Sonnet User's Guide - Sonnet Software

Sonnet User’s Guide
First Condition: Make both of the lateral substrate dimensions greater than one
or two wavelengths.
When using the Open Waveguide Simulator, we view the sidewalls of the shielding
box as forming a waveguide whose tube extends in the vertical direction, propagating
energy from the antenna toward the “Termination” as shown on page 275.
Radiation is then approximated as a sum of many waveguide modes. If the tube is
too small, there are few, if any, propagating modes, violating the First Condition.
There is an easily made mistake when modeling radiation from small discontinuities.
Discontinuities are usually small with respect to wavelength. For a discontinuity
analysis, the sidewalls are usually placed one or two substrate thicknesses
from the discontinuity. In this case, the substrate dimensions are unlikely to meet
the First Condition. If the sidewalls form below a cut-off waveguide, there is no
radiation.
Second Condition: Make sure the sidewalls are far enough from the radiating
structure that the sidewalls have no affect.
Another way to look at this condition is to consider the image of the structure (discontinuity
or antenna) created by the sidewall. Position the sidewall so that the image
it forms has no significant coupling with the desired structure.
Usually two to three wavelengths from the sidewall is sufficient for discontinuities.
For single patch antennas, one to three wavelengths is suggested. Requirements
for specific structures can easily be greater than these guidelines. If the First
Condition requires a larger substrate dimension than the Second Condition, it is
very important that the larger dimension is used.
If you are using the far field viewer, the larger the box the better. The far field
viewer assumes that S-parameters from em are from a perfect open environment.
If some of the power is reflected due to a box that is too small, the input power
calculated by the far field viewer will be slightly incorrect. The far field viewer
then calculates antenna efficiencies greater then 100%. If this occurs, the box size
should be increased.
Third Condition: Place the top cover outside the fringing fields (i.e., near field)
of the radiating structure, preferably a half wavelength.
If this condition is violated, the resistive top cover becomes involved in the reactive
fringing fields which form the near field of the radiator. This changes what
would have been reactive input impedance into resistive input impedance, overestimating
the radiation loss.
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