Building the Rectangular Waveguide

We used 100mm x 50mm rectangular hollow box section aluminium with 3mm thick walls (off cuts from the 8+8 antenna). This gives us an internal size of 94mm x 44mm. We chose to build the antenna for 2.442GHz, or channel 7, as this is close to the center of the 13 channels available to us (US designs use channel 6 (2.437Ghz), as they have 11 channels). I followed Rob Clark's design. I have included a key lengths calculator in the form below. Nb.Circular waveguides (cans) use a different formulae (see Can Waveguide).

Frequency
GHz
Wide Sidemm (Inside Measurement)
Short Sidemm (Inside Measurement)
 
Free Space
Wavelength
mm
Waveguide
Wavelength
mm
Max
Frequency
GHz
Min
Frequency
GHz
Cut Off
Frequency
GHz
Probe Lengthmm
Open End to Probemm (Open End to probe center)
Probe to Reflectormm (Probe center to Reflector)
Inside Lengthmm (Reflector to Opening)
Outside Lengthmm (Length including Mount)

The final product only needs to be accurate to about 1mm as this will only shift the frequency within the range of the 802.11 spectrum. A smaller cavity length will shift the frequency response to that slot to a higher channel. A longer slot cavity length will shift it down.

Methods and madness

I measured to .1mm using vernier callipers, on the assumption that errors in cutting might bring that closer to the 1mm error mark. There are only two to make, and you can't go too wrong, the probe (N-Socket) position and the placement of the reflector (end plate).

Misc Notes

Not very exciting, but works well out to ~2Km. (with a cone probe it was working well at 3km) Shows the reflector mounting
N-Socket with 1/4 wavelength copper wire
(including the length of the bit of the
N-Socket it is soldered into).

This becomes the probe for the antenna.

Shows the probe mounted.
A conical feed is a much better probe. It is made from a 20mm x 40mm piece of copper and rolled into a cone. It is slightly harder to make, but it is worthwhile.

Cut the red area from the copper and roll into a cone. Solder onto an N-Connector so that the cones base will be exactly half way across the cavity.

Nb.The 10mm hole in the waveguide for the Nconnector will be too small for a cone probe. You will need to drill it out to 16mm. Do this in small increments, widening the hole one or two drill bit sizes at a time. It will make a much neater hole.