Yunjia Textile · Technical guide
EMF Shielding Guide: Materials, Fabrics and Enclosures
This guide explains how RF shielding textiles work, how to read a dB result, and why fabric, seams and openings all decide what a finished item can do.
Key takeaways
- Shielding works when a conductive layer forms a continuous path around the space. Gaps break it.
- Every 10 dB means 10 times less transmitted power: 20 dB = 99% blocked; 40 dB = 99.99% (unit conversion, not a product result).
- A fabric result describes the fabric. A tent or canopy has to be tested as a finished item.
- A phone's signal bars cannot measure shielding performance.
What shielding actually does
A shielding fabric is a thin conductive layer. Radio waves that hit it are mostly reflected, some energy is absorbed, and whatever finds a gap leaks through.
Shielding changes how a wave travels from one side of a barrier to the other. At the surface, some energy returns toward the source as reflection. Some enters the material and is dissipated as heat through absorption. The remainder is transmitted through the layer or finds a route around an edge or through an opening.
The material and the geometry work together. A conductive textile can provide a useful barrier along one path, while an opening provides another path into the same space. Following both paths helps explain why a good fabric result and a finished enclosure result can differ.
Shielding, grounding and sensing are different jobs. This page covers shielding only; grounding is explained in our Grounding Sheet Guide, and conductive structures in the Conductive Textile Structure Guide.
Reading a dB number
dB counts by factors of ten. 10 dB lets through 1/10 of the power, 20 dB 1/100, 30 dB 1/1,000. A higher dB number means much less gets through.
SE = 10 log10(P0 / P1)
P0 = power without the shield; P1 = power with the shield, using the same setup and frequency.
Every 10 dB means ten times less power reaches the other side.
| SE | Remaining power | Blocked |
|---|---|---|
| 10 dB | 1/10 | 90% |
| 20 dB | 1/100 | 99% |
| 30 dB | 1/1,000 | 99.9% |
| 40 dB | 1/10,000 | 99.99% |
| 60 dB | 1/1,000,000 | 99.9999% |
| 80 dB | 1/100,000,000 | 99.999999% |
99% and 99.99% look almost the same, but they differ 100 times in how much gets through.
For field quantities (V/m or A/m), SE = 20 log10(E0/E1). Under matched conditions it gives the same dB number as the power form.
Layer 1 — What makes a material shield
A material shields when electric current can flow freely across it. The incoming wave drives currents in the surface; those currents send the wave back. Wherever the current path is broken, energy leaks through.
When a radio wave reaches a good conductor, its magnetic field drives a current along the surface. That current radiates its own wave, reducing the field behind the conductor and sending energy back as reflection. Some of the current's energy is lost as heat in the material — that is absorption. Both depend on a conductive path that the current can follow without interruption.
This is why the rest of this guide follows the current. In a fabric, the path runs through the conductive fibers and their contact points (Layer 2). In a finished item, it has to cross seams, zippers and overlaps (Layer 3). An ordinary stitched seam can interrupt this path if the panel edges lose conductive contact. A gap at a closure or an open bottom lets the wave leak or bend through.
Continuous path
Broken path
Engineering notes — surface current and skin depth
Js = n × H
Here n is the outward unit normal and H is the total magnetic field just outside the conductor. On a good conductor, the tangential electric field is close to zero, so the induced surface current Js re-radiates the reflected wave.
The wave's magnetic field sets the surface current; the current produces the reflection.
Engineers often write shielding as SE = R + A + B: reflection, absorption, and a correction for multiple internal reflections in thin layers.
δ = 1 / √(π f μ σ) A ≈ 8.686 · t / δ (dB)
Frequency f, permeability μ and conductivity σ set skin depth δ; t is material thickness. Higher frequency and higher conductivity mean the current stays closer to the surface.
At 50 Hz, copper's skin depth is about 9.35 mm (8.53 mm at 60 Hz) — far thicker than any textile coating, so thin fabrics do little against power-line magnetic fields. This copper calculation uses σ = 5.8 × 107 S/m and μ = μ0.
How conductive fibers and coatings are made is covered in the Conductive Textile Structure Guide.
Layer 2 — Fabric
For fabrics, the key number is sheet resistance (Ω/sq): lower usually means a better current path. But pores, weave and coating quality also matter, so the tested dB result is what counts.
Sheet resistance describes resistance across a square of a thin conductive layer, written Ω/sq. For an ideal uniform sheet, the same number applies to a large square or a small square: increasing its length also increases its width. On a textile, the measured path includes the conductive network and the contacts between its parts.
A woven, knitted or mesh structure leaves different openings and contact points. Two fabrics with similar sheet resistance can therefore produce different shielding results. The coating must connect across the structure, and the openings still need to be considered at the frequencies being measured.
Choose the sample and test range together. A resistance figure helps describe the current path; the shielding test measures how the whole fabric sample behaves in the stated arrangement.
Engineering notes — an ideal sheet
SE ≈ 20 log10(1 + Z0 / (2 Rs)) Z0 ≈ 376.73 Ω
A teaching estimate for an ideal, gap-free sheet. Real fabrics have pores and finished items have seams, so this number cannot predict or replace a measured result.
Lower sheet resistance Rs means more attenuation in this ideal model. Example: Rs = 1 Ω/sq → SE ≈ 45.5 dB.
Layer 3 — Seams and openings
A finished item is only as good as its weakest joint. Seams, loose closures and open edges can interrupt the current path, allowing waves to leak through gaps.
A seam has to join conductive surfaces as well as hold fabric together. Insulating thread alone does not establish that connection; the panel overlap and contact need attention. Conductive sewing thread can help maintain a path when the joint is designed to keep the surfaces in contact.
Closures need the same continuity. A zipper or hook-and-loop strip should make conductive contact along its working length, including the points where the closure meets the wall. An overlap covers an opening only while its conductive layers stay in contact.
The floor counts too. When the bottom is made of the same conductive layer and sewn into the walls, the enclosure is closed on every side, and the seams and the zipper are what is left to get right.
Engineering notes — openings and wavelength
λ = c / f
Wavelength λ is the wave speed c divided by frequency f. A gap starts to matter as its longest side approaches a sizeable fraction of the wavelength.
| Frequency | Wavelength | λ/2 |
|---|---|---|
| 900 MHz | 33.3 cm | 16.7 cm |
| 2.4 GHz | 12.5 cm | 6.25 cm |
| 5 GHz | 6.0 cm | 3.0 cm |
Where a joint is designed to keep conductive surfaces in contact, these parts can help keep the current path continuous: 
conductive sewing thread, a 
silver-plated zipper and 
conductive hook-and-loop.
Layer 4 — Finished solutions
Bed canopies, tents, pouches, curtains and clothing use similar fabrics but very different shapes. The shape, the closures and whether it fully surrounds the space decide the result.
Follow this chain when choosing which evidence applies. A material property helps describe the starting point; a fabric measurement includes its textile structure. Connections then join that fabric into an assembly. The final shape determines which paths remain open and what a finished-item test needs to include.
Bed canopies
Many canopies hang over a bed and stay open at the bottom. That opening is large compared with RF wavelengths, so signals can enter from below unless a shielding floor layer overlaps the walls. A closed enclosure, floor included, removes that path.
Tents and enclosures
The EMF Tent is built as a closed enclosure, floor included: the floor is a sewn-in sandwich of two nylon layers with the same silver mesh as the other sides between them. That leaves the seams and the zipper as the weak points. A number for the finished item still needs its own test.
Bags and pouches
Small, fully closed, and easy to check by fold and seal. The closure is usually the weak point.
Curtains and single panels
A panel reduces signal along the direct path, but waves also bend around its edges and reflect off walls. A panel's fabric dB does not describe the room.
Clothing
Garments cover part of the body and have open edges by design; treat any fabric value as a material property, not a garment result.
How shielding is tested
Real shielding numbers come from controlled lab tests at stated frequencies. At home you can only compare before and after with the same tool, in the same place, at the same band.
Common sources sit at very different frequencies — power wiring at 50/60 Hz, cellular from several hundred MHz to a few GHz (higher in some 5G bands), Wi-Fi at 2.4, 5 and 6 GHz — so a result only applies to the band it was measured at.
| Home check | What it can show | What it cannot show | Band it covers |
|---|---|---|---|
| Call / text test | Whether the phone can still connect at that moment | Any dB value; small vs large reductions; other bands | The cellular band the phone happens to use |
| Wi-Fi signal app (RSSI, dBm) | The change in one access point's signal at one spot, before vs after | Cellular or other bands; results are coarse and depend on phone position | 2.4 / 5 / 6 GHz Wi-Fi |
| RF meter | A relative change within the meter's frequency range | A standard SE value; anything outside its range or below its noise floor | Per the meter's specification |
A repeatable before-and-after check
- Use one fixed source you control (for example your own router), not passing cell traffic.
- Mark the measuring spot and keep the same height and orientation.
- Use the same device, app or meter and the same mode (peak or average).
- Take several readings without the shield, then several with it fully closed.
- Compare the averages. With readings in dBm, the difference is the change in dB (for example, −40 dBm to −70 dBm is a 30 dB drop).
- Write down the date, frequency band and setup, and repeat the same way next time.
This shows the change in your setup. It is not a lab shielding value.
Match the method to the object
| Standard | What it tests | Typical scope |
|---|---|---|
| ASTM D4935-18(2026) | Planar materials (fabric samples) | 30 MHz – 1.5 GHz |
| GB/T 30142-2013 | Planar materials | As stated in the standard / report |
| IEEE 299.1-2013 | Enclosures and boxes with all dimensions 0.1–2 m | Inactive-Reserved since 2024; revision P299.1 in progress |
| IEEE 299-2006 | Enclosures with all dimensions ≥ 2 m | 9 kHz – 18 GHz (extendable 50 Hz – 100 GHz); Inactive-Reserved |
IEEE 299.1 covers small enclosures and boxes with all dimensions between 0.1 m and 2 m, separate from large-room methods. Select the method by the actual dimensions of the item.
Planar material
Finished enclosure
A fabric test measures a specimen in its defined fixture. A finished-item test includes the assembly's walls, seams, closures and openings, with specified source and measurement positions. Ask what was inside the test boundary before comparing two reports.
What to ask for in a report
- Exact item tested: fabric or finished product
- Frequency range and measurement points
- Method or standard, including its edition
- Laboratory and report number
- Sample state: new, washed or otherwise conditioned
Frequently asked questions
Does shielding fabric need to be grounded to block RF?
No. RF shielding comes from a continuous conductive layer around the space; grounding does not create that layer. Grounding a shield is a separate engineering topic (low-frequency electric fields, electrical safety) and is not what makes RF shielding work. Earthing your body is not shielding — see our Grounding Sheet Guide.
Does one shielding fabric block everything — cellular, Wi-Fi, Bluetooth, GPS?
Shielding depends on frequency. A result measured in one range does not automatically apply to another. Check that the tested frequency range covers the bands you care about, and remember that seams and openings usually limit a finished item more than the fabric does.
Can I just use aluminum foil instead of fabric?
Foil is a good conductor and can shield radio waves. In practice it tears, creases open at joints, does not breathe, and is hard to close reliably. The same rule applies to any material: the continuity of the whole enclosure matters more than the sheet itself.
Does RF shielding fabric block 50/60 Hz power-line fields?
Not in the same way. Low-frequency magnetic fields pass through thin conductive textiles because their skin depth is millimeters, not microns. They are usually handled by distance from the source or by magnetic materials designed and measured for that purpose.
Which material is best: silver, copper, nickel or stainless steel?
Each can conduct well enough to shield. Choose by tested data at your frequencies, sheet resistance, durability, handling and breathability rather than by metal name. How each conductive material is built is explained in our Conductive Textile Structure Guide.
Is one panel or curtain enough, or do I need a full enclosure?
A single panel only reduces signal on the direct path. Radio waves also bend around edges and reflect off walls, ceilings and floors. A full enclosure with closed seams controls all paths, which is why panel and enclosure results are not comparable.
Why can a tent or canopy measure lower than its fabric?
A fabric test measures a flat, gap-free sample. A finished item adds seams, zippers, overlaps and sometimes an open bottom, and each can leak. That is why a finished item needs its own test at stated frequencies.
Does shielding wear out or wash out?
Conductive coatings can wear at folds, closures and rubbing points, and washing can affect them. Follow the product's care instructions. To check an item over time, repeat the same before/after comparison described in the testing section.
Does lower sheet resistance always mean better shielding?
Usually it helps, but not always in a finished product. Pores, weave structure, coating uniformity and seams can all change the result, so sheet resistance is a guide and the tested dB value is the evidence.
What should I specify before requesting a test report?
The exact item (fabric or finished product), frequency range, test standard or method, sample condition (new or washed), and lab and report number. Without these, a dB number cannot be compared with another.
References and scope
- LearnEMC: Practical Electromagnetic Shielding. Connections, apertures and field-dependent limitations.
- NIST SP 811, Chapter 8, §8.7. Logarithmic field and power quantities. 2022 CODATA constants supply the free-space constants used in teaching calculations.
- Peng and Qin: Clarification of Basic Concepts for Electromagnetic Interference Shielding Effectiveness (2021 preprint). Model definitions and interpretation of decomposition terms.
- LearnEMC: Plane-Wave Shielding Theory. Uniform-barrier models and good-conductor approximations.
- Tosaka et al.: Method for Estimating Sheet Resistance of Thin Shielding Sheets Using DFFC (URSI, 2008). Thin-sheet transmission model.
- ASTM D4935-18(2026). Official public scope; the full standard is needed for the complete procedure.
- IEEE 299-2006 and the P299 revision project. Official status and scope listings.
- GB/T 30142-2013. Official standard identity and current status.
- IEEE 299.1-2013 and the P299.1 revision project. Small-enclosure scope and status.
Scope and limits. This guide explains RF shielding physics and test methods. Formulas and conversions are teaching tools, not product results. Any shielding value applies only to the item, frequency range and method stated in its test report; fabric results do not transfer to finished products. Home checks show relative change only. Shielding performance is not a health or medical claim, and this page does not assess exposure levels. Our silver fiber products use silver-plated fibers; a silver fiber percentage is the share of plated fiber, not silver metal content.