Hack 90. Build a Slotted Waveguide Antenna

Make a high-gain, horizontally polarized omni or unidirectional antenna that looks cool.

Unlike wideband antennas such as the Biquad [Hack #88], slotted waveguides are resonant antennas and have a relatively narrow operating frequency range. The designs described in this hack have an adequate bandwidth for any 802.11b/g wireless LAN, but they have been carefully designed and must be equally constructed with care.

The major attraction of a slotted waveguide design is its simplicity. Once you have built the first one, it is very simple to build many more. The gain varies little across the 2.4 GHz spectrum, dropping a little bit at the extreme ends. Figure 6-18 shows a finished 8-element directional.

Figure 6-18. An 8-element slotted waveguide

6.9.1. How a Waveguide Antenna Works

A waveguide is a low-loss transmission line. It allows us to propagate signals to a number of smaller antennas (slots). The signal is coupled into the waveguide with a simple coaxial probe; as it travels along the guide, it traverses the slots. Each of these slots allows a little of the energy to radiate. The slots are in a linear array pattern, and the total of all the radiated signals adds up to a significant power gain over a small range of angles close to the horizon.

In other words, the waveguide antenna transmits almost all of its energy at the horizon, usually exactly where we want it to go. Its exceptional directivity in the elevation plane gives it quite a high-power gain. Additionally, unlike vertical colinear antennas, the slotted waveguide transmits its energy using horizontal polarization, which is the best type for distance transmission.

6.9.2. Unidirectional Waveguide Antennas

This hack describes two unidirectional designs. The first has 8 slots and is about 30 inches long. The second has 16 slots and is about 5 feet long. Simple to construct, the 8-slot has been provided as a good starting point for an antenna novice. I built my 8-slot prototype using only hand tools.

The 16-slot design has been made to radiate over a wider beamwidth by the addition of wings to each side of the guide, flush with the front (slotted) surface. It is, of necessity, higher Q, and the higher gain is obtained over a narrower bandwidth. The wings can be expanded aluminum or sheet metal and should extend 9.6 inches beyond the sides of the guide. They act as a ground plane for the slots. Do not change this dimension; it is two electrical wavelengths.

6.9.3. Omnidirectional Slotted Waveguide Antennas

The slotted waveguide has achieved most of its success when used in an omnidirectional role. It is the simplest way to get a real 15-dBi gain over 360 degrees of beamwidth.

Horizontal polarization [Hack #100] in a wide area network can often double the number of users that can interconnect without interference. When using horizontally polarized Biquads or patch antennas (provided that they have been tested for good cross-polarization performance) at the client site, these omnis will be 20 dB stronger than the signal from a similar vertical collinear. Conversely, vertically polarized receiver antennas prefer the vertically polarized colinear over the slotted waveguide by a similar amount. Transmission on an immediately adjacent channel (say, channels 5 or 7), normally not permissible because of interference, is now possible. So, a judicious intermingling of horizontally polarized clients can talk with a horizontal central station on the same or adjacent channels that other clients are using with vertical polarization.

To make the unidrectional antenna radiate over the entire 360 degrees of azimuth, a second set of slots is cut in the back face of the waveguide. When looking straight at the face of the waveguide, you will be able to see clearly through both slots.

Unfortunately, unless a lot of slots are used, the antenna becomes more like a bidirectional radiator, rather than an omnidirectional. This antenna was invented in the 1940s, and as our simulation and measurement technologies have become more accurate, it is apparent that the slotted waveguide designs we used in the past are far from optimum. The most common defect is a tilt in the radiation pattern at the extreme ends of the frequency range. This occurs when the wavelength of the signal traveling down the guide differs from the slot spacing.

My current favorite uses 32 slots to get 15 dBi of gain, radiated in a uniformly omnidirectional manner. The large number of slots makes it easier to dissipate the energy from the waveguide. As with the 16-slot unidirectional, two sets of wings (one set at each slot surface) are required to get equal radiation of energy over a full 360 degrees. Note that a higher Q is necessary to get all the slots illuminated evenly.

Note that the gain-versus-frequency curve peaks at 2,440 KHz, and it radiates well over all 14 possible 802.11b/g channels.

6.9.4. Highly Directional Slotted Waveguide Antennas

Sometimes, it is useful to have a highly directional antenna. For example, when installing a point-to-point link between two buildings, it is not desirable to have a wide angle of coverage. Any interference from other 802.11b devices (or microwave ovens) that are in the radiation zone will affect your link integrity.

The ideal antenna for such a situation is a dish, such as Primestar's. When using a Biquad feed [Hack #88], it is possible to reject interference outside the dish's primary 5-degree cone by 30 dB or more.

But if a 16-slot waveguide antenna is turned to a horizontal position, parallel with the ground, it radiates vertical polarization. Its directivity in this plane is extremely good. So, if you don't have a dish handy, using a pair of these slotted waveguides, parallel to the ground, will work well.

6.9.5. Construction Details for the 8-Slot Unidirectional Antenna

The base extrusion for all of my slotted waveguides is 4" x 2" O.D. rectangular aluminum tubing with approximately 1/8-inch thick walls. Inside dimensions are 3.756 x 1.756 inches (95.4 mm x 44.6 mm). These inside dimensions are critical, and must be within +/- 0.040 inches or +/-1mm if the antenna center frequency is to be +/- 1 channel. I cut the end inserts from a 5/16" x 1 3/4" flat aluminum bar extrusion.

Waveguide antennas are fairly critical in their constructional dimensions, and are easiest to make with a CNC milling machine. I have computed these designs so that they would be easy to replicate; if you are plus or minus 1 mm, the design will work fine, but you must be careful. I used a jig, a hand operated DeWalt heavy duty cut-out tool, a 1/4-inch router bit, and lots of water to machine the slots. This worked fine (even if it was a little tedious). Really, folksplus or minus 1 mm will not kill your antenna!

6.9.6. Coupling the Signal into the Waveguide

As mentioned previously, we are propagating the WLAN signal down a waveguide and then using it to excite a number of elemental radiators, or slots. The first task is to get the signal into the waveguide with a feed probe. First, obtain a suitable N connector. Take a piece of 20 mm x 40 mm copper or brass shim, and form it into the shape of a cone. Use Figure 6-19 as a template.

Figure 6-19. A template for the feed cone

Solder it to the inner conductor of your Type N connector. Its length should be 20 mm, and its largest diameter should be about 15 mm. When soldered to the N connector, it should protrude exactly into the center of the waveguide and no further. Figure 6-20 illustrates the finished feed cone.

Figure 6-20. The completed feed cone

Both ends of the waveguide need to be terminated for RF. The easiest way I found to do this was to cut 3.75-inch pieces of 5/16" x 1.75" aluminum bar stock. I do not recommend that you make the end plugs sloppily, but good electrical contact is not required.

Remember not to have any screws protruding into the waveguide for more than 1/8 inch, especially the screws holding down the N connector. They will affect performance.

6.9.7. 8+8 Slot Omnidirectional Antenna

The total length of air inside the 8+8 slot omnidirectional waveguide, from end to end, is 765 mm. Mount the N connector in the center of the widest side: 27.5 mm from one end (the base) of the airspace in the waveguide, and offset 10 mm from the center line of the face, in the direction as the offset of the first slot. The wavelength of the radiation passing down the waveguide is longer than a wavelength in free air (it is 161 mm in this design).

The first slot is centered 1.0 wavelength from the base, at a maximum of the H field in the waveguide. This length is 161 mm from the base of the airspace. It is the H component of the field that induces the energy into the slots and makes them radiate. Each slot is 59 mm long, and extends outwards from the centerline for a width of 17 mm. The waveguide excites each edge of the slot, depending on its position across the wide surface of the guide. If it straddled the exact center, each edge of the slot would be excited in antiphase (the waves cancel each other out), and there would be no radiation. So as we offset the edges of the slots, the more the offset, the greater the energy that is dissipated into each slot. The electrical length of each slot should be 59 mm. Do not allow too much kerf at the ends (it should be 2 mm radius max). I recommend finishing the cut with a 1/8-inch router bit (or a file). Or you might use the 1/8 bit in a CNC machine to cut the entire rectangular outline. Remember, even though these slots are arranged vertically, they radiate horizontal polarization.

For the 8+8 slot omnidirectional, slots 2 through 8 are centered at distances of 241, 322, 403, 483, 564, 644, and 724 mm from the base of the airspace, staggered across the centerline. It doesn't matter which direction the first one is cut, but they must alternate. The end plate should create a 765 mm airspace. Looking straight on at the front of the guide, you can see right through both the front and back slots.

6.9.8. 8-Slot Unidirectional Antenna

The total length of air inside the 8-slot unidirectional, from end to end, is 760 mm. Mount the N connector in the center of the widest side, 25 mm from the base of the airspace in the waveguide. The wavelength of the radiation passing down the waveguide is 160 mm in this design. The first slot is centered 1.0 wavelength from the base, at a maximum of the H field in the waveguide. This length is 160 mm from the base of the airspace. Each slot is 58 mm long, and extends outwards from the centerline for a width of 20 mm.

The waveguide excites each edge of the slot depending on its position across the wide surface of the guide. If it straddled the exact center, each edge of the slot would be excited in antiphase and there would be no radiation. So, as we offset the edges of the slots; the more the offset, the greater the energy that is dissipated into each slot. The electrical length of each slot should be 59 mm. Do not allow too much kerf at the ends. Remember, even though these slots are arranged vertically, they radiate horizontal polarization.

Slots 2 through 8 are centered at distances of 240, 320, 400, 480, 560, 640, and 720 mm from the base of the airspace, staggered across the centerline. It doesn't matter which direction the first one is cut, but they must alternate. The end plate should be to create a 760 mm airspace.

6.9.9. Construction Details for 16- and 16+16-Slot Design

The correct wavelength for these designs is 161 mm. The gain for the 16-slot unidirectional is 15 dBi17 dBi, verified on my test range, across the whole band. On the range, the 16 slotter gives slightly higher gain than my Hyperlink Technologies model 2419G Mesh Parabolic, which is rated at 19.1 dBi gain.

The slot width for the 16 slotter is 15 mm, and it is 12 mm for the 32 slotter; otherwise, the key dimensions are the same.

Trevor Marshall