<math> \mathbf{H^{ff}(r)} = \frac{1}{\eta_0} \mathbf{ \hat{k} \times E^{ff}(r)} </math>
Far fields are typically computed in the spherical coordinate system as functions of the elevation and azimuth observation angles θ and φ. Only far-zone electric fields are normally considered. When your physical structure is excited using a lumped source, a waveguide source, a distributed source, a short dipole source, or an array of such sources, the far fields represent the radiation pattern of your source(s) in the far zone. In that case, you need to define a '''Radiation Pattern - Far Field Observable''' for your project. When your physical structure is illuminated by a plane wave source or a Gaussian beam source, the far fields represent the scattered fields. In the case of a plane source, you can compute the radar cross section (RCS) of your target structure. In that case, you need to define an '''RCS - Far Field Observable''' for your project.
[[Image:Info_icon.png|40px]] Click here to learn more about '''[[Maxwell%27s_Equations#Definition_of_the_Far_Radiation_Zone | Computing the Far Fields & Radiation Characteristics]]'''.
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[[Image:Info_icon.png|40px]] Click here to learn more about '''[[Data_Visualization_and_Processing#Far-Field_Observables | Far Field Observables]]'''.
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[[Image:Info_icon.png|40px]] Click here to learn more about '''[[Data_Visualization_and_Processing#Computing_Radar_Cross_Section | Computing Radar Cross Section]]'''.
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In the FDTD method, the far fields are calculated using a near-field-to-far-field transformation of the field quantities on a given closed surface. EM.Tempo uses rectangular boxes to define these closed surfaces. You can use EM.Tempo's default radiation box or define your own custom box. Normally, the radiation box must enclose the entire FDTD structure. In this case, the calculated radiation pattern corresponds to the entire radiating structure. Alternatively, you can define a custom radiation box that may contain only parts of a structure, which results in a partial radiation pattern.
[[Image:FDTD_FF2.png|thumb|400px|EM.Tempo's Far Field Acceleration dialog.]]
The default radiation box is placed at an offset of 0.1λ<sub>0</sub> from the largest bounding box of your physical structure. You can change the offset value from the "Far Field Acceleration" dialog, which can be accessed by clicking the {{key|Acceleration...}} button of EM.Tempo's Radiation Pattern dialog. Calculation of far-field characteristics at high angular resolutions can be a very time consuming computational task. You can accelerate this process by setting a lower '''Max. Far Field Sampling Rate''' from the same dialog. The default sampling rate is 30 samples per wavelength. A low sampling rate will under-sample the mesh grid points on the radiation box.
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[[Image:Info_icon.png|40px]] Click here to learn more about '''[[Maxwell%27s_Equations#Definition_of_the_Far_Radiation_Zone | Computing the Far Fields & Radiation Characteristics]]'''.
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[[Image:Info_icon.png|40px]] Click here to learn more about '''[[Data_Visualization_and_Processing#Far-Field_Observables | Far Field Observables]]'''.
[[Image:Info_icon.png|40px]] Click here to learn more about '''[[Data_Visualization_and_Processing#Visualizing_3D_Radiation_Patterns | Visualizing 3D Radiation Patterns]]'''.
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[[Image:Info_icon.png|40px]] Click here to learn more about '''[[Data_Visualization_and_Processing#Computing_Radar_Cross_Section | Computing Radar Cross Section]]'''.
[[Image:Info_icon.png|40px]] Click here to learn more about '''[[Data_Visualization_and_Processing#2D_Radiation_and_RCS_Graphs | Plotting 2D Radiation & RCS Graphs]]'''.