EMI, EMC, and Shielding Advanced EMC Topics Informational

What is the effect of cable shield termination technique on the shielding effectiveness at high frequencies?

The cable shield termination technique has a dramatic effect on the shielding effectiveness at high frequencies because the manner in which the cable shield connects to the equipment enclosure determines whether the shield current flows through a low-impedance path (maintaining SE) or through a high-impedance pigtail that acts as an antenna (radiating the noise). The key termination methods are: 360-degree (circumferential) termination (the cable shield connects to the enclosure through a full 360-degree contact with the connector backshell or a conductive gland; this provides the lowest impedance path and highest SE: > 40-60 dB up to several GHz; this is the recommended method for all applications above 10 MHz), pigtail termination (the cable shield is stripped back and a short wire connects the shield braid to a ground point on the enclosure; the pigtail wire has inductance, approximately 10 nH/cm, which creates a high impedance at high frequencies: a 5 cm pigtail has approximately 50 nH, which is 300 ohms at 1 GHz; SE is degraded by 20-40 dB above 100 MHz compared to 360-degree termination; pigtail terminations should never be used above 10 MHz), and drain wire termination (a drain wire sewn into the shielded cable is connected to a PCB ground pad; similar degradation to pigtail; acceptable only below 1 MHz). The SE degradation from a pigtail termination is approximately: delta_SE = 20 log(1 + j omega L_pigtail / Z_transfer), where L_pigtail is the pigtail inductance and Z_transfer is the cable's transfer impedance.

Category: EMI, EMC, and Shielding

Updated: April 2026

Product Tie-In: Shielding, Gaskets, Absorbers, Filters

Cable Shield Termination for High-Frequency Performance

Cable shield termination is one of the most common errors in EMC design. A perfectly shielded cable can lose 30-40 dB of shielding effectiveness from a single pigtail termination. Understanding and implementing proper shield terminations is essential for meeting radiated emission limits.

Termination Methods

360-degree backshell (best): The cable shield braid is clamped around the full circumference by a conductive connector backshell. The current flows uniformly around the connector without creating any loop or antenna. Used with: MIL-DTL-38999 connectors, shielded D-sub connectors with backshells, and BNC/SMA/N-type RF connectors (which inherently provide 360-degree termination)
Conductive cable gland (good): A gland fitting that clamps the cable shield braid to the enclosure wall with 360-degree contact. Less expensive than backshell connectors. EMC performance: similar to backshell termination. Used in industrial equipment where cost is a factor
Pigtail termination (poor): A wire from the braid to a ground screw. The wire radiates. SE degrades above approximately 10 MHz. Only acceptable for low-frequency applications (power cables, audio) or as a temporary fix

Cable Shield Termination Parameters

Pigtail inductance: L = 10 nH/cm × length [nH]
Pigtail impedance at frequency f: Z = 2pi f L
5 cm pigtail at 100 MHz: Z = 2pi × 100e6 × 50e-9 = 31 ohms
5 cm pigtail at 1 GHz: Z = 314 ohms (very high, SE severely degraded)
SE degradation: delta_SE ≈ 20 log(Z_pigtail / Z_transfer_cable)

Common Questions

Frequently Asked Questions

Can I use a pigtail if it is very short?

A shorter pigtail is better than a longer one, but even a 1 cm pigtail has approximately 10 nH of inductance, which is 6 ohms at 100 MHz and 63 ohms at 1 GHz. This still degrades SE by 10-20 dB at GHz frequencies compared to a 360-degree termination. For frequencies above 100 MHz: any pigtail is too long. For frequencies below 10 MHz: a 1-2 cm pigtail is acceptable because the inductance is negligible at low frequencies.

How do I terminate a shielded cable to a PCB?

Best methods: use a shielded connector that makes 360-degree contact with the PCB ground plane (e.g., a shielded RJ-45, USB, or HDMI connector where the shell contacts the PCB ground pads around the entire connector footprint). Second best: use a metal cable clamp that presses the cable shield braid against a large ground pad on the PCB near the connector. Worst: solder a pigtail wire from the braid to a single ground pad.

Does double shielding help if the termination is poor?

No. A double-shielded cable with pigtail terminations performs no better than a single-shielded cable with pigtail terminations, because the leakage path is at the termination, not through the cable shield braid. The pigtail inductance bypasses both shield layers equally. Double shielding only helps when both shield layers are properly terminated with 360-degree connections.

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