Electromagnetic Interference Shielding With Perforated Metals

Background

Perforated metal can be used to provide EMI/RFI shielding. To determine which perforated pattern is best suited for this function, 16 samples of perforated metal were tested at 9 frequency levels and their performance evaluated. Information about the samples, the frequencies used, and the results of the tests are provided in the tables that follow. The results are expressed in dB of Shielding Attenuation and in % of Attenuation.

Shielding Test Results

The test results show that Shielding Effectiveness of 40 dB provides 99.000% attenuation of the electromagnetic (EMI/RFI) radiation while a Shielding Effectiveness of 92 dB, the highest Shielding Effectiveness found in the tests, provides 99.997% attenuation. A Shielding Effectiveness of 40dB is the targeted minimum in most applications. Very effective shielding was provided by most samples up to frequencies of 7 GHz. Above that frequency, some of the samples dropped below 99.000% effective, but most samples stayed comfortably above 95% effective even at the highest frequency level of 10 GHz. The obvious conclusion to be reached is that there are many perforated patterns that designers can choose from to meet their design requirements.

The largest single source of leakage is along contact surfaces between two parts. If a tightly sealed electrical connection is not made, the leakage through the interface can be greater than through the structure.

Table 1. (Part A) EMI/RFI shielding for various perforated metal materials.

Hole Diameter (inches)	60° Centre Spacing	Open Area (%)	Thickness (inches)	Material	Sample Size (inches)
.040	.055	48.00	.022	Alum	8x8
.062	.093	40.00	.027	C.R. Steel	24x24
.062	.125	23.00	.025	Alum	8x8
.072	.100	47.00	.038	C.R. Steel	8x8
.075	.125	32.70	.049	C.R. Steel	8x8
.078	.109	46.10	.027	C.R. Steel	24x24
.079	.115	42.80	.036	C.R. Steel	8x8
.100	.187	25.90	.040	Alum	8x8
.125	.187	40.30	.030	Stainless	24x24
.125	.187	40.30	.030	Alum	24x24
.125	.187	40.30	.060	Alum	24x24
.125	.187	40.30	.125	Alum	24x24
.156	.187	63.00	.057	C.R. Steel	24x24
.156	.250	35.40	.057	C.R. Steel	24x24
.187	.250	51.00	.027	C.R. Steel	24x24
.187	.250	51.00	.057	C.R. Steel	24x24

Table 1. (Part B) EMI/RFI shielding for various perforated metal materials.

Shielding Effectiveness – Attenuation in dB
MHz			GHz
30	100	300	1	2	4	6	8	10
70	70	80	56	55	48	57	35	48
61	67	66	56	45	48	46	44	33
92	84	90	68	65	60	67	35	45
64	70	90	62	55	52	40	33	37
72	70	90	72	68	63	68	43	48
66	75	72	63	55	54	45	41	39
60	64	88	66	60	56	62	33	40
66	70	77	69	62	62	67	41	48
58	62	57	49	40	37	34	31	26
62	63	61	51	44	38	34	31	28
71	72	69	58	49	48	39	35	34
85	84	84	73	68	84	73	63	51
57	58	59	48	40	35	31	29	26
59	64	61	53	43	43	41	36	31
52	52	49	37	31	24	21	19	18
55	56	55	46	36	33	30	27	24

Table 1. (Part C) EMI/RFI shielding for various perforated metal materials.

Shielding Effectiveness - % Attenuation
MHz			GHz
30	100	300	1	2	4	6	8	10
99.97	99.97	99.99	99.84	99.82	99.60	99.86	98.22	99.60
99.91	99.96	99.95	99.84	99.44	99.60	99.50	99.37	97.76
100.0	99.99	99.97	99.96	99.94	99.90	99.96	98.22	99.44
99.94	99.97	99.97	99.92	99.82	99.75	99.00	97.76	98.59
99.98	99.97	99.97	99.98	99.96	99.93	99.96	99.29	99.60
99.95	99.98	99.98	99.93	99.82	99.80	99.44	99.11	98.88
99.90	99.94	100.0	99.95	99.90	99.84	99.92	97.76	99.00
99.95	99.97	99.99	99.92	99.92	99.92	99.96	99.11	99.60
99.87	99.92	99.86	99.65	99.00	98.59	98.01	97.18	94.99
99.92	99.93	99.91	99.72	99.37	98.74	98.01	97.18	96.02
99.97	99.98	99.86	99.87	99.65	99.60	98.88	98.22	98.01
99.99	99.99	99.99	99.98	99.96	99.99	99.98	99.93	99.72
99.86	99.87	99.89	99.60	99.00	98.22	97.16	96.45	94.99
99.89	99.94	99.91	99.78	99.29	99.29	99.11	98.42	97.18
99.75	99.75	99.65	99.59	97.18	93.69	91.09	88.78	88.78
99.82	99.84	99.82	99.50	99.42	97.76	96.84	95.53	93.69