Site VSWR (S_VSWR) Measurements

Site Voltage Standing-Wave Ratio (S_VSWR) measurements are used to validate a test site (Anechoic chamber or Open Area Test Site) for performing radiated emissions measurements above 1 GHz (below 1 GHz, NSA measurements are used for chamber validation).

The S_VSWR measurement is made in accordance with CISPR 16-1-4 and its purpose is to check for the influence of any reflections that might adversely affect emission measurements.

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

The S_VSWR measurement is a comparatively new measurement and some older chambers may require modification in order to meet the Svswr requirements.

New anechoic chamber validations

For verifying new anechoic chamber installations, using an independent measurement service provider gives you confidence that your new chamber is validated accurately, impartially and efficiently.

We accurately measure the S_VSWR performance of new chambers using our in-house developed software and measurement methods to give you accurate results, quickly and efficiently. Our depth of experience also ensures that we can help you to configure your chamber to achieve the optimum measurement results.

In addition to new chamber installations, we also provide S_VSWR measurements for chamber relocations, upgrades and modified chambers worldwide.

Regular S_VSWR validation of anechoic chambers

Some EMC laboratories choose to measure the S_VSWR of their chambers themselves, however this can actually be very consuming and requires specific equipment so it is often more efficient to use an independent company specializing in these measurements.

• Our in-house custom software and measurement procedures ensure that the S_VSWR measurements are completed quickly, resulting in minimum chamber downtime for you and less disruption to your laboratory testing schedule.

• Our engineers have been measuring S_VSWR in chambers for many years, even before it was a requirement in the standards. By taking advantage of our expertise, your laboratory personnel are freed-up for more productive work without any distractions of needing to understand infrequent, unfamiliar measurements.

• S_VSWR measurements require special antennas with specific radiation patterns as described in CISPR 16-1-4. We supply all of the required equipment so that you do not need to own and maintain any special equipment to undertake these measurements.

• Our depth of experience in chamber validation measurements enables us to help you configure your chamber to achieve the optimum measurement results.

Contact us to discuss your S_VSWR and other chamber calibration requirements

S_VSWR measurement method

For The S_VSWR measurement is used to validate test sites for radiated emission measurements above 1 GHz. Measurements are commonly made up to 6 GHz, which is the upper frequency for some commercial EMC standards however for new chambers, the measurements are sometimes performed up to 18 GHz to ensure that future requirements are also covered. Measurements are usually made at a nominal 3m measurement distance although occasionally other distances may be used.

Measurements are performed with antennas oriented in both horizontal and vertical polarisations.

The purpose of the S_VSWR procedure is to check for the influence of reflections from the chamber construction and any objects that are permanently installed in the chamber.

The S_VSWR is evaluated by performing measurements of the signal level using a receive antenna with a transmit antenna located at a sequence of six positions along a line to the reference point of the receive antenna.

Interference between direct (intended) and any reflected signals will result in variations in the received signal at the six locations. The S_VSWR is defined as the ratio of maximum received signal to minimum received signal from the six measurements.

S_VSWR test volume

The S_VSWR is evaluated in a ‘test volume’, similar to the test volume required for the NSA measurement. This volume is a cylinder covering the largest diameter and height of the Equipment Under Test (EUT). The exact positions to be measured are defined in CISPR 16-1-4 and will depend on the size of the test volume. Typical positions are shown in the diagram below:

In this case, the S_VSWR is evaluated at the centre, front, left and right of the test volume, and six locations are used for the transmit antenna for each test position.

Note that the receive antenna remains fixed for each of the measurements (as it would be in normal emission measurements). In particular, the receive antenna is not aligned with the transmit antenna when measuring the side positions. This can cause some difficulties with the measurement, especially when evaluating larger test volume diameters, since the direct received signal is lower than it would be with the antennas aligned. The contribution from reflected signals compared to the direct signal are therefore higher. It is not uncommon for the S_VSWR  results to be higher at the left and right positions.

It is also important that the receive antenna is carefully selected for these measurements since antennas with a narrow beamwidth can result in poor S_VSWR  performance. CISPR 16-1-4  states that the receive antenna used for the S_VSWR evaluation shall be of the same type as used for EUT emissions measurements.

Test volume height

The S_VSWR positions are measured at either the middle height of the test volume or 1.0m above the bottom of the test volume, whichever is lower. An additional measurement is required at the top of the test volume if this is more than 0.5m above the lower measurement height. Measurements at this additional height are only required for at the front position of the test volume.

The receive antenna is also adjusted so that both transmit and receive antennas are at the same height.

It should also be noted that CISPR 16-1-4 also has a requirement that no more than 30cm of the test volume should be obstructed by the floor absorber, which limits the height of the absorber or requires the bottom of the test volume to be raised above the floor.

S_VSWR measurement results

The acceptance criterion for 1 GHz to 18 GHz site validations is for an S_VSWR ≤6.0dB
Results are usually presented graphically, showing the S_VSWR result for all of the measurement positions compared to the 6dB limit:

Additional chamber measurements and validations

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

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