Electrical testing, in its most basic form, involves applying voltage or current to a circuit and comparing the measured value to an expected result. Electrical test equipment verifies the calculations within a circuit, and each piece of test equipment is designed for a specific application.
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It is the responsibility of a test technician to know which piece of test equipment to use for the task at hand and also to understand the limitations of the test equipment they are using. In this article, we will examine the most common pieces of test equipment used in the field.
Electrical test equipment should be considered a source of lethal electrical energy. Technicians must observe all safety warnings and follow all practical safety precautions to prevent contact with energized parts of the equipment and related circuits, including the use of appropriate Personal Protective Equipment.
Also known as a VOM (Volt-Ohm meter), a multi-meter is a handheld device that combines multiple measurement functions, including voltage, current, resistance, and frequency, into a single unit.
Multi-meters are primarily used to diagnose electrical issues in a wide range of industrial and household devices, including electronic equipment, motor controls, domestic appliances, power supplies, and wiring systems.
Digital multi-meters are the most common type of meter used today. However, analog multi-meters are still preferred in certain cases, such as when monitoring rapidly changing values or making sensitive measurements, like testing for CT polarity.
Megohmmeters are one of the most frequently used pieces of test equipment. Photo: TestGuy
Most commonly referred to as simply a megger, a megohmmeter is a specialized type of ohmmeter used to measure the electrical resistance of insulators.
Resistance values measured by megohmmeters may range from several megohms to several million megohms (terao-ohms). Megohmmeters generate high voltages through battery-powered internal circuitry or a manually operated generator, with outputs ranging from 250 to 15,000 volts.
Megohmmeters are among the most commonly used pieces of test equipment and can be employed to measure the insulation of various types of equipment, including circuit breakers, transformers, switchgear, and cables.
Related: Basic Test Equipment: Insulation Resistance Tester
Often referred to as a DLRO in the field, the low-resistance ohmmeter is used for making highly precise resistance measurements below 1 ohm. Low-resistance ohmmeters generate low-voltage DC currents using battery power, with outputs of up to 100A.
Resistance measurements are obtained using four terminals, known as Kelvin contacts. Two terminals (C1, C2) carry the current from the meter, while the other two (P1, P2) enable the meter to measure the voltage across the resistor. With this meter type, any voltage drop caused by the resistance of the first pair of leads and their contact resistances is disregarded by the meter.
Low-resistance ohmmeters are among the most commonly used pieces of test equipment and can be employed to measure the resistance of various types of equipment, including circuit breaker and switch contacts, cables and busways, transformers and generators, motor windings, and fuses.
Dielectric withstand (or hipot) testing checks for good insulation in medium and high-voltage equipment, which is the opposite of a continuity test. During this test, insulation is subjected to stress above nominal values to ensure minimal current leakage from the insulation to the ground.
Hipot test sets comprise a high voltage lead, a return lead, and a ground lead. The high voltage lead is connected to the device under test, with all other components grounded, and the resulting current is measured through the return.
If too much return current flows, the internal protection of the test set will trip. The hipot test is a go, no-go test, which means that the leakage current must not trip the test set, but there is no specific minimum acceptable value.
The output voltage can vary from 1 kV to over 100 kV, either AC at line frequency or DC, depending on the device under test. Very low frequency (VLF) withstand testing involves the application of an AC sinusoidal waveform, typically at 0.01 to 0.1 Hz, to evaluate the quality of electrical insulation in highly capacitive loads, such as cables.
Related: Power Cable Testing and Diagnostics Overview
A high current test set may consist of two components known as a control unit and an output unit, or these functions may be integrated into a single package. Low-voltage, high-current outputs are utilized for primary-injection testing of low-voltage circuit breakers.
The high-current or primary-injection test set comprises large transformers that reduce line voltage (e.g., 480V) to a much lower level, typically ranging from 2V to 15V. This significant reduction in voltage enables a substantial increase in available current output, often exceeding 15kA, particularly for short-duration testing.
Current output is regulated by a tap changer and a variable resistor. Integrated timers show the time between current activation and deactivation, indicating the time it takes for a circuit breaker to trip.
Circuit breakers can be directly connected to the high-current test set through a bus or cable. Depending on the size, this type of test equipment can also be used to test ground fault and other current relays by connecting directly to the switchgear bus.
Secondary test sets are designed by trip unit manufacturers to be used with a single style or family of trip unit using a proprietary connection. Photo: Switchserve
Circuit breakers equipped with solid-state and microprocessor trip units can be tested by injecting secondary current directly into the trip unit, rather than passing primary current through the CTs using a high-current test set. The primary limitation of the secondary current injection test method is that it only tests the logic and components of the solid-state trip unit.
Secondary test sets are designed by trip unit manufacturers to be used with a single style or family of trip units, typically using a proprietary connection. Test kits can vary from simple handheld, push-button style designs to more sophisticated suitcase units that operate similarly to a primary injection test set.
Handheld units are frequently used to bypass trip unit protective functions, like ground fault protection, when testing circuit breakers through primary injection.
Related: Primary vs. Secondary Injection Testing for Circuit Breakers
These are power system simulators used for testing protection devices in industrial and power systems. Relay test sets are equipped with multiple sources to test solid-state and multi-function numerical protection. Each voltage and current channel operates independently to simulate various power system conditions.
High-end relay test equipment can assess not only simple voltage, current, and frequency relays but also complex protection schemes, including communication-assisted line protection and protection schemes that utilize IEC -compliant IEDs (intelligent electronic devices).
Related: Protective Relay Testing and Maintenance Overview
Power Factor Test Sets offer a comprehensive AC insulation diagnostic test for high-voltage equipment, including transformers, bushings, circuit breakers, cables, lightning arrestors, and rotating machinery.
Test voltages are typically 12kV and below. The power factor test set measures the voltage and current of the device under test using a reference impedance. All reported results, including power loss, power factor, and capacitance, are calculated from the vector of voltage and current.
Tests are conducted by measuring the capacitance and dissipation factor (power factor) of a specimen. The measured values will vary when undesirable conditions are present, such as moisture in or on the insulation, the presence of conductive contaminants in insulating oil, gas, or solids, and the occurrence of internal partial discharges.
Test connections consist of a single high-voltage lead, (2) low-voltage leads, and a ground. Safety switches and a strobe light are included for operator protection, and a temperature sensor is employed to adjust test values. Power factor test sets are typically operated with a laptop computer connected via USB or Ethernet.
Related: 3 Basic Modes of Power Factor Testing Explained
Winding resistance measurements are a crucial diagnostic tool for evaluating potential damage to transformer and motor windings. In transformers, winding resistance can change due to shorted turns, loose connections, or deteriorating contacts in tap changers.
Measurements are obtained by passing a known DC current through the winding under test and measuring the voltage drop across each terminal, in accordance with Ohms Law. Modern test equipment for this purpose employs a Kelvin bridge to achieve accurate results. You can think of a winding resistance test set as a large, low-resistance ohmmeter (DLRO).
Winding resistance test sets typically have (2) current leads, (2) voltage leads, and (1) ground lead. The typical current range of a winding resistance test set is 1A to 50A. Higher currents have been found to reduce test times on high-current secondary windings.
Related: Transformer Winding Resistance: Test Methods and Procedures Explained
Three-phase TTR testing connection diagram. Photo: EEP.
The TTR test set applies voltage to the high-voltage winding of a transformer and measures the resulting voltage from the low-voltage winding. This measurement is known as the turns ratio. In addition to the turns ratio, these units also measure excitation current, phase angle deviation between the high- and low-voltage windings, and percent ratio error.
Transformer turns ratio test sets are available in various styles and test connections. However, all turns ratio testers have at least two high leads and two low leads. The excitation voltage of a TTR test set is typically less than 100V.
Related: Introduction to Transformer Turns Ratio Testing
CT test sets are compact, multifunctional units designed to conduct demagnetization, ratio, saturation, winding resistance, polarity, phase deviation, and insulation tests on current transformers. High-end CT test equipment can directly connect to multi-ratio CTs and conduct all tests on all taps with the press of a button, without the need to change leads.
Current transformers can be tested in their equipment configuration, whether mounted in transformers, oil circuit breakers, or switchgear. Modern CTs with multiple voltage and current outputs can also function as a relay test set when operated with a laptop computer.
Related: 6 Electrical Tests for Current Transformers Explained
Traditional field testing of vacuum interrupters uses the high-potential (hi-pot) test to assess the dielectric strength of the bottle. This test produces a go/no-go result, which doesnt determine when or if the gas pressure inside the bottle has dropped to a critical level. Unlike the hi-pot test, testing vacuum interrupters using magnetron atmospheric condition (MAC) principles can offer a viable way to assess the condition of vacuum interrupters before they fail.
The magnetic field test is set up by placing the vacuum interrupter into a field coil, which generates a constant DC current during the test. A continuous DC voltage, typically 10 kV, is applied to the open contacts, and the current flow through the vacuum interrupter is measured.
Related: Magnetron Atmospheric Condition (MAC) Testing Principles
The ground resistance test set functions by injecting a current into the earth between a test electrode and a remote probe, measuring the voltage drop caused by the soil to a designated point, and then using Ohms Law to calculate the resistance.
Ground resistance test sets come in various styles, with the most common ones being the 4-terminal unit for soil resistivity testing and the 3-terminal unit for fall-of-potential testing. Copper rods or similar stakes are employed to establish contact with the earth, along with spools of small stranded wire to facilitate long-distance measurements.
Clamp-On Ground Resistance Testers measure ground rod and grid resistance without the need for auxiliary ground rods. They provide accurate readings without disconnecting the ground system under test, but they do have limitations.
Related: 4 Important Methods of Ground Resistance Testing
Power recorders are devices used to gather voltage and current data that can be downloaded into software for the analysis of electrical system conditions. These are troubleshooting tools employed to identify electrical issues such as voltage swells, sags, flicker, and poor power factor.
Power recorders can also be utilized to measure power consumption over a period of time, which is valuable for engineers planning system expansion or for customers looking to audit their energy bills. There are various types of power recorders available, varying in size, accuracy, and storage capacity.
Installation of a 3-phase power recorder involves wrapping conductors with split-core CTs and clipping a set of leads to system voltage and ground. The recorder is set up to measure according to the system configuration for a specified time period and can also be viewed in real time using a PC or integrated screen.
Related: Power Quality Analysis: Basic Theory and Applications Explained
Thermal imagers are cameras that detect invisible infrared radiation and convert that data into a colored image on a screen. Infrared cameras are primarily used for inspecting the integrity of electrical systems because the testing procedures are non-contact and can be carried out swiftly, even while the equipment is in service.
Comparing the thermal signature of normally operating equipment to the one being assessed for abnormal conditions provides an excellent means of troubleshooting. Even if an abnormal thermal image is not fully understood, it can still be used to assess whether further testing may be necessary.
Thermal imagers are categorized based on their accuracy and detector resolution. High-end infrared cameras offer high-resolution image capture and temperature accuracy down to a tenth of a degree or less.
Related: Infrared Thermography for Electrical Distribution Systems
Vibration analyzers are used to identify and locate the most common mechanical faults (such as bearings, misalignment, unbalance, and looseness) in rotating machinery. When mechanical or electrical faults develop in motors, vibration levels increase. These increases in vibration and noise levels occur at varying degrees of severity as a fault develops.
Accelerometers are used to capture vibration measurements while the equipment is in operation, and the data is then imported into software for analysis. As the machine under test operates, the accelerometer detects its vibration along three planes of movement: vertical, horizontal, and axial.
Arcing, tracking, and corona all result in ionization, which disrupts the surrounding air molecules. An ultrasonic tester detects high-frequency sounds produced by these emissions and converts them into the human audible range.
The sound of each emission is listened to using headphones, and the signals intensity is observed on a display panel. These sounds can be recorded and analyzed through ultrasound spectral analysis software for a more precise diagnosis.
Normally, electrical equipment should be silent, although some equipment, such as transformers, may produce a constant hum or steady mechanical noises. These sounds should not be confused with the erratic, sizzling, frying, uneven, and popping sounds of an electrical discharge.
Ultrasonic detectors are also valuable for detecting air leaks in transformer tanks and gas-insulated circuit breakers.
Load banks are utilized for commissioning, maintaining, and verifying electrical power sources, such as diesel generators and uninterruptible power supplies (UPS). The load bank applies an electrical load to the device under test and dissipates the resulting electrical energy through resistive elements as heat. These resistive elements are cooled with motorized fans within the load banks construction.
Multiple load banks can be interconnected if necessary. Some load banks are entirely resistive, while others may be purely inductive, purely capacitive, or a combination of the three. Load banks are the most effective way to simulate, confirm, and validate the real-life demands on critical power systems.
Battery Impedance Test Equipment is primarily employed in substation and UPS applications to assess the condition of lead-acid cells by measuring important battery parameters such as cell impedance, cell voltage, inter-cell connection resistance, and ripple current. All three tests can be conducted with a single unit.
The battery impedance tester operates by applying an AC current signal across an individual cell and measuring the AC voltage drop caused by that current, as well as the current within the individual cell. It then calculates the impedance. The standard lead set used is a dual-point, Kelvin-style lead. One point is for applying the current, and the other is for measuring the potential.
Related: 3 Simple and Effective Tests for Battery Systems
The time-domain reflectometer (TDR) is used to determine the characteristics of electrical paths by transmitting a signal and observing reflected waveforms along a conductor. If the conductor is of a uniform impedance and is properly terminated, then there will be no reflections and the remaining incident signal will be absorbed at the far-end by the termination. If there are impedance variations, due to a fault or poor terminations, then some of the incident signal will be reflected back to the source. The operating principle of a TDR is similar to that of radar.
An optical time-domain reflectometer (OTDR) is the optical equivalent of an electronic time domain reflectometer (TDR). The OTDR injects a series of optical pulses into the optical fiber under test and extracts light that is scattered or reflected back from points along the fiber. The strength of the return pulses is measured and integrated as a function of time, and plotted as a function of length of the fiber. This is equivalent to the way that an electronic time-domain meter measures reflections caused by changes in the impedance of the cable under test.
If youre considering an anechoic chamber (some call it an EMC chamber) for in-house emissions, immunity or wireless testing, this guide will help you to determine the important factors to consider when youre choosing a make and model. But even if youre not in a position to purchase a chamber, this guide will help you to understand more about the chamber at your 3rd party testing providers facility.
Here are some of the topics were going to cover in this post:
What are anechoic/EMC chambers and how do they work?
What types of EMC chambers are available and how do they differ?
What are the design characteristics of an EMC chamber and how do they affect performance?
Typical test equipment required for radiated emissions testing and radiated immunity testing in a chamber
EMC chamber costs: new vs. used
Used EMC chambers for sale
Dismantling, re-assembly and transportation costs
The word anechoic more or less means without echo. The first part of the word an, meaning not or without and the second part echoic coming from the Latin word echo which itself stems from the Ancient Greek word χώ (ēkh) and the related word χή (ēkh, sound).
The key characteristic of an anechoic chamber is that its designed to absorb reflections of waves within the chamber rather than have them bounce off the walls which would cause an echo. These chambers, if designed and assembled correctly, also do a great job at keeping waves from entering the chamber i.e. they provide shielding from outside interference.
If youve ever been in a huge room or cave and shouted echo!, youll know that you usually get some kind of reverberation where you hear your voice repeat a few times.
To avoid this phenomena, anechoic chambers are lined on the inside with materials that absorb waves (which well get into below).
There are many types of anechoic chambers that are designed for different applications. Some of the most common uses and types are for things like audio recording, radiated emissions testing, radiated immunity testing, wireless transmitter (RF) testing, antenna testing and specific absorption rate (SAR) testing.
Audio chambers are the odd man out here because they deal with absorption of sound waves rather than electromagnetic energy which is common to all of the other types of chambers. In this guide well be dealing with electromagnetic (EMC/EMI) anechoic chambers.
In terms of electromagnetic testing, which is of course my focus here at EMC FastPass, there are a few different types of chamber that can be super useful. Your choice really depends on your application (and also the size of your wallet!).
The most common type of EMC testing chamber by far is the semi-anechoic chamber. The word semi indicates that its only partially able to absorb electromagnetic energy and one of the reasons for that is that the floor of the chamber is reflective rather than absorptive. Why is it reflective? For a couple of main reasons:
But, that isnt to say fully anechoic rooms (FAR) dont exist. An effort has been underway for many years to develop and adopt a standard for making repeatable measurements in a FAR (such as that outlined in EN -3).
One of the reasons for the push is that if measurements made in a FAR can be proven to have good correlation to an OATs (see below) or a SAC, then there could be significant cost savings to be had. The cost savings relate to:
You may also need a fully anechoic room if youre attempting to remove all reflections for transmitter/receiver testing, which helps to remove multipath artifacts. Its a lot easier to create a FAR for a specific transmitter because youre predominantly interested in absorbing a very tight band of frequencies.
So, the quest for a lower cost FAR test chamber exists, but studies have highlighted repeatability issues as well as correlation issues to measurements made at the more trusted SAC and OATS test facilities.
Semi-anechoic chambers and OATS are the preferred method for almost all radiated emissions testing standards such as the ubiquitous ANSI C63.4 and CISPR 16.
The standard choice for in-house or 3rd party emissions testing facility is either a semi-anechoic chamber or an open area test site (OATS).
In many ways the OATS is preferable to the SAC because there are no walls in the vicinity of the measurement area. Even with plenty of absorbing material on the walls of a SAC, there will still be a portion of the wave energy that gets through the absorber and reflects back off the metallic surface of the chamber wall. The receiving measurement antenna in that case picks up the wave coming from the equipment under test (EUT), the reflection off the floor, and the partial reflection off one or more walls.
So it can be harder to achieve acceptable measurement accuracy (typically +/- 4 dB according to ANSI C63.4) in a SAC than it is at an OATS.
The main downside of an OATS of course is that since there is no shielded chamber, the measurement antenna also picks up whatever is floating around in the air at the test site. That can include things like the FM radio band, cell bands, air traffic, maritime radio, emergency services and much more. That background noise (called ambient noise) often makes it hard to see the emissions coming from an EUT so testing usually goes a lot slower compared to testing in a SAC.
The ambient noise problem gets worse the more transmitters you have in the vicinity, so within or near a large city it makes testing on an OATS next to impossible. The ambient noise also prevents automated emissions testing because human intervention is required to distinguish between noise from the EUT and background noise. Long story short testing at a SAC goes much faster.
The other downside of an OATS is that you cant legally perform radiated immunity testing because its illegal to broadcast at the power levels typically required to generate the field strengths called out by the RI root standard (IEC -4-3) e.g. 3 V/m, 10 V/m or higher for MIL/aerospace. So if you want to do radiated immunity testing as well, youre going to need some kind of chamber (ideally a SAC, but there are other options which Ill discuss below).
A poor mans SAC is a GTEM which stands for Gigahert Transmissive Electromagnetic cell. These are much smaller than a SAC and much more cost effective, but they have their limitations.
I went into detail on GTEMs in previous articles here and here.
You can legally use GTEMs to measure radiated emissions for FCC part 15B and 18 devices (with some caveats) and perform radiated immunity testing according to IEC-4-3 appendix D. In terms of radiated emissions testing, IEC/EN-4-20 outlines the usage and restrictions of a GTEM for testing purposes.
The two main downsides of a GTEM are the limited EUT size and the measurement error at lower frequency ranges (approximately under 200 MHz). Measuring emissions coming off cabling, which is a dominant failure mode due to common mode currents, can be really tricky in a GTEM. This paper outlines the effects of different cable configuration on measurements within a GTEM.
GTEMs come in many sizes ranging from 30cm or so all the way up to 20ft long. If youre going this route, its important to get a GTEM that can cope with your largest product, both in terms of fitting it in the door and having a sufficiently large uniform test volume.
To compensate for the increased measurement uncertainty at frequencies below 200 MHz, I would advise using a current clamp as well to measure the common mode current present on every cable in the 30 MHz 200 MHz frequency range. In that band, most of the radiated emissions failures are due to common mode current on external cabling, so between the GTEM and the cable clamp, youre going to be able to find and quantify absolute emissions levels quite accurately.
A SAC is definitely preferable to a GTEM, but if you have space and/or budget concerns, a GTEM might be a better way to go. Some companies have both, reserving the GTEM for pre-compliance testing and the SAC for full-compliance testing.
A good app note from Nokia compares measurements from a GTEM to a SAC to an OATS. And York EMC have another excellent app note on GTEMs here.
An electromagnetic reverberation chamber (RVC) (or mode-stirred chamber (MSC)) is where t
he whole chamber is made to resonate. They are predominantly used as a cavity resonator to perform radiated immunity testing. Due to the high Q-factor of the chamber and the almost complete reflectivity of the walls and floor, an electromagnetic field of a given strength can be created using a much smaller power amplifier compared to that needed in a SAC.he whole chamber is made to resonate. They are predominantly used as a cavity resonator to perform radiated immunity testing. Due to the high Q-factor of the chamber and the almost complete reflectivity of the walls and floor, an electromagnetic field of a given strength can be created using a much smaller power amplifier compared to that needed in a SAC.
Power amplifiers can easily cost tens or hundreds of thousands of dollars, especially if you require the high field strengths called out by standards such as MIL-STD-461 and DO-160 (200 V/m). In contrast field strengths of tens of thousands of V/m can be produced in a reverberation chamber.
In one of these things it would be pretty easy to exceed the RF exposure guidelines set out in the FCCs OET65 (Human exposure to RF EM fields) so I wouldnt recommend walking into one while its on if you ever plan on having kids.
Reflectors like the ones shown in the picture are used to stir the standing waves so that all points of an EUT can be immersed in a field of a given strength.
An RF shielded room forms the basis for a semi-anechoic chamber. Its a well sealed metal box, that offers electric and magnetic field shielding effectiveness over a given range of frequencies.
This picture on the right is from a 10m chamber that I bought and dismantled earlier this year.
There are of course different shielding materials and configurations, which carry with them different shielding performance. Factors such as the material type and thickness affect electric and magnetic field attenuation as you can see in the table below.
Sometimes copper mesh is adequate, as in the case of a screened room (like the one shown on the right). If you only need 50 or 60dB of attenuation, these will save you a lot of money compared to a full shielded room.
If you need maximum shielding effectiveness, youll want double electrically isolated shielding, which has two layers of shielding material (one on the inner surface and one on the outer surface), typically connected at only one point the main grounding point.
Inquiring about the cost of a full size semi anechoic chamber, for many manufacturers at least, often induces mild heart palpitations.
Engineers searching for alternatives may come across a small shielded enclosure (e.g, 2ft x 1ft x 1ft) and wonder whats stopping them from using that for emissions or immunity testing instead.
One of the main issues with these shielded boxes is reflections. The physical size will limit the dimensions of the absorbing material and therefore it wont be possible to fit absorbing cones in that are very effective at lower frequencies.
Also, with such small dimensions, youll be measuring or transmitting in the near field for any frequencies under a couple of GHz.
Both of these factors lead to measurement of unintentional emissions being pretty useless.
So with these limitations, what can they actually be useful for?
Lets not forget that a semi-anechoic chamber isnt just useful for emissions testing. With the right types and configurations of absorber material, its perfectly acceptable to do RF transmitter testing, either for transmitter characterization (such as emissions bandwidth, power spectral density and spurious emissions measurements), but also for testing links between 2 or more transmitters.
If you want to learn more, please visit our website Hengfeng.
There are special types of anechoic chambers designed specifically for characterizing the field patterns from antennas. While I think its possible to make antenna pattern measurements in a standard semi anechoic chamber, a specialized antenna measurement chamber is likely to provide more accurate data along with faster scan times.
Something like the antenna measurement chamber shown on the right shows an example of a modular antenna measurement chamber with a full treatment of hybrid RF absorber.
We sometimes have used antenna measurement chambers for sale. Check out our online store for current inventory.
One of the main considerations with a semi-anechoic chamber is the physical distance between the EUT (equipment under test) and the measurement antenna.
For RF emissions and immunity testing purposes, separations of 3 meters, 5 meters and 10 meters are the most common.
The separation is important because it defines the frequency at which you transition from measuring within the near field to measuring within far field.
Ideally you always want to measure in the far field because its only at that point that any wave is setup as a plane wave.
In the near field, the field strength may diminish at 1 over r cubed or r squared and may vary wildly between 2 points (where r is the separation between the source and the antenna). In the near field, the behaviour is very much dependent on the source of the emission. If its high dV/dt, the near field will be predominantly E-field. If its high dI/dt, the near field will be predominantly H-field.
Whereas in the far field, the field is stable and diminishes quite predictably at 1 over r.
Often particular standards mandate a specific separation distance, but will allow you to use different separations by extrapolating the limits to the new separation. For example, in the Industrial, Scientific and Medical standard CISPR 11, allowable separations vary depending on the size of the EUT as well as the class (A or B) and the group (1 or 2).
A larger separation is generally considered to be more accurate because its more likely that youll be measuring in the far field rather than the near field.
3m separation would be my recommendation for minimum separation for radiated emissions testing in the 30 MHz 1 GHz range.
If your chamber is for antenna measurements, the separation between the AUT (antenna under test) and the measurement antenna will be guided by your transmission frequency and the size of your antenna. If youre transmitting at 2.4 GHz, 1 wavelength is only 12.5cm, so you can get away with much smaller separations (and much smaller anechoic chambers).
Some chambers use a reflector to increase the measurement distance within a fixed chamber size to get closer to a far field measurement.
The next thing to consider is what frequencies you need to measure within your chamber.
Chambers are fairly well known to not perform very well at lower frequencies (e.g. below 100 MHz) due to limitations to pyramidal absorber length (discussed in more detail below). Thats why its often a good idea to re-measure the lower end emissions on an OATS.
But you can really affect the upper frequency range by your choice in absorber materials and also the effectiveness of the shielding.
For RE testing of unintentional radiating devices, youll typically want to measure from 30 MHz up to several GHz, with the upper limit depending on the highest frequency present in your design.
The FCC defines the upper measurement range for unintentional emissions measurements, which as you can see from the table above depends on the highest frequency generated or used in your design.
For RI testing, youll typically want to go from 80 MHz up to MHz. Your standard may define higher or lower values than this, but for many devices, 80 M 2.7 G is typical. Expansion to 6 GHz is planned for some standards, so it may be prudent to design your chamber up to at least 6 GHz if you plan to undertake radiated immunity testing.
For RF (or wireless transmitter) testing, its often required to measure spurious harmonic amplitudes up to the 10th harmonic. E.g. for a 2.4 GHz transmitter, youll need to verify all harmonics up to 24 GHz are within compliance.
The FCC mandates measurement of spurious emissions from Part 15 wireless transmitters up to the 10th harmonic
Semi-anechoic chambers often come speccd up to 40 GHz, but you can go higher or lower depending on your requirements. This will typically affect your choice of absorber material.
The quiet zone characteristic defines an area within your chamber that your EUT must fit within, if the measurement accuracy characteristics are to be met. The larger the EUTs you want to accommodate, the larger your chamber needs to be to maintain measurement accuracy.
As you can imagine, increasing the quiet zone means you need to increase your chamber width (and possibly length and height) which increases amount of expensive absorber material to line the walls.
The RF absorbing material that lines the inside walls of your chamber is crucial to the overall performance. Its goal is to absorb 100% of any incident electromagentic waves and convert that energy to heat.
In reality all absorbing materials are not perfect at absorbing RF energy and their performance is very dependent on a number of factors such as the incident angle of the waves, the frequency and the power levels. Reflections will occur, but your choice of absorbing material will define how bad the reflections are and how much energy is absorbed.
Ferrite tile typically lines all of the walls and ceiling of an anechoic chamber. Ferrite works best at lower frequencies (e.g. below 100 MHz) and it works in combination with hybrid absorber foam (see below) to minimize reflections over a wide band of frequencies.
Looking at the absorption profile of a typical ferrite tile, you can see that its fairly useless if the incident angle of the wave deviates from normal by any significant degree.
The tiles are usually mounted onto a piece of wood before being screwed into the chambers shielding panels. The wood spacer actually acts as a dielectric spacer between the tile and the grounded wall panel, creating a capacitive effect.
Sometimes for emissions pre-compliance chambers as well as immunity chambers, hybrid absorber wont be required on top of the tile. It really depends on the upper frequency range of the signals you want to measure.
Also, you can get away with less than full coverage (specular coverage) at the cost of measurement accuracy. Ferrite tile is one of the most expensive components in a chamber, so if you dont need a fully compliant room, this may be a good area to save money.
If your EUT emitted only 1 frequency, it would be possible to absorb all of the wave energy with 1 sheet of paper coated with a substance of surface resistivity of exactly 377 Ohms (the wave impedance of free space), placed at exactly 1/4 wavelength from the reflecting wall (assuming the wave was incident at right angles to the wall). This configuration is called the Salisbury Sheet.
But of course, were trying to absorb a large band of frequencies at many incident angles, so we eventually end up with a tapered cone that absorbs different wavelengths of incident waves. Also, the pyramidal shape allows from a gradual transition of impedance from free space impedance (377 ohms) up to the impedance of the lossy ferrite tile absorber base.
Just as in the case of high speed digital design on a PCB, any sudden change of impedance will cause reflections, so the pyramidal structure is an attempt to minimize this effect.
This type of absorber can be made from a number of materials such as polyurethane or polystyrene, with either of these materials typically loaded with a lossy material such as carbon.
There are lots of papers on how absorbers work, but this one from Glen Dash is one of the best (i.e. most concise and easy to understand) Ive found.
To be able to maintain excellent shielding effectiveness, everything entering or exiting (penetrating) the walls or floor of an anechoic chamber must be filtered or isolated.
That includes everything from the power supply input for the EUT and turntable/mast, RF cables (connecting to the antennas or LISNs) and any other I/O and control cables.
If you just fed the cables through a tight hole, the chances are that RF leakage would occur in both directions. The effect of that is that you would begin to see ambient RF signals within the chamber when the whole purpose of a chamber is to keep them out.
The amount of RF leakage youll get, or the amount of reduction in shielding effectiveness, would depend on the dimensions of the hole or aperture, the wavelength of the signal that encounters the hole, and whether the source is in the near or far field.
The doors used to enter and exit an anechoic chamber are a common area where the shielding effectiveness can be weakened.
Since it isnt possible to have a perfect press fit of metal to metal around the door, finger stock (shown below) is the preferred method to ensure that there is a tight seal the whole way around the door.
A crucial factor in determining the performance of your anechoic chamber is measurement accuracy. There are many factors that affect measurement accuracy to take into account:
For EMC pre-compliance measurements, you may be content with a measurement thats +/- 6 dB accurate. Whereas for full compliance measurements, the typical required accuracy is at least +/-4 dB.
Relaxation of measurement accuracy generally equates to a lower overall cost of a chamber because you can use cheaper absorbing material or less of it.
Knowing which types, sizes and configurations of absorber to select is a tricky business. Typically this process is taken care of now by simulation software.
Usually a chamber needs to be fully lined with ferrite tiles (at least on the walls and ceiling), but its often possible to use only specular anechoic coverage of absorbing foam material, whilst still meeting the +/- 4dB measurement criteria. To be able to determine which absorber material and size to pick, as well as where to place it, thats determined by a number of factors including:
Your chamber manufacturer will likely offer several stock packages or use software to customize according to your requirements and budget.
Ground plane flatness is essential for ensuring a repeatable reflection off the ground. In ANSI C63.7 (Construction of Test Sites for Performing Radiated Emission Measurements), the criteria or rule of thumb for ground plane flatness is called the Rayleigh roughness criteria which approximates that the ground plane should not have bumps or valleys exceeding 5cm at 3m antenna separation, to maintain measurement accuracy.
Using a flat, sturdy sub-floor beneath your metallic groundplane will help to ensure that the surface stays flat. Also the type and quality of welds or joints used to join together the metal sheets will have an impact.
Its arguable that ground planes at an OATS are far more susceptible to warping due to expansion and contraction caused by direct sunlight than a chamber.
Choosing the right size of semi-anechoic chamber is critical because it defines what measurement accuracy you can expect as well as which standards you can perform full compliance testing to.
One of the main considerations in selecting your chamber size is the maximum size of the EUT youll be testing. If youre testing small products, you can get away with a smaller quiet zone as I discussed before, meaning the chamber width can be kept small.
Most chamber manufacturers quote the quiet zone dimensions for each of their standard chamber sizes.
As I mentioned earlier, if you only need to make pre-compliance measurements, you can get away with a much smaller chamber. This allows you to make big savings on not only floor space, but also the cost of ferrite tile and absorbing materials.
ETS Lindgren has the Spacesaver series for example, which are significantly smaller than their standard 3m, 5m or 10m chambers. Typically with small chambers like these, the antenna height will be limited or even fixed, which certainly limits the measurement accuracy. But, for getting a good idea of emissions, or troubleshooting known emissions problems, they can be a good option.
For many standards, the anechoic chamber size is not clearly defined. The litmus test is often whether the chamber meets the NSA requirements (see below) of the standard for a given EUT size.
But, in general a chamber sized to accommodate a 3m EUT to measurement antenna separation (e.g. approx 30ft x 20ft x 20ft) will be adequate for most international radiated emissions measurement standards.
5m separation is considered superior to 3m separation because the measurements generally have better correlation to measurements made at 10m separation (the gold standard of chambers). But of course as you increase the antenna separation, the amount of absorbing material lining the walls greatly increases which bumps up the cost.
The maximum height of your chamber will be defined by not only the exterior chamber dimensions, but also whether its built on top of a pit. Typically an anechoic chamber will have at least 2 ft below the ground plane to allow for:
If you have the luxury of pouring a concrete pit, thats great the groundplane can be made to be flush with the rest of the floor (externally).
Other options are to build on top of a floor and have steps up to the chamber door, or in some cases, you can raise the whole floor of the room to bring it flush to the chamber floor.
These decisions are of course dependent on the constraints of the space youre working with.
Youve probably gathered by now that chambers are far from perfect. To define how far from perfect they actually are, there are a couple of standard tests that determine two main aspects of their performance.
The first is to determine how good the metal shell is at preventing RF signals getting inside. There are a number of areas where RF signals can leak into the chamber, such as at panel joints, cable penetrations and doors.
The shielding effectiveness is usually tested at 1 GHz (unless the customer requests a more in depth test at other spot frequencies) by placing a transmit antenna outside the chamber and a measurement antenna inside the chamber.
The difference between the transmit power and the receive power defines how effective the shielding is.
The diagram to the left shows how electric and magnetic near fields behave in proximity to a breach in the shield.
Pro-tip: to find the source of a leak, dial down the transmit power until the signal is barely perceptible inside the chamber. Then move the receive antenna around the chamber to find the maximum amplitude. This normally allows you to isolate the leakage area.
There is a publicly searchable database of every FCC listed/listed & accredited measurement facility in the world. For each facility, you can view their submitted data to see how the measurement accuracy varies with frequency.
A dirty little secret of the EMC test lab industry is that even where labs meet the +/-4dB measurement accuracy specification, lab to lab variance can be much more than 8 dB. The FCC performs round-robin testing of EUTs around every lab involved in their TCB program. Statistics of the round robin are not published, but discussions in webinars by TCB participants indicate that variations of more than 40dB are not unheard of.
Often EMC test labs with more than 1 anechoic chamber will purchase an identical size, make and model of chamber so that internally at least, the chambers have good correlation. This helps them to avoid an awkward conversation justifying differences in measurements with their customer.
The VSWR method has been used for decades to qualify characteristics of anechoic chambers such as the size of the quiet zone and the reflectivity of the absorbing material. Some also prefer the VSWR method to the NSA to characterize measurement accuracy.
Although there are many ways to perform a VSWR test, most are variations of a method based on moving a transmitting source along a measurement axis to sample a standing wave of an electromagnetic field.
Check out this and this for more information.
Now that you have your semi-anechoic chamber, what else do you need? Unless you like sitting in a very quiet room, listening to nothingness (or in my case some very loud, high pitch tinnitus), youre gonna need some gear to make the most of the space.
What youll need depends on the testing you want to do in there, but below Ill outline the main pieces of gear for emissions, immunity and RF testing.
The type of measurement antenna you get really depends on what frequency ranges you need to measure. I outlined the various types of antennas used for EMC testing in the radiated emissions section of the EMC testing beginners guide. Typically though, youll be looking at a biconical antenna for sub 300 MHz, a log-periodic antenna from 2 or 3 hundred MHz up to 1 or 2 GHz, then a horn antenna for >1 GHz.
Many labs use a biconilog/bilogical antenna which is a combo of a biconical and log period, to cover a larger range with only one antenna.
The other main piece of equipment youll need is either a spectrum analyzer or EMI receiver. These two pieces of gear essentially do exactly the same thing i.e. they measure the voltage picked up from the measurement antenna across a range of frequencies. However the receiving circuitry inside them is really different, giving them each pros and cons.
If you want to do full compliance testing, youll typically need an EMI receiver (CISPR compliant). For all other testing, a much lower cost spectrum analyzer equivalent will most likely suffice.
Choosing the right RF cable is critical. Select one with low loss all the way up the highest frequency of interest. Since cable losses vary proportionally with cable length, youll want to factor in how far your receiver is from the antenna when working out what magnitude of cable attenuation is acceptable.
Youll most likely want some kind of automation software to (a) add/subtract factors for antennas, amplifiers, attenuators and cables (b) speed up testing (c) produce professional looking test reports.
See RF section below for more suggestions and information.
Radiated immunity testing uses mostly the same equipment as radiated emissions testing, with only a couple of additions.
You can use exactly the same antennas (e.g. hybrid, horn) as used for radiated emissions testing, only they are driven hard rather than used in receive mode. The only caution here is to ensure that your antennas can handle the power levels that you need to drive to generate the field strengths called out in the applicable standards.
If your antennas arent up the job, youre gonna have an expensive day.
The signal source for the radiated field is always a signal generator. For standard radiated immunity testing according to EN-4-3, the signal is typically swept from 80 MHz up to a few GHz. The carrier frequency is a sine wave, and the signal is usually amplitude modulated with a 1 kHz signal (80% AM). These parameters can change depending on the standard, so check yours to determine the spec of signal generator youll need.
Sometimes itll be necessary to swap out a signal generator mid way through radiated immunity testing to accommodate a different frequency band. If you want to get really fancy, you can buy GPIB controlled switches and invest in good automation software to deal with signal generator and power amplifier swapping automatically.
Power amplifiers are necessary to generate the power levels required to drive antennas hard enough to create the field strength levels called out in the standards.
For example, you might need a 50W amplifier to achieve 10 V/m at 3m across the 80 MHz to 1 GHz band. The power of amplifier you need is very much dependent on the specifications of your antenna (i.e. gain, VSWR) as well as the attenuation in the cabling.
Again, automation software is pretty necessary for any kind of radiated immunity testing other than troubleshooting at specific spot frequencies. For radiated immunity automation, the software needs to store the amplitude settings for the signal generator that correspond to a given field strength at a fixed distance from the transmit antenna.
Using an isotropic field probe, the software can calibrate the field levels across a band of frequencies and store those sweeps. Any changes to your test setup after calibration will probably totally nullify your calibrated field. That includes changes to: in-line atteunuation, antenna separation/orientation, RF cable type/length and basically anything else involved in generating the field.
RF switches arent necessary, but if you want a really smooth and fast setup then theyre a good option. They allow the automation software to select between different signal generators and/or power amplifiers so that you can sweep the whole frequency range without having to manually switch over cables and equipment. If youre doing lots of RF testing, it could be worth the investment.
Attenuators are useful for a couple of reasons:
An isotropic field probe is necessary to confirm that youre generating adequate field levels at all locations within the test volume. If I remember correctly, the field levels (measured in V/m) are calibrated by measuring at 0.5 m intervals within the test volume (e.g. 1.5 m cubed), with acceptable levels being defined at between -0dB (i.e. cant be lower than specified)/+6dB at not less than 75% of measured grid points (e.g. 12 of 16).
These days, the standard necessitates that the field is quickly verified at one point before each EUT is tested which ensures that you didnt do anything daft with the cables, connections or software.
Before you even think about investing in a radiated wireless test set up, make sure to check out how useful conducted RF testing can be. Its so much cheaper (<$5k as opposed to >$100k for a chamber) and covers 90% of the RF tests that youll need to do.
For wireless testing, youre probably going to need some additional hardware:
The type of antennas you need very much depends on the frequencies you need to measure. One hybrid broadband antenna can cover the frequency range 30 MHz up to a couple of GHz, but you may need to go higher and lower than that band.
Often youll need to measure spurious radiated emissions up to the 10th harmonic of your fundamental transmission frequency, in which case youll need at least one horn antenna.
When youre measuring low level radiated emissions, often youll need to boost the signal received from the measurement antenna to elevate the signal above your analyzers noise floor.
Conversely to amplifiers, attenuators help to ensure that input signals meet the input requirements of your analyzer. In the case of a spectrum analyzer, that typically means saturation of the sensitive diode mixer circuit on the input.
If you do overdrive your analyzers input, the best case scenario is that you get some gain compression which results in inaccurate measurements. Worst case scenario is that you toast something expensive on the input stage.
Either way, its a good idea to add enough attenuation on the input signal path to ensure that your analyzer input stays within the specified operating range.
Its usually better to use external attenuation rather than rely on the attenuators internal to your analyzer, for a couple of reasons. The first is that you can size your external attenuators to have an increased ability to dissipate power. Internal attenuators are usually very limited in their power handling capability, so without carefully calculating the powers involved, its relatively easy to cook them.
The second reason that its usually better to use external attenuators is that the noise floor will appear to be lower. If you use internal attenuation, the spectrum analyzer will compensate by adding the attenuation value to not only the signal but also the noise. So, if youre trying to measure a weak signal, internal attenuation may swamp the reading. With an external attenuator instead, the signal will be attenuated, but the noise floor will remain the same on the analyzer screen.
Automation software is typically necessary to store and compensate for transducer and amplifier factors. Without compensating for these factors, you wont be able to calculate the absolute magnitude of the signal.
Automation software also allows you to generate a nice test report with a plot and tabulated data. To use automation software, youll need to connect your PC to your analyzer, which is usually accomplish via a GPIB communications port. PCI cards are available from National Instruments to give your PC a GPIB port. External USB to GPIB converters are available, but I could never get mine to work.
Manufacturers such as Rohde & Schwarz and Keysight (Agilent) often package their own automation software with their equipment. Typically it costs a fortune and there is an annual support fee.
Alternatively, you might want to investigate a free option called EMCware from Amplifier Research.
You could also perceivably write your own software in Labview or even excel. Theres a GPIB toolkit from KE5FX that could be useful if you want to go that route.
Theres no getting around the fact that buying a semi-anechoic chamber is an expensive proposition. For larger companies it can be a no-brainer in terms of ROI, but for smaller to medium sized hardware companies, the decision to buy one and train up staff vs continuing to use a 3rd party test lab isnt an easy one.
Occasionally youll find a used chamber for sale, and they often represent a huge saving.
Due to the high cost of absorbing materials (tile and foam) and the increase in wall/roof area with increasing antenna separation, a 5m chamber is significantly more expensive than a 3m chamber and a 10m chamber is significantly more expensive than a 5m chamber.
But heres a ballpark of what you can expect for pricing. These are for fully compliant chambers.
Antenna Separation New Used
3m $300k-$750k $250k-$450k
5m $500k-$1m $350k-$750k
10m $1m-$2m $500k-$1m
If youre looking for used chambers, drop me a line and we can chat about your requirements.
If youre looking for used anechoic chambers, navigate over to our test equipment store for current inventory.
We buy and sell new and used anechoic chambers.
So as you can see, there are great savings to be had if you find a used anechoic chamber for sale, but there are a few things to keep in mind:
Ways in which your installer can screw up the shielding effectiveness:
Ways in which your installer can screw up the absorption performance (affecting measurement accuracy):
There are some additional costs to keep in mind if youre purchasing a used semi-anechoic chamber.
Dismantling a chamber can easily take 5-30 days depending on the size and the amount of absorber/tile you need to take off the walls.
Pricing from professional chamber installers varies wildly, but expect to pay $25k-$100k to dismantle a chamber once all costs and equipment have been factored in and almost double those figures for re-assembly due to the extra time needed.
Transportation is generally via one or more large (53 ft) trucks or in some cases via shipping container. Expect to pay $1k-$5k for shipping costs per truck within North America. Also the amount of time it takes to load up trucks or containers can be very significant.
So even if you find a used anechoic chamber bargain, keep in mind that there are potentially significant costs for relocation.
Check out my current inventory of used chambers. Ive also partnered with a chamber manufacturer to be able to offer new chambers.
Ok that was a not so brief outline of several types of anechoic chamber. Did you find what you were looking for? Have any anechoic chamber experiences of your own to share? Let me know in the comments below.
Andy Eadie is a former senior hardware design engineer and former EMC test lab owner. Hes had a weird fascination with magnets since 4 years old and has been publishing articles, eBooks and online courses since he founded EMC FastPass in .
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