EMC and Circuit Protection
Two measurements of the transmission coefficient took place, one with the material in place on the Mini Box and one with nothing at all between the two antennas (IEEE-STD-299 open reference). Identifying the difference between the two transmission coefficient readings would determine the shielding effectiveness of the material.
Here again, the same test sample was positioned across the aperture at the front of the enclosure. One antenna, serving as the transmitting antenna, sat at the opposite end of the opening inside the enclosure. Another antenna, naturally functioning as the receiving antenna, sat outside of the Mini Box in close proximity to the material under test.
Fig. 2. Diagram showing various components of the Mini Box test set-up
The IEEE STD-299 ‘free space’ reference test method facilitates testing in the main chamber and Mini Box. In both cases, the transmit and receive antennas pointed directly at each other at the same distance apart as in final shielding effectiveness measurements.
After final testing, it was possible to calculate the shielding effectiveness based on the difference between the value obtained during the open reference measurement and the final measurement, expressed in decibels (dB).
Tried and tested
The results show impressive shielding effectiveness for all Premier conductive plastic UL-94 V-0 and Super Temperature (ST) grades, specifically: 71-99 dB (Premier A230-ST); 63-99 dB (Premier A230-FRHF); 71-104 dB (Premier A240-ST); and 63-100 dB (Premier A240-FRHF).
Fig. 3. Testing took place using the ‘free space’ reference test method
This type of small-scale IEEE-STD-299 test set-up evaluates shielding effectiveness at frequencies up to 115 GHz. Unfortunately, large test chambers used for shielding effectiveness tests are generally not suitable for such high frequencies because of equipment cable lengths and the use of standard equipment. In addition, due to the test fixture and sample size, transfer impedance testing techniques are typically not useful beyond 1-10 GHz.
The moulded Premier ‘load’ and ‘reference’ test samples measured 133 mm in diameter. An important benefit of this sample size is that the very same diameter is suitable for
www.cieonline.co.uk.
testing to ASTM D4935 and IEEE-STD-299 using either the main Parker Chomerics test chamber or the Mini Box. As a result, the collection of test data from 30 MHz to 110 GHz necessitates just one test sample. The ASTM D4935 tests utilized a coaxial fixture designed specifically to test homogeneous materials above a frequency range of 30 MHz to 1.5 GHz. In terms of configuration, a test sample sat between two halves of the test fixture. Using a conventional spectrum analyser, the first test took place with the ‘reference’ disk, while the second shielding effectiveness test used the ‘load’ test sample.
IEEE STD 299 tests
IEEE STD 299 testing utilised two different set-ups. First, the main chamber tested sample at frequencies from 800 MHz to 40 GHz. This chamber is a fully welded steel enclosure that measures 3.7 x 6 m in size with two sections and a common wall featuring an access panel upon which the test sample sits. One side of the test chamber holds the transmit test equipment and antenna, while the other side holds the receive equipment and antenna. Second, the Mini Box tested frequencies from 50 MHz to 110 GHz using a Keysight N5225A network analyser.
Sure enough, there are a few differences in the results, but the comparison is actually quite remarkable considering the differences in test techniques. It is possible to conclude that
conductive plastics for the 5G marketplace are highly effective solutions in EMC design. Dependent upon the specific frequency of the 5G application, shielding effectiveness spans a typical range from 40 to 100 dB. OEMs and end users can rely on this data as Parker Chomerics designs shielding effectiveness test fixtures and methods to mirror the conductive coating product used in real-life application. The company, which can call upon many years of experience, reports that end users always achieve similar shielding effectiveness results to that of the published test data.
www.parker.com/chomerics Components in Electronics September 2023 67
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