Shielding Effectiveness in RF Cable Assemblies Explained

Published on 7/27/2026 • Updated on 8/1/2026

The outer conductor of a coaxial cable has two jobs, and they are the same job seen from two sides. It has to keep the RF signal inside the cable so it does not leak out and radiate. And it has to keep external RF energy from getting in and corrupting the signal. Both jobs matter, and how well a cable does them is captured in a single number: shielding effectiveness, measured in decibels.

At sub-GHz frequencies you can get away with a lot. At microwave and mmWave, you cannot. A cable with mediocre shielding will show up as a noise floor that is a few dB higher than it should be, as channel-to-channel crosstalk in a multi-radio system, as PIM products on cellular uplinks, and as radiated emissions that fail EMC testing. Every one of those problems has been chased by an engineer who assumed the interconnect was clean.

This guide covers what shielding effectiveness actually is, how the four cable families compare, and how to pick a shield architecture that matches your system.

What Shielding Effectiveness Really Means

Shielding effectiveness (SE) is the ratio of RF energy hitting one side of the shield to the energy that makes it through to the other side, expressed in dB. If a shield lets one-millionth of the incident energy pass, that is 60 dB of shielding. One-billionth is 90 dB. Every extra 10 dB means ten times better isolation.

The number goes both directions. The shield that keeps your RF signal from leaking outward is the same shield that keeps external interference from leaking inward. So when you specify SE, you are simultaneously specifying two things: how much your cable can radiate before it becomes an EMC problem, and how much external EMI the cable can absorb before it becomes a noise-floor problem.

Why It Matters at High Frequencies

At low frequencies, most shields look pretty solid. A basic braid on an RG-58 gives you 55 to 60 dB, and for a lot of RF work that is fine. The trouble starts as frequency climbs and the physics changes.

Two things happen. First, the skin effect drives the RF current into a very thin layer on the inside of the outer conductor. That layer becomes electrically important, and any interruption in it (a gap in the braid weave, an oxidized strand, a connector interface with a poor ground) starts to look like an antenna. Second, the wavelength of the signal gets smaller. At 10 GHz the wavelength in air is 30 mm; at 40 GHz it is 7.5 mm. Braid gaps that were electrically invisible at 100 MHz are efficient radiators at 40 GHz.

This is why the same cable, tested at different frequencies, will show different SE numbers. Almost every SE spec on a datasheet will be for a specific frequency or frequency band. Compare like with like.

Reading Shielding Effectiveness in dB

A working sense of what the numbers actually mean helps a lot when reading datasheets.

Note that 20 dB is not a small difference. Going from 60 dB to 80 dB means the shield now leaks 100 times less energy. Going to 100 dB is 10,000 times less. That is why the choice of shield architecture is not incremental. It is a set of very different tiers.

The Four Cable Families Compared

Almost every coaxial cable you will meet fits into one of four construction families. Each has a fundamentally different shield architecture and a fundamentally different SE ceiling.

RG-Series Cables

The RG-series names go back to the U.S. military RG (Radio Guide) numbering system: RG-58, RG-174, RG-316, RG-142, RG-213, and the rest. Modern RG cables use a single or double braided shield of tinned or silver-plated copper, typically at 85 to 95 percent coverage. That is the crux of the shielding limitation. There are always gaps in a braid weave, even a good one. Every gap is a small leak, and the leaks add up.

RG cables are the right answer for short lab jumpers, short internal chassis wiring, low-frequency test setups, and any application where extreme isolation is not required. Above a few GHz or in a dense EMI environment, the residual leakage becomes the limiting factor and you should be looking at LMR-class or better.

LMR and Low-Loss Flexible Cables

Modern low-loss families (LMR-195, LMR-240, LMR-400 and equivalents from other manufacturers) fundamentally rethink the shield architecture. Instead of a single or double braid, they use a two-layer construction: a solid aluminum foil bonded to the dielectric, plus a tinned-copper braid over the foil.

The foil gives you 100 percent physical coverage against high-frequency leakage. The braid gives you mechanical strength and better performance at low frequencies where the thin foil alone starts to become resistive. Combined, they push shielding effectiveness to 90 to 100 dB and often better. This is why LMR-class cable is the default for outdoor 5G jumpers, cellular infrastructure, satellite ground segment, and most radar and communications ground applications.

Semi-Rigid Cables

Semi-rigid takes the shield architecture to its logical extreme: replace the braid entirely with a solid metal tube (copper, tin-plated copper, sometimes aluminum). Because the outer conductor is now continuous metal with no gaps of any kind, shielding effectiveness routinely exceeds 120 to 140 dB across the full microwave range. There is nothing for RF to leak through.

Common diameters are 0.085", 0.141", and 0.250" (referenced by outer conductor OD). The trade-off is mechanical. Once formed, the shape is permanent. You cannot flex a semi-rigid repeatedly without work-hardening the tube and cracking it. That is why semi-rigid dominates inside radar modules, aerospace electronics bays, and high-isolation microwave instruments where the cable will be shaped once at manufacture and never moved again.

Rigid Coaxial Lines

True rigid coaxial lines are two concentric metal tubes with dielectric spacers holding them in precise alignment. Diameters run from under an inch to more than 8 inches. Shielding effectiveness is essentially perfect. The only leakage paths are at the connector interfaces and expansion joints, and those are engineered to minimize it.

This is not a general-purpose cable. It is used for high-power broadcast transmitters, radar feeders, cellular tower trunk runs, and other applications where power handling and lowest possible loss dominate. Direction changes require machined elbows, tees, and transitions. You do not "install" a rigid coax line the way you install a flexible one. You build it, piece by piece.

Side-by-Side Shielding Comparison

How to Select the Right Cable

Six questions get you 90 percent of the way to the right cable. Answer them before you look at any datasheet.

The Connector Is Part of the Shield

This is the section most articles skip. It matters more than any of the cable choices.

A cable with 120 dB shielding effectiveness terminated in a connector with 60 dB isolation is a 60 dB system. You built a nickel wall around a hole. Every SE spec you read is meaningful only if the connectors, the connector-to-cable attachment, and the mating interfaces preserve it.

Practical implications:

Installation Practices That Preserve Shielding

The best cable you can buy will not deliver its rated shielding if it is installed badly. A few habits worth having:

Maintenance and Service Life

A well-installed cable assembly with quality connectors will typically deliver 10 to 25 years of service. The exact number depends on environment more than anything else.

Basic maintenance that adds years to the number:

• Periodic visual inspection for jacket cracks, abrasion, or connector corrosion
• Clean connector interfaces with isopropyl alcohol and lint-free swabs. Never touch mating surfaces with bare fingers
• Verify connector pin depth with a gauge after every 100 or so mating cycles, or whenever performance degrades
• Replace any assembly with permanent kinks, crushed sections, or rising VSWR or insertion loss
• Retorque threaded connectors that have loosened due to vibration

The connectors are usually what limit the life of an assembly, not the cable itself. A high-cycle test cable may need replacement after a few thousand matings; a fixed semi-rigid inside a benign enclosure can last decades.

Application-Specific Recommendations

Common Shielding Mistakes

Frequently Asked Questions

Bottom Line

Shielding effectiveness is not a secondary spec at high frequencies. It is the difference between a cable that behaves like a transmission line and one that behaves like an antenna. Choose the shield architecture (single braid, double braid, foil-plus-braid, semi-rigid, or rigid) that matches the isolation your system actually needs, and match the connector quality to it. Torque properly, weatherproof outdoors, respect the bend radius, and inspect periodically.

Do those things and the cable assembly disappears from your problem list. Which, for RF hardware, is the highest compliment you can pay it.