Shielding effectiveness (SE) is a key parameter when considering microwave coaxial cable assemblies and connectors for applications with strict electromagnetic-compatibility (EMC) requirements. Measuring the shielding or screening (as the IEC prefers¹) effectiveness of microwave coaxial cables is not trivial, however, and requires a full understanding of the terminology and techniques practiced essentially by a distinct subset of the microwave industry concerned with EMC.

EMC parameters include emissions, immunity (susceptibility), and crosstalk. Emission defines the impact of the cable on the environment. Immunity shows the ability of the cable to meet specified performance in a given EMI environment. Finally, crosstalk is EMC interaction between the cables themselves. Although coaxial cable is a passive linear device and according to the reciprocity theorem, emission and immunity tests should lead to the same results, this doesn't always occur. A typical coaxial cable is not always a linear and, therefore, reciprocal component.

EMC parameters can be measured by many methods designed to detect low-level signals, although test results are often nonrepeatable. EMC test responses can depend on the type of test system chosen.

The most common shielding parameter is surface transfer impedance, Z_T, which was first introduced by Schelkunoff from Bell Labs in 1934. It can be defined as a ratio of induced series electromotive force inside the shield in the secondary (disturbing) circuit (V₂) to the disturbing circuit current (I₁) flowing in the shield in the primary (outside disturbing) circuit of an electrically short piece of cable.

Unlike most cable parameters that determine the signal propagation along the cable, Z_T characterizes the energy propagation across the cable through the shields. The importance of using an electrically short piece of cable is critical. For cable, the surface transfer impedance has a unit of Ω/unit length (typically mΩ/unit length):

Equation 1 is defined for the cable only. For connectors and electrically short cable assemblies, the surface transfer impedance is defined for the whole length of the assembly.

The surface transfer impedance applies to the SE against current. According to Ampere's Law, current is related to magnetic field. Therefore, the surface transfer impedance relates to magnetic of galvanic coupling. But capacitive coupling is also possible (for example, through the holes in cable braids). To include this coupling, there is also a definition of equivalent transfer impedance, which includes effects of galvanic, magnetic, and capacitive coupling. Thus, in this case the effective transfer impedance (Z_TE) can be defined as:

where:

Z_F = the capacitive transfer impedance.

It is important to note that Z_T depends only on the screening properties of the shield and doesn't depend on the nature of the outer circuit (in this case, the testing cavity surrounding the leakage source). Because of this, the surface transfer impedance is established as a primary screening parameter in the literature. The capacitive transfer impedance depends on the outer circuit geometry and permittivity (as any capacitance does). In practice, capacitive coupling should not be a concern for high-performance microwave cables, only for single-braided cables similar to RG316. According to ref. 3, capacitive coupling can be normalized in a way that Z_F will be invariant to the outer circuit under typical conditions. Figure 1 shows typical transfer impedance data for microwave cables. Because surface transfer impedance measurements require an electrically short device under test (DUT), they are not suitable for microwave transmission lines, but can be used for microwave connector measurements.

Screening/shielding attenuation is defined as the ratio of the maximum power in the secondary (outer) circuit to the power propagating to the primary (inner) circuit. Since shielding attenuation measurements don't have a requirement for electrically short DUTs, they can be used for electrically long objects such as microwave cables. Screening attenuation is actually defined for electrically long objects only and doesn't depend on the mechanical length. Shielding attenuation measurements depend not only on the screening properties of the DUT but also on the measurement system, such as the velocity and impedance differences between the test cavity and the DUT. According to ref. 7, shielding attenuation is a secondary screening parameter, with a great deal of uncertainty inherent in the interpretation of the test results.

There are two important additional functions: the transfer function and a summing function. The general coupling transfer function is defined as the square root of power measured related to the square root of power sent to the system:

The coupling transfer function for cable measurements is different for the near and far ends (T_n and T_f). Additionally, it is a complex number. Phase effects are expressed by the summing function S which has the form of (sin x)/x and it has some difference for the near and far end S_n and S_f (Fig. 2). For low frequencies, the summing function becomes equal to unity while for high frequencies, the envelope can be calculated as:

where the near and far end cutoff points are

The point of intersection between the asymptotic values is the so-called "cutoff frequency." This frequency gives the condition for electrically long samples. Parameters ε_r1 and ε_r2 are the relative dielectric permittivity of the inner and outer systems and l is the cable length.

There is some confusion in the definition for cutoff frequency. Traditionally, in transmission-line theory, the cutoff frequency is a frequency from which the excitation of the next higher-order mode of propagation is possible. For example, in a coaxial line this frequency is when the TE₁₁ mode can theoretically be excited along with a principal mode TEM. In the case of EMC measurements, the cutoff frequency is a frequency point where the summing function crosses the axis, and the DUT can be assumed to be electrically long (which has nothing to do with the traditional definition for cutoff frequency).

As a result, the cutoff frequency has a single definition for two different effects. This can be confusing since the high-frequency triaxial test setup has some issues with regard to both effects.

MIL-C-17 is the main military specification for the US coaxial-cable industry but it doesn't include RF screening requirements. MIL-T-81490 and MIL-C-87104 apply to special high-power coaxial assemblies for airborne applications, and both specifications have similar RF shielding test specifications and test methods. The test methods are based on a special triaxial cavity with the shorted both outer circuit ends (thus the outer coaxial system forms a coaxial resonator).

The mode-stirred method covered in MIL-STD-1344 (method 3008) is a connector test. The coaxial connector shielding covered by MIL-PRF-39012 includes a triaxial cavity method. There are some international specifications developed by the International Electrotechnical Commission, Technical Committee 46, Working Group 5. IEC TC46 WG5 has been working for over 30 years and is going to release some standards regarding to the coaxial-cable screening effectiveness. There is a Special International Committee on Radio Interference (CISPR) that is related to the IEC TC77. The European Standards (EN) were created by the European Committee for Electrotechnical Standardization (CENELEC). The Society of Cable Telecommunication Engineers (SCTE) also published its own documents. At present, most cable standards have been developed for low-frequency coaxial cables. Typical applications include a transmission line for a wireless base station or for a CATV system. Therefore, most of the test methods can be used for frequencies to 2 GHz. Until now, international shielding standards have had little influence on the US cable industry, but that may change in the future, most likely for companies that are involved in export or with manufacturing facilities overseas.