Assuming the two antennas have ports you can look at the transmission between the two ports.

If the two antennas are in the same model, the the S parameter, e.g. S21, will give you the transmission.

If they are different models and in each other's far-field then you will need to use the far-field gains calculated for each one, in the Frijs equatuion.

If they are in separate models and in each other's near-field then you need to do a lot of learning before you can do what you want.

Mohamed K. Emara Malcolm White Thank you

Malcolm White Dear sir, how can i put two models in each other's farfield in CST? and if i want to put two antennas in the same model, one for transmitting and one for receiving, how to set the ports of the two antennas?

Malcolm White oh one more question, is it possible to detect time signals in farfield region by using CST? thank you in advance( You may not know that you have helped me answer my questions here before. thank you very much

To be in each other's far fields the antennas must be further apart than twice the largest width or height of either antenna, squared, divided by half a wavelength.

=2D^2/wavelength.

Sometimes this is possible in one simulation, but usually it makes the problem too big.

Just put a port on each antenna. When you run the simulation there is a box to choose which port will be excited, but it makes no difference because S21 = S12

CST help says

Frequency/Animation (Time/Animation) frame

This frame is only available for broadband farfields.

Frequency / Time: Sets the desired plot frequency or the plot time. The time setting does not include the time delay due to the finite distance to the farfield origin.

Set Time:Toggles between frequency domain farfield and transient field display.

Start / Stop: Click here to start or stop the animation.

Settings…: Opens a dialog box where you can specify field animation settings like step width.

It looks like you should be able to do it, but I haven't ever done it.

When you set the far-field monitor, you can set a Transient broadband monitor instead of a frequency monitor

Malcolm White thank you, I'll try that!

Malcolm White how are you today, sir? I've set the transient broadband monitor and the 3D result looks exactly the same as a frequency monitor shows, not a time domain signal, how to solve this?

Read the help. It says this. Read more help and try some things if this is not good enough.

Field Monitor

📷 Simulation: Monitors > Field Monitor

This dialog box gives you the opportunity to define field monitors that you might need to obtain additional information on the electromagnetic field distribution inside your structure. It is possible to define frequency as well as time monitors. After a calculation, you can observe your field monitors by selecting them in the navigation tree.

Labeling frame

Name: Displays the name for the field monitor either as a user input or automatically generated.

Automatic labeling: This check button enables or disables the automatic labeling for the defined field monitor. The automatically generated label consists of  the selected monitor type, including either the specified frequency or time sample settings of the frequency or time monitor, respectively.

Type frame

E-Field: The electric field vectors will be stored.

H-Field and Surface current: Selecting this monitor creates two monitor entries in the result tree. The first one shows the magnetic field vectors. The second one (labeled with "Surface Current") uses the magnetic fields on the surfaces of  "PEC" or "lossy material" solids to calculate the surface currents there.

Surface current (TLM only): Just the surface currents will be calculated. Only supported by TLM Solver.

Powerflow: This monitor stores  the Poynting vector of the electromagnetic field. The monitor represents the maximum value (peak value) of the power flow at every spatial point, encountered within one period of time. Therefore, this type of monitor is time independent.

Current density: If there are electric losses inside of the calculation domain, the currents inside of these materials are stored in this type of monitor. Please note that the time monitors only consider currents generated by electric conductive material model whereas the frequency monitors can take into account any kind of electric dispersive materials.

Power loss density/SAR: If there are electric or magnetic losses inside of the calculation domain, the sum of the electric and magnetic power dissipated inside these materials will be recorded.

Please note that the time monitors only consider losses arising from electric and magnetic conductive material model whereas the frequency monitors can take into account any kind of dispersive materials.

Concerning the Specific Absorption Rate (SAR) please refer to the SAR calculation overview page for detailed information.

Electric energy density / Magnetic energy density: Choose one of these monitors if you want to store the electric/magnetic energy density throughout the monitor volume. These monitors represent the maximum values (peak values) of the energy density within one period of time. Therefore they are time independent.

Please note that the time monitors only consider energy density stored in lossless material or in electric/magnetic conductive material model whereas the frequency monitors can take into account any kind of dispersive materials.

Farfield/RCS: For RCS calculations use a plane wave excitation. See Farfield Overview for detailed information.

Note: Farfield monitors must not have two parallel magnetic/electric boundaries, one of two parallel boundaries must be of the type open (PML).

Field source: This monitor records the tangential fields on a prescribed box surface. The recorded data can be used as a near field source in other projects.

The data is saved in the results folder as "name of the source.fsm". This monitor is available for the hexahedral transient solver, the tetrahedral frequency domain solver, and the integral equation solver. It records the data at the specified frequencies. It is recommended to use the proprietary FSM nearfield data format in software products of CST. In order to use the recorded fields outside of CST products it is recommended to use the NFS nearfield scan data exchange format. See here for details on how to export FSM nearfield data as NFS nearfield scan data exchange format.

Space charge: This monitor records the charge density in non-conductive regions.

Particle current density: This monitor records the charged particle current density.

Specification frame

Here, the field monitor domain can be specified, selecting between frequency and time.

Frequency: The monitor records the chosen field type only for a specific frequency. If the transient solver is used, this is done by use of a DFT. Information about a possible normalization can be obtained in the section Spectral results from transient solvers.

Frequency: Enter a valid expression to specify the recording frequency of the frequency field monitor. It should be within the frequency range entered in the Frequency Range Settings dialog box, otherwise this monitor will be ignored during the calculation.

If you are defining field monitors (Except of type field source) and not editing one, you can define multiple monitors by choosing one of the following options:

Step width (Linear): Choose the width between the samples in the range of Fmin and Fmax.

Samples (Linear): Choose the number of equidistant samples to define between Fmin and Fmax.

Samples (Log.): Choose the number of samples to define in the range of Fmin and Fmax. The width between the samples varies logarithmically.

Freq. samples: For field source monitors, you can choose a number of equidistant frequency samples between Fmin and Fmax where frequency domain field sources are monitored.

Fmin/Fmax: This shows the minimum and maximum frequencies available for the monitors. It can be adjusted for the field source monitors.

Time: The monitor records the chosen field type at several equidistant time samples. Note that  this feature is not available for farfield monitors for frequency domain solvers.

Start time: Enter here the start time for the time monitoring.

Step width: Enter here the desired step width for the time monitoring. Together with start and end time this value determines the full number of recorded time samples. Note that time monitors possibly need a great amount of disk memory if the step width is chosen too small.

End time: Selecting this check box offers the possibility to define a specific endtime for the recording. If it is not selected, the recording will continue up to the end of the calculation.

Average rep. period: Activating this check button allows the calculation of averaged values over time for a power loss time domain monitor. By default the monitor is integrated over time and averaged by its number of sample steps. However, it is also possible to define a repetition period which is then used for normalization of the averaged power loss monitor. In any case, the duration and sample step of the integration is defined by setting the start and end time as well as the step width.

Please note that this monitor type is only available for the transient solver and can be further used for an averaged SAR calculation as well as an imported source field for the thermal solver of CST MPHYSICS®  STUDIO.

Transient Broadband: If a farfield monitor was selected in the type frame a broadband farfield can be calculated during the transient solver run. The broadband farfield monitor is based on an expansion of the farfield in terms of spherical waves. The origin of the expansion is the center of the bounding box. It allows to obtain the farfield at any frequency in the specified range as well as at any time of the simulation. Frequency domain and time domain results can be accessed by selecting plots of a broadband farfield monitor and by the plot properties dialog.

Freq. samples: Choose a number of equidistant frequency samples between Fmin and Fmax where frequency domain farfields are calculated. Results for any frequencies in between are interpolated to obtain broadband information. For a more accurate interpolation over the frequency band increase this value e.g. to "31". An odd number assures that the result at mid frequency is not obtained by interpolation.

Accuracy: Defines the desired accuracy of the farfield. Together with the Fmax and the size of the structure it determines the number of modes required to represent the farfield.

Note that leaving out higher order terms by choosing a lower accuracy is equivalent to low pass filtering the farfield solution. This saves memory and computation time. However, the farfield result has usually less detail.

Transient farfields: This check button activates the additional calculation of transient farfield information which can be displayed afterwards for a certain time in the post processing. Note that in order to accurately obtain time domain farfields the computational effort is higher than for broadband farfields only.