Normalised Site Attenuation (NSA)

Normalised Site Attenuation measurement in a 3m semi-anechoic chamber

Normalised Site Attenuation (NSA) measurements are used to determine the suitability of a test site (Semi-anechoic chamber or an Open Area Test Site) for performing radiated emissions measurements as required by international standards including CISPR 16-1-4 and ANSI C64.4.

These measurements are required for validating new chamber installations and also for regular recalibration of existing facilities.

New anechoic chamber validations

For new chamber installations, it is advisable to use the services of an independent measurement supplier to give you confidence that your new chamber is validated accurately and impartially by a competent 3rd party.

We accurately measure the NSA performance of new chambers as well as chamber relocations, upgrades and modified chambers worldwide.

Regular recalibration of anechoic chambers

For regular recalibration some laboratories choose to measure the NSA performance themselves, however this can be more time consuming and a less accurate option than using an independent company that specializes in these measurements.

  • We use custom software and measurement procedures to ensure that the NSA measurements are completed quickly, resulting in minimum chamber downtime for you and less disruption to your laboratory testing schedule. Typically we measure the NSA of a chamber within one day.
  • Our engineers have many years of experience of measuring NSA in chambers of all shapes and sizes. By using our measurement services your laboratory personnel are available to do more productive work without needing to fully understand unfamiliar measurements that they would only be performing infrequently anyway.
  • NSA measurements require matched antenna pairs with special calibrations. We supply all of the required equipment so that you do not need to own and maintain specific antennas for these measurements.

Contact us to discuss your NSA or other chamber measurement requirements

Normalised Site Attenuation measurement method

The method described here is applicable to semi-anechoic chambers which use a ground-plane. The method used for Fully Anechoic Rooms (FARs) is described on a separate page here.

Normalised Site Attenuation is used to validate test sites for radiated emission measurements usually over the frequency range 30 – 1000 MHz.

The basic principle behind the Normalised Site Attenuation (NSA) measurement is that the loss is measured between a pair of antennas and then this loss is then compared against the loss for a‘perfect’ site and any difference is deemed to be a deviation caused by imperfections in the site.

The geometry for the site attenuation measurements is shown in the diagram below. A transmit antenna is positioned at a fixed height and a receive antenna is swept in height while the measuring instrument records the maximum signal level. The reason for sweeping the antenna height is to ensure that the maximum signal is recorded when the direct wave and the wave reflected from the ground plane interfere constructively (this principle is similar to the reason for height sweeping the receive antenna when making normal EMC radiated emission measurements).

NSA geometry and measurement configuration

The height of the receive antenna is usually swept from 1 – 4 m in fully compliant chambers; however this height may be reduced for pre‑compliance chambers with lower ceiling heights.

Measurement distances are usually 3m or 10m although 5m measurement distances are also sometimes used.

Volumetric NSA

If the site attenuation is measured only at a single position (e.g. at the centre of the turntable), then it is possible that any reflections from the chamber construction might not be picked-up. To avoid this possibility, a series of measurements at different positions,polarizations and heights are required.

The exact measurement geometries are described in the relevant standards (e.g. CISPR 16-1-4, ANSI C63.4) but will consist typically of a number of positions around the turntable area (orthe space where the EUT will be located). The size of this area will depend on the chamber design and the size of turntable or EUT.

The measurements are made in both horizontal and vertical polarisation and the transmit antenna is typically located at the following heights:

1.0 m and 2.0 m in Horizontal polarisation

1.0 m and 1.5 m in vertical polarisation

Volumetric NSA measurement geometry

NSA measurement results

The site attenuation measurement results are conventionally given in terms of deviation from an ideal test site. The usual specification for compliant chambers is that the deviation should be less than ±4dB.

Typical NSA measurement result graph

Additional chamber measurements and validations

Further information on our chamber validation measurement services can be found on the following pages:

Anechoic chamber validation Validation of fully anechoic chambers (FAR validation) Field uniformity calibration of anechoic chambers Site VSWR measurements (Svswr)

If you have any questions or need advice on the best chamber calibration for you then Contact us to discuss your requirements.

Normalised Site Attenuation – Key facts:

  • NSA is used to validate an EMC test site for making radiated emissions measurements
  • Common measurement standards include:
    • CISPR 16-1-4
    • ANSI C63.4
    • EN 50147-2
    • EN 55022
  • Typical frequency range of measurements: 30 – 1000 MHz
  • Measurements are performed over a ‘test volume’ which defines the largest EUT size
  • Results should be within ±4dB for a compliant test site
Anechoic chamber validation
Validation of fully anechoic chambers (FAR validation)
Field uniformity calibration of anechoic chambers
Site VSWR measurements (Svswr)

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