Circular waveguides and the can-antenna for 802.11

Circular waveguides used as antenna.

TE11 mode
D = 0.586*lambda_o

TMo1 mode
D = 0.766*lambda_o

TE21 mode
D=0.97*lambda_o

Below cutoff, the signal is attenuated.

Best to operate in TE11 mode. If other modes are excited, the antenna pattern will became skewed and the maximum will not be on the boresite. TM01 modes seem ok though. But for TE21 modes and higher, the pattern is skewed. The pattern information came from an online microwave antenna book.

The length of the can matters somewhat too. Ideally the aperture should be at a standing wave maximum point. So the length ought to be 3/4 LG (guide wavelength, not free space wavelength)