So what's the difference?
This antenna uses round holes rather than slots. They are much easier to make! A normal slotted waveguide antenna has the slots in the broad face of the waveguide. The broad face (of a vertical waveguide) has both vertical and horizontal currents, so a vertical slot has to be used if you want to radiate only horizontal polarisation. The short face carries only horizontal currents so the shape of the holes does not matter much.
I searched all the antenna books I could find, but no one mentioned an antenna like this. But after I convinced myself that it should work, I made one, and it did work.
What are the Disadvantages?
The holes are not resonant at 10 GHz, so they do not radiate as readily as resonant slots. The result is a high "Q" sharply tuned antenna.
The radiating elements on opposite sides of the waveguide are further apart, so the omnidirectionality is not as good.
The vertical spacing of the elements is greater than one wavelength in free space which leads to less than optimum gain for the length of the antenna, and larger sidelobes in the vertical plane.
Dimensions relate to WG16 for 10.368 GHz.
The waveguide must be exactly a multiple of one half guide wavelength long between short circuits at the operating frequency.
The holes are 3/8" (9.5mm) diameter (largest possible)and spaced one guide wavelength apart, starting one quarter guide wavelength from the short circuit.
Holes at the same height on opposite faces will give opposite contributions in the directions of the broad face, resulting in a null in that direction. To get the side lobes, as in the pattern shown, the holes on opposite faces are offset by one half wavelength vertically.
A tuning screw is required to set the resonant frequency. The antenna is fed at the centre by a small probe in the centre of the broad face, rather like any other co-axial to waveguide transition.