Electric fields with different frequencies, total intensity? What happens when electric fields with different frequencies are combined? Is it possible to calculate the intensity of the total electric field?
For a project, I need to simulate a brain treated with microwaves from antennas operating at different frequencies. I would like to calculate the specific absorption rate (SAR), but for that I need the magnitude of the total electric field.
 A: The intensity of the total electric field will just be the sum of the two separate intensities, because the interference term will oscillate at a very fast frequency (namely $\omega_1 - \omega_2$) and its mean value will thus be zero.
See this awnser for more details.
A: Electric fields obey the superposition principle.  So let's call the two fields $\mathbf{E}_{1}\left( \omega_{1} \right)$ and $\mathbf{E}_{2}\left( \omega_{2} \right)$.  Then the total field, $\mathbf{E}_{t}$, is given by:
$$
\mathbf{E}_{t} = \mathbf{E}_{1} + \mathbf{E}_{2} \tag{1}
$$
Note that the field intensity or energy density is given by:
$$
W = \frac{ \varepsilon_{o} \ \left( \mathbf{E} \cdot \mathbf{E} \right) }{ 2 } \tag{2}
$$
where $\varepsilon_{o}$ is the permittivity of free space.  You can see that $W_{t} \neq W_{1} + W_{2}$ by noticing that:
$$
\begin{align}
\left( \mathbf{E}_{t} \cdot \mathbf{E}_{t} \right) & = \left( \mathbf{E}_{1} + \mathbf{E}_{2} \right) \cdot \left( \mathbf{E}_{1} + \mathbf{E}_{2} \right) \tag{3a} \\
& = \left( \mathbf{E}_{1} \cdot \mathbf{E}_{1} \right) + \left( \mathbf{E}_{2} \cdot \mathbf{E}_{2} \right) + 2 \left( \mathbf{E}_{1} \cdot \mathbf{E}_{2} \right) \tag{3b} \\
& \neq \left( \mathbf{E}_{1} \cdot \mathbf{E}_{1} \right) + \left( \mathbf{E}_{2} \cdot \mathbf{E}_{2} \right)  \tag{3c}
\end{align}
$$

Is it possible to calculate the intensity of the total electric field?

Yes, add the two input fields together then calculate the intensity using Equation 2 above.

I would like to calculate the specific absorption rate (SAR), but for that I need the magnitude of the total electric field.

Calculate $\mathbf{E}_{t}$ using Equation 1 above and then find its magnitude, which is the total electric field magnitude.
