JAWUG

Waveguide Theory for normal people

Ok, this is a work in progress. I will be adding sections as I get round to them. I am planning on trying to explain why rather than just tell you to chop holes in a piece of pipe with some arbitrary values with a promise it works nice. Hardly any wifi sites go into any detail about how these things work, and many of the sites that do the information is somewhat dodgy.

• What is a waveguide
  

A Waveguide is basically just a hollow conductive tube - it can be round, rectangular or elliptical. It is used to carry high frequency radio signals, usually anywhere from 1GHz to about 100GHz (microwave). Since we are interested in slotted waveguides, the fact that round and elliptical waveguides exist is about all the information you will get about them here.

• What are waveguides used for?
  

Waveguides are not usually used for antennas, most waveguides are used as transmission lines for microwave frequencies to avoid transmission losses which would happen if normal conductors such as coaxial cable were used. The reason for this is microwave is more about electromagnetic fields than about voltage and currents typical of conventional theory. So, a waveguide when used with microwave is the least loss transmission line. Additionally a waveguide can carry more power than coaxial cable.

• Spatial relationships

The physical size of the waveguide is important. Not any pipe will do, but a certain amount of variation is possible. For a rectangular waveguide a few requirements must be met - The width of the waveguide must be at least 1/2 the wavelength. For WiFi this means about 6.5cm. The height of the guide needs to be less than the width, so square waveguide are not cool. The reason or this is that we want the waveguide to operate in dominant mode. The height should be kept less than 1/2 of a wavelength, so anything under about 6cm is good. The guide can be made as long as you want. As you might have guessed the wide size is what dictates the frequency range the waveguide will operate at. The ideal wide size is apparently 0.7 of a wavelength in order to ensure dominant mode of operation. The shorter length is less important, it determines the amount of power that the guide can carry - since wifi stuff is very low power this is pretty unimportant to us. The other important dimension is how long the guide is, for slotted waveguide antennas the ends are usually closed with a flat piece metal, this will cause the signal to bounce back down the waveguide when it hits the top, so it is important to dimension the waveguide such that the bounced waves don't mess up the ones still going up. Usually the waveguide will be a multiple of the 1/4 wavelength size.

• Physical properties

The waveguide should be made of conductive material. Usually metal such as copper or aluminium. The inside of the waveguide should be smooth and free from any surface anomalies such as bumps, scratches, cracks and the like. A good rule of thumb is that if it is nice and shiny it will work well.

• Wavelengths
  

WiFi equipment works between 2.412GHz and 2.484GHz, this means the wavelength in air is about 12.4cm to 12cm. You will notice I said in air - the wavelength in a confined space such as in a waveguide changes depending on the size of the container. I will come back to this at a later stage.

• Waveguide probes

In order to get a signal into the waveguide a probe must be used, the most common is a simple 1/4 wavelength antenna, usually just a short piece of copper wire about 3cm long. The probe is placed in the center of the wide side and 1/4 of a wavelength from the bottom of the guide. You may have seen other types of probes, the most common being a conical shaped probe. A conical shaped probe is better because it has a larger surface area in which to generate the E field into the guide. A larger probe has two additional properties; it increases the power that can be transmitted, and also the bandwidth. So using a conical probe is recommended. The other types of probes you probably won't come across are loops, these are used to generate a H field which in turn will induce the E field, loop probes are usually placed at the edge of the waveguide. The one other way of getting a signal into the guide is through a slot/hole in the wall of the guide through which a signal can enter.

• What is the E field?
  

The E field is the is an electric field which is created by a potential difference between two points, in a waveguide this is the walls.

• What is the H field?
  

The H field is the magnetic field which is created by current flowing in the conductive parts of the guide, again the walls.

• Energy propergation within the guide
  

Although the pictures look rather static, the E fields and H fields are constantly in flux. The signal travels along the waveguide, and as it goes the E fields and H fields follow, since the E fields and H fields actually are the signal, this is somewhat logical.

• Slots
  

Yes, finally we get to the slots, but to understand how the slots work you really had to know about those damn fields.