using these cantennas, it appears that it makes sense to drill a small hole in the can just behind the N-type connector. Thus any rain or condensation which finds its way into the can has an easy route out. The hole should not affect the performance of the antenna.


Mounting

This antenna has a beamwidth of around 30 degrees and needs aiming. Also the polarisation is important, that is whether the driven element inside is pointing skywards (vertical) or sideways (horizontal) - this needs to match the antenna it is communicating with. We have been mounting them around a standard 25 mm television pole with a U-bolt attached to an adjustable mounting from a television antenna shop - this allows adjustment in the horizontal and vertical plane. This has then had a short piece of stainless steel tubing clamped in, attached to the can with glue and gaffer tape, or cable ties. None of these are a very good solution.

It was nescessary to aim the antenna before tightening the bolts fully, and check the polarisation was correct. This involved having the N-type cable attached to a PCMCIA (Personal Computer Memory Card International Association) card in a PC (Personal Computer) or laptop with someone down below monitoring the signal, although I have taken a laptop up on the roof - it is not adviseable. An ideal solution would be a hendheld PC with 802.11b card, and pigtail....

Note - always point a can-tenna away from you, and do not look down into the can whilst running. This recommendation is based on caution rather than known danger. Human eyes have very little cooling, and hence are the parts of the body most likely to absorb, and fail to dissipate, excess microwave energy.
We maximised the signal by aiming the can-tenna roughly by landmarks, and the use of a compass, and then moved it bit by bit sideways and then up and down until the maximum signal to noise, or link quality was achieved. This required the use of a wireless tool for the PC. I used Wavemon for GNU/Linux, but most wireless card drivers have some form of link quality read-out. Depending on where the monitoring PC was, we needed extra people, or walkie talkies, mobile phone etc. to relay the signal strength after each adjustment. Once the maximum signal was achieved, we clamped up the bolts tight, and celebrated a job well done.