Should testing of radiated emissions be performed on a 30m, 10m, 3m or 1m test site has been a controversial debate for a long time in the world of electromagnetic compatibility (EMC) compliance testing. In this article, we will skip military (MIL-STD 461, RTCA DO-160 etc) and automotive (CISPR 25) type testing as this testing is typically conducted at a 1m distance over the entire frequency (usually 10kHz to 18GHz).
Radiated Emissions on a 10m Open Area Test Site
Our focus here is on consumer and commercial Radiated emissions EMC testing. Particularly when the old preferred methods described in many of these EMC testing standards were based on performing radiated emissions on a 10m Open Area Test Site (OATS) for the frequency range of 30MHz to 1GHz. Also note that for frequencies under 30MHz, it was noted that a preferred measurement distance of 30m is required. The FCC addressed this issue for FCC Part 15C (intentional radiators) with the 40dB/decade reduction for frequencies below 30MHz or the two measurement distance approach. Most international standards also specify a 3m measuring distance for Radiated emissions above 1GHz.
This effectively resulted in driving out to a field with a weatherproof hut and a ground plane and performing radiating emissions in the elements with many ambient RF emissions. As wireless technologies became more available and widely used it made these measurements ever so more difficult due to the increase in ambient emissions.
Radiated Emissions on an Indoor Open Area Test Site
Over time with advancements in equivalent free space condition test sites as an OATS site i.e. indoor Open Area Test Site (iOATS) it made radiated emissions testing in an ambient free environment which made testing much easier and faster. Test site semi-anechoic materials and technologies allowed increased performance over ever-increasing frequency ranges.
Initially, as with Fully Anechoic Chamber (FAR), the indoor Open Area Test Site (iOATS) chambers met some scepticism. However over the last few decades, iOATS chambers have become the norm for EMC testing of radiated emissions due to their many benefits, some of these benefits include:
- No ambient RF emissions from the outside world
- Decreased measurement times as pre-scans and formal final measurements can be performed on the same site with the same product setup.
- Can be installed in a building thus can be climate controlled and weatherproof
- Able to piggyback another chamber used to locate and isolate support equipment (such as we have at EMC Bayswater).
10m Radiated Emissions test site
After the use of iOATS test sites, was established and widely accepted among the EMC testing community the focus remained on the required minimum test distance. The main issue here lies with if the antenna is in the “near-field” over time the standards have slowly introduced limits and guidelines on how to approach this issue. There are still issues surrounding the near-field effects with respect to the largest dimension of the product been tested and/or the measuring antenna as per the CISPR 16 standards.
As technologies advance and the EMC testing standards evolve with modern testing methods it is becoming more acceptable to test at a standard 3m distance for non-large items. In the past, most standards would only list a 10m radiated emissions measuring distance. However, a 10m radiated emissions test site is still required for large products for testing laboratories to be able to issue accredited reports. We will go over the product dimension restrictions later in this article.
3m Radiated Emissions test site
The defining of limits to a 10m limit with the statement in the standards to adjust the limit i.e. 20dB/decade for the new measuring distance i.e. 3m has gradually been changing. A good example is that many of the common standards now list 3m measurement distances such as:
- EN 55032 of CISPR 32 – Multimedia equipment (also called up by ETSI 301 489 series, and replaced EN 55022 / CISPR 22 and EN 55013 / CISPR 13
- EN 55011 or CISPR 11 (as called up by EN 61326-1 and EN 61326-2-6)
- FCC Part 15.109 for all digital device expect Class A (Class A product limits can be increased (20dB/decade) to cater for a 3m measurement distance)
Most of the common consumer/commercial EMC standards referenced by the standards above such as EN 55015 / CISPR 15 etc and now allow radiated disturbance measurements to be performed at a 3m distance; however, this is usually still dependent upon the size of the Equipment Under Test (EUT).
So the main focus now is not on the measuring distance itself i.e. either 10m or 3m, but the size of the Equipment Under Test (EUT).
What size equipment (EUT) is allowed to be tested on a 3m Radiated emissions test site
Within EN 55032 / CISPR 32 it states that the maximum width of the EUT, local auxiliary equipment and associated cabling shall be within the test volume as demonstrated during the Normalized Site Attenuation (NSA) test site validation. The validated measurement volume does not need to encompass any local auxiliary equipment and associated cabling.
EN 55011 / CISPR 11 states that a 3m test site can be used for small equipment. The definition of small equipment is defined within the standard. The EUT can either positioned on a tabletop or be standing on the floor which, including its cables fits in a cylindrical test volume of 1.2 m in diameter and 1.5 m above the ground plane. This means if the EUT is floor standing equipment it can have a 1.2m diameter and be 1.5m in height from the ground reference plane (GRP). Or if the EUT is tabletop equipment it can have a 1.2m diameter and be 0.7m in height assuming a table height of 0.8m above the GRP.
To use a radiated emissions test site to test for FCC Part 15 requires the test site such as an iOATS chamber to be calibrated in accordance with ANSI 63.4: 2009 and C63.4: 2014 required for FCC testing (North America) and ICES testing (Canada).
Other important aspects to be considered are the physical dimensions of the antenna and the product. The larger the dimensions of the antenna and the product itself the more the near-field effect is present. Also, the Antenna 3dB beam-width must be considered and the area of measurement illumination of the product at the measuring distance. We determined that a hybrid antenna such as a Sunol JB-5 worst case 3dB beam-width at 1000MHz gave an illumination of 2.2m using a 3m measuring distance.
Alternative measuring distances and future corrections to limits
In EN 55032 / CISPR 32 they have included validation of 5m radiated emissions test sites. EMC Bayswater’s test site can accommodate 5m measurements, the 5m measuring distance is a great compromise between a 10m and a 3m measurement. It allows larger turntable sizes and thus larger products to be tested whilst maintaining the compact chamber size.
As previously stated the higher the frequency measurements above 1GHz are usually made at a preferred measurement distance of 3m. However, due to the inherent measuring system noise floor at higher frequencies, this distance may need to be reduced further.
There has been much talk about applying a compensated correction applied to the limit (not just using the 20dB/decade conversion) to allow 3m measurements on larger sizes EUTs for some time. Hopefully, at some stage, these will see their way into the EMC standards and make everyone’s life easier.
Conclusion
A 3m measuring distance for radiated emissions test site is valid for the majority of all products. The product under test and measuring antenna dimensions should be considered by the testing laboratory carefully. EMC Bayswater’s Normalize Site Attenuation (NSA) has a 3m diameter/volume. Products tested to EN 55011 or CISPR 11 referenced standards should have a maximum diameter of 1.2m and be less than 1.5m in height above the GRP in order to issue an accredited test report when tested at 3m. The antenna should be selected for the measurement should illuminate the product, thus various different antennas may be required. In addition to this certain specific antenna, requirements may apply for particular product standards and an example would be EN 60945.
If your product is a large product or is very tall we can advise you and recommend the best solution at the quoting stage. The actual limiting factors governing each test site and associated equipment is complex, we have determined our maximum site EUT dimensions and will advise accordingly. This could be either a 3m or 5m iOATS measurement at EMC Bayswater or through either a laboratory referral or sub-contracting for testing on a 10m iOATS or OATS. If needed we can make all the arrangements for the testing on a 10m measurement test site on your behalf.
If you like some further reading and more information, here is a very useful article from the interference Technology: Radiated Emission Measurements at 1/3/5/10/30 Meters.
