EMI Shielding Calculator

MAJR Products EMI Shielding Calculator is a FREE tool to help you determine shielding effectiveness of printed circuit board shields and other enclosures.

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The program provides an estimate of the resultant shielding effectiveness due to ventilation holes needed for heat management or folded metal that form slots on the sides of a board level shield.

Graphing of the resultant calculated data is plotted as shielding effectiveness (dB) vs. Frequency (MHz).  The graph is located at the right side of the data entry spreadsheet section.  As for shielding materials, this program may approximate shielding effectiveness and be utilized for other flat shields such as screens and perforated panels.

For tooling / manufacturing cost and design time it is advantageous to determine the minimum shielding effectiveness of the shielding enclosure prior to fabrication.

This program is used as an internal design tool by engineers at MAJR Products Corporation as a service to our customers.  This service provides an estimate of shielding effectiveness of a new or existing shield design(s) without costly pre-testing.  Error may always be present when calculating shielding effectiveness (near field, edge effects, reflection, or waveguide effects are not addressed); therefore final radiated emission testing prior to new product release or redesign of an existing product is necessary for valid EMC compliance.

The static layout of the Aperture Attenuation Modeling Program is shown in Appendix A.

Note:  The program will exhibit a negative shielding value for a shielding effectiveness level when less than 0dB attenuation is calculated.  This computed response is highlighted and identified in red within the cell location and should be evaluated as a 0dB attenuation level for that particular frequency.


Program Layout

The shaded cells are for data entry by the user.  The non-shaded cells are protected and are used by the program to locate data for calculations.

Program Instructions

Three steps are stated at the bottom of the spreadsheet to instruct a correct data input format as follows:

  1. Enter from 1 (minimum), to 10 (maximum), frequencies of interest in a MHz format.

Because of the calculation used for wavelength, the frequency(s) need to be in a MHz format; as an example: 5 kHz shall be entered as .005; 5 GHz shall be entered as 5000.

Figure 1 (Data entry of Frequency)

The program is set-up to utilize the same frequencies for the holes and slots calculating sections.  If part of a shield is sectioned and utilizes different frequencies, a separate spreadsheet program will need to be run and treated as two separate applications.  When entering frequency in the shaded cells of column 1, the same frequencies will automatically fill in the frequency column for slots.  This automatic entry is to eliminate repeated number entry by the user, and to establish equal comparison of data between holes and slots.

 

  1. Enter the number of holes and/or slots within the calculated l/2 (1/2 wavelength) circular area for each frequency entered.

When the frequency is entered in the shaded cells of column 1, the program calculates the wavelength (l) and divides the number by 2.  With knowing l/2 (inches) from column 3, the number of apertures within a circular area of l/2 shall be entered buy the user in the shaded cells of column 4.

Figure 2 (Data entry of Aperture(s) within l/2)

  

  1. Enter the hole and/or slot maximum size.

When the maximum aperture size (hole diameter or slot length) is entered in column 5 (first cell only), the program places the entry into the corresponding cells below the first cell.  This automatic entry is to eliminate repeated number entry by the user.

Figure 3 (Data entry of maximum aperture size)

If multiple aperture sizes are within l/2 a separate spreadsheet program will need to be run for each aperture size.  This will allow the user to identify the shielding effectiveness for each hole and/or slot size.  The shielding effectiveness is determined by the size as well as the quantity of apertures within the stated frequency(s) half wavelength area.  If this multiple size aperture scenario exists, the data to be concerned with is the lowest shielding effectiveness level as seen in the graph of the spreadsheet program.  If the shielding application only has holes, or only has slots, the hole or slot section that is not needed may be voided by entering “0” in column 5 (aperture size).


Data Entry Table / Calculation for Shielding Effectiveness

The shielding effectiveness calculations for both holes and slots are performed by the program in columns 6 and 7.  The resultant shielding effectiveness is placed in column 8 as shown in Figure #4.  

Figure 4 (Program calculations for shielding effectiveness)

Notes presented at the bottom of the program section of the spreadsheet specify program attributes such as cell entry and recommended aperture characteristics per frequency.


Shielding Effectiveness Graph

The graph includes a plot of holes and/or slots, which shows the shielding effectiveness (dB) vs. frequency (MHz).

The graph is the modeled (calculated) shielding effectiveness data from column 8 on a Y-axis, and frequency from column 1 on an X-axis.  If both holes and slots are used, the graph identifies them as separate curves.  The lowest curve on the graph is of interest because this indicates the minimum shielding effectiveness level of the printed circuit board shield or enclosure shield.

As with any modeling program, when testing components within a test fixture, there will be deviations between actual test data and theoretical test results.  The Aperture Attenuation Modeling Program incorporates a “Test Fixture Factor” entry area for holes and slot; this number as shown in Figure #5 is used to adjust the multiplier of the Log calculation function use by the program.  As an example: For Holes, 40 is the theoretical multiplier and for slots, 20 is the theoretical multiplier.  In actual testing, one may find that the test data reflects 10 dB to 15 dB higher on average over a specific frequency range vs. the theoretical modeled data.  In this case, increasing the multiplier will increase the shielding result; thus harmonizing to “real” tested data.  As expected, in reverse, a decrease in the multiplier will decrease the shielding effectiveness model results.  If the modeling program is adjusted to reflect an actual test fixture, with all other major parameters constant, the modeling program will become a customized program for a specific product and/or test lab set-up.

Figure 5 (Test Fixture input multiplier)

See Appendix A for an example of the Aperture Attenuation Modeling Program.

Appendix A