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. They can be expanded aluminum or sheet, 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.

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 15dBi gain over 360 degrees of beamwidth.

Horizontal Polarization 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 will 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 are cut in the back face of the waveguide. When looking straight at the face of the waveguide you will be able to see straight 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 40's, and as our simulation and measurement technologies have become more accurate it is apparent that the slotted waveguide designs we have 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 15dBi of gain, radiated in a uniformly omnidirectional manner. The large number of slots makes it easier to dissipate the energy from the waveguide. Like 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.