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RF Absorbers

V.20160405 1

by

Jos Westhof-Jacobs Business Development

RF Absorbers © Siepel/JWJ

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• Electromagnetic Absorbers

• Types, characteristics

– Reflectivity

– Power handling

– Fire retardant properties

• Performance measurement & Validation

Agenda

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Reason for Anechoic?

• To achieve a reflection-free environment

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Because of the wave impedance

the near field region fields can

range from predominantly

magnetic to predominantly

electric.

RFI Fieldstrength measurements

are done in the Far Field

The free space impedance is 377

Ohms (120 x pi)

Free Space

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How does an absorber work?

• The Shield = 0 Ohms

• Free Space = 377 Ohms

• => Need for matching

• An absorber is a broadband series impedance

between free space and short circuit

• an absorber ‘hanging in the air’ is doning everything

excepted what you expect

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Ferrite FE

Pyramidal foam APM

Truncated APX

Hybrid HY

Truncated Hybrid HYT

Convoluted APC

High Power AHP

Wedge ADM

Walkway API

Multilayer AT/AH

Reticulated CR/PR

Weatherproof AI

Narrowband ASI/APU

Multilayer MI

Flat P

Epoxy Absorber AE

Siepel Absorber Portfolio

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Electromagnetic Absorbers

• Applications & Selection

– EMC

– Radio

– Antenna

– RCS

– Compact Ranges

– PIM

– Outdoor Cf. Anechoic Chamber Section

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Electromagnetic Absorbers

• Most used for EMC Chambers

– FE: Ferrite (30 MHz – 1 GHz)

– APM: Pyramidal Foam Absorber (80 MHz – >40 GHz)

– ADM: Microwave Wedge Absorber (80 MHz – >40 GHz)

– HY: Hybrid = Ferrite + Pyramidal (30MHz – +40 GHz)

– P: Flat Absorber (80 MHz – +40 GHz)

– API: Walkway Absorber (200MHz – 18 GHz)

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Electromagnetic Absorbers • FE10 - Unique Ferrite Absorber solution by Siepel

– Tiles 100 x 100 x 6,7mm are pre-glued

on metal backed galvanized sheet panel

(3x3 tiles; 300x300mm), with 5 fixing holes

– Each panel is screwed on a horizontal

metal rail system which are fixed to the

vertical panel stiffeners

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Electromagnetic Absorbers • Pyramidal foam absorber, 80MHz - 40GHz (and

above)

– High tech polyurethane foam

– 90% open cells

– Numerical simulation tools

• Full wave methods: Microstripes – CST (TLM)

• Asymptotic or hybrid methods: XGTD – Remcom (GTD/UTD)

– Sophisticated carbon-based aquous solution

– Standard power handling capability 2 kW/m²

– Unique manufacturing process & quality control on

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Electromagnetic Absorbers • APM Pyramidal Foam absorbers

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Electromagnetic Absorbers • APX and HYT

– APX = Truncated APM

– HYT = Truncated HY

– HYT+ = Truncated HY with improved performance

Benefits:

Space saving

Less susceptible to damage

Ideal on and around doors

(larger clear opening)

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Electromagnetic Absorbers • HY: Hybrid (Wideband EMC Absorbers)

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Electromagnetic Absorbers

• ADM: Wedge

– 80 MHz – +40 GHz

• API: Walkway

– 200 MHZ – 18 GHz

• AH/AT: Multilayer Flat

– 450 MHz – +40 GHz

• ASI & API: Resonant Flat Elastomer

– 10% of the central frequency 1.5-26GHz

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Absorbers Performances • Measurement 30 MHz-1 GHz

– Coaxial line (14m long) for S11 using VNA & Time Domain

– Precise reflectivity measurements on a variety of (cascaded)

absorbers, influence of gaps & other imperfections

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Absorber Performances • Measurement > 1GHz

– Measurement of S21 using VNA & Time Domain

– Detailed measurement of incident and reflected wave

over various angles)

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Electromagnetic Absorbers • Performances acc. to incidence wave

– APM Absorbers keep their performances for incidence angles

<45°

Example: APM66, 2GHz, 50° APM66

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Absorbers Power Handling • Power handling = Maximum surfacic RF Power an absorber is able to

support

• RF Power / Surface = Watt / m2 or Watt / cm2

• (1 Watt/cm2 = 10.000 W/m2)

• RF Power turns into heat

• Max. Power is related to field: P = E2 (V/m) / Z (377 Ohms)

• In practice this 377 Ohms is only theory and never met and depends

on antenna characteristics, distances, modulation, harmonics, etc…)

• For Pulsed signals, power handling value can be increased (e.g.

Short pulses/Low puls repetition frequencies

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Power Handling vs. Absorbers

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Testing Power Handling

• Siepel tests:

– Continuous wave CW 5 minutes

– At the end of the test, the sample is cut to check for

“internal smolding”

– Upon request this is done for indiviual absorbers (not

included in the standard Factory acceptance Test)

– Only done from 5-18 GHz, for lower frequencies: very

expensive, since we need kilowatts of RF power

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AHP High Power Absorber

• Made of honeycomb and phenolic resin

• Black colour

• Handles up to 20 kW/m2 !!

• For Military and Satellite testing, the absorbers

can be ventilated for higher powers

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Electromagnetic Absorbers • Fire retardancy

– ISO 11925-2 Euroclass E

– NRL 8093 tests 1, 2 & 3

– DIN 4102 B2

– UL 94 HBF

– ISO 4589-2 LOI

• In-house test laboratory

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Electromagnetic Absorbers

• Coating – Standard: aquous blue paint

• Commonly used hi-tech colour, available in other colours as well

– Optional: plastic blue paint, featuring Optimized carbon binding: no finger marks, no pollution or

carbon dust, keeps the chamber clean

Suitable for Class 100.000 Clean Rooms (Aerospace, Chips, …)

High flexibility and excellent shape memory

Extreme softness: very sustainable

No impact on RF performances (Paint = RF transparant)

Longer lifespan (in particular for movable absorbers on the floor)

Improved aestetics (brightness)

Available in any colour (corporate colour, logo, …)

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