Simulating Antenna Crosstalk on an Airplane’s Fuselage

Antenna crosstalk, or co-site interference, is problematic when multiple antennas are used on a single large platform. In this example, the interference between two identical antennas at VHF frequency is studied with an S-parameter analysis of different configurations of a receiving antenna installed on an airplane fuselage. The 2D and 3D far-field radiation patterns of a transmitting antenna are computed and the lit and shadow areas on the airplane surface are also visualized.

Figure 1: An airplane with ~20 m length of fuselage is simply depicted as an all-metallic structure except for the antenna enclosure. The surrounding air domain and perfectly matched layers are not shown.

Model Definition

The airplane is composed of two parts: a simplified metallic body and two antennas on its fuselage. All metal surfaces are modeled as perfect electric conductor (PEC). The PEC condition is automatically applied to all exterior boundaries. Removing the airplane body causes its surfaces to become effectively exterior and the PEC condition is applied by default. The antenna is made out of very thin metal strips inserted inside a dielectric (εr = 4.3) block. The antenna is miniaturized with a meander line design, which decreases the antenna input impedance. To match the initial low input impedance with regard to conventional 50 Ω, a folded monopole antenna (effectively a folded dipole) on a large ground plane method is used.

The entire airplane is enclosed by a spherical air domain which is finished with perfectly matched layers on the outermost parts. This mimics the antenna testing in infinite free space without causing unwanted reflection from the outer walls.

On each antenna, a lumped port is assigned on the gap between the metallic meander line and the airplane’s fuselage. S-parameters are calculated from two lumped ports that show the antenna matching properties as well as the amount of interference in the given configuration.

The receiving antenna is located at three difference places on the bottom side of the fuselage, and this location variation is modeled using a parametric sweep.

Results and Discussion

Figure 2 shows the E-field norm distribution in dB scale on a vertical cut plane at each location of the receiving antenna. Regardless of the location, the antenna on the bottom side of the fuselage is reacting to the field from the transmitting antenna on the top of the fuselage, so there is no complete shadow region with any of these three locations.

To identify lit and shadow areas, the E-field norm is visualized on the airplane surface in Figure 3. By adjusting the display color range, the difference between the two categories is emphasized. The areas around turbines, rear wings, and some regions of the bottom side of the main wings and fuselage are relatively less affected by the transmitting antenna.

See Figure 4 where the E-field norm is plotted in dB scale while the receiving antenna on the fuselage bottom is moved from the rear to the front side. From the XY view, the shadow area is visible, located at the rear end of the fuselage bottom.

The amount of interference can be also quantitatively described in terms of S-parameters (Table 1) and the computed S21 gives a more clear clue about where to install the second antenna to minimize the crosstalk to the first antenna.

Figure 2: E-field norm plot in dB scale on the yz-plane with the receiving antenna at locations on the fuselage bottom from the rear to front side.

Figure 3: E-field norm on the airplane surface. The visualization color range is adjusted to emphasize the lit and shadow regions due to the transmitting antenna.

Table 1: S-parameters.
Position	Front	Middle	Rear
S11	< -10 dB	< -10 dB	< -10 dB
S21	-20.5 dB	-23.9 dB	-21.2 dB

Figure 4: E-field norm plot in dB scale from the XY view with the receiving antenna on the fuselage bottom from the rear to front side. The shadow area is located on the rear side.

RF_Module/EMI_EMC_Applications/airplane_antenna_crosstalk

Modeling Instructions

From the menu, choose .

New

In the window, click

Model Wizard

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In the tree, select .

Click .

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In the tree, select .

Click

Study 1

Step 1: Frequency Domain

In the window, under click .

In the window for , locate the section.

In the text field, type 30[MHz].

Global Definitions

Parameters 1

In the window, under click .

In the window for , locate the section.

In the table, enter the following settings:


Name	Expression	Value	Description
a_y	-5[m]	-5 m	Second antenna location

Geometry 1

Import 1 (imp1)

In the toolbar, click

In the window for , locate the section.

Click

Browse to the model’s Application Libraries folder and double-click the file airplane_antenna_crosstalk_body.mphbin.

Click

Import 2 (imp2)

In the toolbar, click

In the window for , locate the section.

Click

Browse to the model’s Application Libraries folder and double-click the file airplane_antenna_crosstalk_radiator.mphbin.

Click

Click the

button in the toolbar.

Mirror 1 (mir1)

In the toolbar, click

and choose .

Select the object only.

In the window for , locate the section.

Select the check box.

Move 1 (mov1)

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and choose .

Select the object only.

In the window for , locate the section.

In the text field, type a_y.

Sphere 1 (sph1)

In the toolbar, click

In the window for , locate the section.

In the text field, type 13.

Click to expand the section. In the table, enter the following settings:


Layer name	Thickness (m)
Layer 1	c_const/30[MHz]/10

A perfectly matched layer (PML) will be configured in this layer. A PML with a thickness of approximately 0.1 wavelengths works well for an incident wave that is normal to the surface.

Difference 1 (dif1)

In the toolbar, click

and choose .

Select the objects , , and only.

In the window for , locate the section.

Find the subsection. Click to select the

toggle button.

Select the object only.

Click

By removing the airplane body from the model domain, the perfect electric conductor (PEC) boundary condition is applied automatically on all surfaces of the airplane.

Definitions

Perfectly Matched Layer 1 (pml1)

In the toolbar, click

Select Domains 1–4 and 8–11 only.

In the window for , locate the section.

From the list, choose .

Electromagnetic Waves, Frequency Domain (emw)

Perfect Electric Conductor 2

In the window, under right-click and choose the boundary condition .

Click the

button in the toolbar.

In the window for , locate the section.

Click

In the dialog box, type 140, 148 in the text field.

Click .

Lumped Port 1

In the toolbar, click

and choose .

Select Boundary 149 only.

For the first port, wave excitation is by default.

Lumped Port 2

In the toolbar, click

and choose .

Select Boundary 141 only.

Far-Field Domain 1

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and choose .

Add Material

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to open the window.

Go to the window.

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Click in the window toolbar.

In the toolbar, click

to close the window.

Materials

Material 2 (mat2)

In the window, under right-click and choose .

Select Domains 6 and 7 only.

In the window for , locate the section.

In the table, enter the following settings:


Property	Variable	Value	Unit	Property group
Relative permittivity	epsilonr_iso ; epsilonrii = epsilonr_iso, epsilonrij = 0	4.3	1	Basic
Relative permeability	mur_iso ; murii = mur_iso, murij = 0	1	1	Basic
Electrical conductivity	sigma_iso ; sigmaii = sigma_iso, sigmaij = 0	0	S/m	Basic