Sonnet User's Guide - Sonnet Software

Chapter 4 Metalization and Dielectric Layer Loss
there are two special metal types available in Sonnet for the Top and Bottom metals:
Free Space and Waveguide Load. See Chapter 19, “Antennas and Radiation”
for a discussion of how and why you would use these types.
To assign a metal type to the Box Top or Bottom metal, select Circuit ⇒ Box Settings
from the project editor main menu. In the Box Settings dialog box which appears
on your display, select the desired metal type for the top metal from the Top
Metal drop list and the desired metal type for the bottom metal from the Bottom
Metal drop list.
Dielectric Layer Loss
The dielectric layer loss calculations in Sonnet are virtually exact, given the substrate
really has a frequency independent conductivity, and/or loss tangent. Our
web site has a lossy conductivity benchmark you can perform on any electromagnetic
solver (or measurement system). See Benchmarking on the Products section
of our web site, www.sonnetsoftware.com.
The dielectric loss is calculated in Sonnet at the beginning stages of the analysis.
The method Sonnet uses starts with the calculation of a sum of waveguide modes.
The exact solution requires an infinite sum of modes, but Sonnet truncates this
sum to some reasonable value (the truncation has never been a source of error).
So, for each mode, if there is a lossy dielectric, the calculation involves complex
numbers instead of just real numbers. This is NOT a discretized function - it is a
continuous function. Therefore, the dielectric loss calculation can be thought of as
exact (only limited to the precision of the machine).
A more reasonable source of error is in the assumption that the conductivity is
constant with frequency. All real dielectrics have frequency-dependant loss (some
smaller than others). Sonnet supplies you with two parameters (Loss Tan and Diel
Cond) to control this frequency dependency. The equation Sonnet uses to calculate
the TOTAL loss is given in “Dielectric Layer Loss,” page 59. There are some
dielectrics with more complicated frequency dependencies, but this equation
works for most dielectrics. Of course, this requires that you know the frequency
dependency of your dielectric. If you have a method of measuring the loss as a
function of frequency (or published data which you can trust), and if it is constant
over your range of frequencies, then dielectric loss is probably not a source of error.
Be careful, however, of published loss data. Verify that the data is valid over
your frequency range.
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