Building Slot Waveguide

We used 100mm x 50mm rectangular hollow box section aluminium with 3mm thick walls. 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). The internal width (the wide side), determines the operating frequency. The internal height (the narrow side) determines the impedance. We used Rob Clark's 5 wavelength design over Trevor Marshall's 4.75 wavelength design simply because we could cut the 5 wavelength design down to get the 4.75 wavelength one if our first try didn't work. Trevor's design also has wider slots and our slots could be made wider if we messed them up too much. We used Paul Wade's spreadsheet (Slotantenna.xls)to calculate the slot sizes and placement (The "Improved from Elliot" figures with a 2% reduction in slot length for square ends (see his text)). I have included a key lengths calculator in the form below.

Top of Form

Frequency	b/g Chnl 1 b/g Chnl 2 b/g Chnl 3 b/g Chnl 4 b/g Chnl 5 b/g Chnl 6 b/g Chnl 7 b/g Chnl 8 b/g Chnl 9 b/g Chnl 10 b/g Chnl 11 b/g Chnl 12 b/g Chnl 13 b/g Chnl 14 -NZ Indoor- a Chnl 34 a Chnl 36 a Chnl 38 a Chnl 40 a Chnl 42 (mid-range) a Chnl 44 a Chnl 46 a Chnl 48 -NZ outdoor low- a Chnl 50 a Chnl 52 a Chnl 56 a Chnl 57 (mid-range) a Chnl 58 a Chnl 60 a Chnl 64 -NZ Outdoor High- a Chnl 149 a Chnl 152 a Chnl 153 a Chnl 157 (mid-range) a Chnl 160 a Chnl 161 a Chnl 165	GHz
Wide Side		mm (Inside Measurement)
Short Side		mm (Inside Measurement)
Material Thickness		mm
NSlots		One Sided
Free Space Wavelength		mm
Waveguide Wavelength		mm
Probe Length		mm
Slot Length		mm
Slot width		mm
Slot Offset		mm (From Center to Slot Center)
Slot Spacing		mm (centers)
Slot to Top		mm (center to top Plate)
Slot to Probe		mm (center to probe center)
Probe to Bottom		mm (Probe center to Bottom Plate)
Inside Length		mm (Top to Bottom Plate)
Outside Length		mm (Top to Bottom)

Bottom of Form

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 slot length will shift the frequency response of that slot to a higher channel. A longer slot length will shift it down. Changes in the slot spacing affects the vertical signal pattern. A smaller spacing (higher channel) results in the lower channels having a slightly downward vertical pattern. A larger spacing (A lower channel) will result in higher channels having a slightly upward tilted vertical pattern. Slot width variation affect the frequency response. The distance of each slot from the center varies with design. I don't know what this affects. One design deliberately changes this distance for each slot so as to reduce the side lobes of the vertical radiation pattern.

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. I did in fact manage to get most slots cut to a few tenths of a mm in length. I used a drill press to make holes at either end of each slot and a hand held router with built on guide to cut between them. I then used a file to square the ends of each slot and to clean the edges up.

Misc Notes

• 100 is added to the bottom (alternately 50mm top & bottom) for mounting (included in form as the outside length).
• I used 3mm aluminium because the N-Socket is 3mm deep. The thickness also alters the slot calculation, but I have no idea how (I'm told the slots would be smaller in length, but I don't know what the exact realtionship is). People have successfully used much thinner material.
• The slots alternate left and right of center, but it doesn't matter which way the first one goes.
• We made the top cap from part of a 44mm offcut and filed it so it was a tight fit (I'm told this isn't that crtical).
• The other side of the offcut was used for bottom plate (We made it an L shape to help in fixing it) and used the callipers to measure how far down the tube it needed to go.
• The probe is a 2mm diameter copper wire soldered into an N-Socket and cut to 31mm (1/4 free space wavelength, including the protruding bit of the N-Socket).
• The N-Socket is held down with nylon screws. I threaded the mounting holes. If you use metal screws, use stainless steal ones and keep them short so they don't protrude into the cavity.
• Water in the antenna, or spider and insects will be a problem. One suggestion is to make the antenna upside down and make small holes in the top plate to let the water out. Another is to cover the holes with thin microwave transparent tape, such as Kapton (source 3M and expensive) or Mylar (3M and others, but a common polyester tape with an acrylic adhesive. Sometimes called Yellow Thermoset tape). Some people have used other tapes. They should be as thin as possible and not heat up in the microwave.

Step by Step pictorial guide

A view from the Front.

Slotted Waveguide Build

• Cut the 100x50mm (3mm thick) rectangular hollow box section aluminum to 5 waveguide wavelengths, with an additional 100mm added for mounting (About 910mm. see double sided waveguide calculator or Single sided Waeguide calculator).
• Mark (with a square and metal scribe) the horizontal center line of each slot and from this, the top and bottom lines of each slot. (Nb. These are all offset 3mm from the top to allow for the insertion of the top reflector.) I used the square to run these lines right around all faces. This ensured the front and rear slots line up.
• Mark the horizontal center line for the N-Socket hole.
• Mark a horizontal line at the top of the bottom reflector.
• Mark the vertical center line of each slot. Nb.They alternate left and right of the center.
• Mark the centers for drilling holes at either end of the slot, 3.25mm in from the top and bottom line.
• Punch each of these marks to help guide the drill.
• Punch the center marks for the N-Socket

• Use a small drill bit to drill a guide hole at each punch point. This helps when drilling the larger holes.

• Drill a 6.5mm hole (slot width), widening each of the guide holes.

• Drill a 10mm hole for the N-Socket, then use a 16mm drill to widen the mouth. The N-Sockets have a conical end. Use an N-Socket to ensure it fits.
• Placing the N-Socket in the hole, mark out the location of the mounting holes.
• Punch the centers of these holes
• I drilled mine to 3mm diameter, so I could thread the holes. I used 4mm nylon bolts, so there is no metal sticking into the cavity.

• Use a router (with a tungsten tipped tool) to join the top and bottom hole of each slot. I only had a 1/4inch cutting tool. This is slightly less than 6.5mm, so I did this in two runs. Nb. Do this slowly so the tool doesn't have to cut too quickly. Also be careful to file away sharp edges at the end of each slot cut.

• Use a file to square the ends.
• Also use the file to clean up the edges. The inside shouldn't be rough.
• We are aiming at 1mm maximum error in length for each slot. Having marked the top and bottom line, it is easy to see where we need file to.

• Using a 44mm wide off cut of the box section, cut an L piece for the bottom reflector and a flat plate for the top reflector.

• Measure half way down the small side of the L section, mark out and drill two mounting holes.
• Using the marked position of the top of the bottom reflector as a reference, do exactly the same to one of the short sides of the main antenna.
• The top reflector is mounted by drilling small holes and using pins to hold it in place (I used small nails).

• The bottom reflector can be riveted in place. It doesn't have to be a perfect fit. Small gaps around the edges will let moisture out.

Nb.The top pictures are from a single sided version, which is mounted top and bottom.
The bottom two are the double sided one. It is mounted on the bottom only, having two brackets for stability.

• The top reflector of the Single Sided one was riveted. I used an L shape, the same as the bottom reflector, cutting the mounting side across the diagonal to fit the cut off top (done to stop water buildup).

For the Double sided version, the Top Reflector was held in place with small panel pins. I drilled 1mm holes through the front and back of the waveguide and into the edge of the top plate and hammered in the panel pins.

I siliconed the edges to ensure the antenna didn't fill up with water.

• The slots were covered by thin polyester tape (microwave friendly and only a micron thick). This stops water getting in and spiders and other insects making the antenna their home. The tape I used is sometimes called "yellow thermoset tape". Test the tape in a microwave oven. It shouldn't heat up (don't blame me if it melts down, catches fire and sticks to the insdes of the oven).

• N-Socket has a 2mm diameter copper wire soldered to it, and is cut so that this is 30.7mm long (including the small bit of the N-Connector it is soldered to).

• A conical feed is a much better probe than the straight wire 1/4 wavelength dipole. 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 along the joint and solder onto an N-Connector so that the cones base will be exactly half way across the cavity. In my case, this was 22mm (including the bit of the Nconnector it was soldered to). I had to cut down the center of the Nconnector slightly.

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.