Referring to Figure 7, note that the additional shielding applied to the connector area improved the 6 to 12 GHz dip yet did not affect the downward slope of the curve, which is likely an artifact of the cable’s 3-mil-thick, served flat-wire outer conductor construction (see Figure 5b), wrapped overall with a thin, metalized, polymer tape. The served flat-wire configuration has continuous helical gaps running the entire length of the cable between each served flat-wire segment. These gaps are openings in the shield that act as electromagnetic radiators. To somewhat remedy this situation, a thin, metalized, polymer tape is applied over the serve, to cover the gaps and improve conductivity between each adjacent flat-wire segment. Because the primary shielding mechanism at low frequency is reflection, this works reasonably well at low frequencies. At higher frequencies, the primary shielding mechanism transitions to absorption, which is a function of the product σrµr, where σr is the material’s conductivity relative to copper and µr is the material’s magnetic permeability relative to copper.2 The polymer tape’s conductivity and magnetic permeability are low compared to copper, and the tape itself is not in intimate contact with the flat wire, which further increases shield resistance. The thinness of the tape, on the order of 1.5 mils, compounds this, as well as the shielding effectiveness being directly related to shield thickness. The result of this construction, shown in the lower curves of Figure 6, is a constant reduction in shielding effectiveness above 1 GHz, falling below the MIL-T-81490A (AS) standard around 7 GHz.

The shielding performance of Gore’s cable assemblies in Figure 6 is relatively flat and well above the 90 dB limit through 18 GHz. This performance can be attributed to the connector design, connector termination techniques and cable construction. The cable assembly uses a durable, helically-wrapped, flat-wire outer conductor; the flat-wire is silver-plated copper, with a thickness of 3 mils. The helical wrap ensures excellent mechanical and electrical contact between the overlapping wraps. The high conductivity of silver-over-copper provides good reflectivity at low frequencies, and the shield’s overall thickness with the overlapping wraps results in excellent absorption at high frequencies.

CONCLUSION

This article addressed the shielding effectiveness of cable assemblies to provide users with a better understanding of construction techniques and how they impact microwave cable assembly performance, using airframe assemblies as an example. The shielding effectiveness of microwave cable assemblies is often ignored, since adequate performance is assumed and rarely verified.

When selecting a microwave cable assembly for airframe use, ask the supplier:

  • Has the cable been expressly designed for airframe applications?
  • Can it withstand the rigors of airframe installation without the RF performance being compromised?
  • Will it meet military shielding effectiveness standards before and after installation?

Microwave airframe cable is a crucial component of many military systems and can shape system performance. Because of this, cable selection should be given careful and thoughtful consideration.

References

  1. S. A. Schelkunoff, “The Electromagnetic Theory of Coaxial Transmission Lines and Cylindrical Shields,” Bell System Technical Journal, 1934, pp. 532-579.
  2. D. D. L. Chung, “Electromagnetic Interference Shielding Effectiveness of Carbon Materials,” Permagon Press, July 2000.