The Mouth and Ears of EMC Test

To evaluate a product for susceptibility to EMI/RFI, the application demands an antenna that can handle high power. When testing for excessive emissions from a new or redesigned product, you need a sensitive antenna to pick up unwanted RF. Yes, for evaluating a product’s susceptibility and emissions, you should have two good antennas—the equivalent of a good mouth and a pair of good ears.

In a search for the mouth and ears of an EMC test system, of course you want ideal equipment. The dream antenna has a very wide frequency range to cover the entire spectrum of interest so you don’t have to change antennas during a test sequence. It must have a high power rating to generate the field levels needed for immunity tests and fairly high gain. The impedance should be 50 W and the VSWR at or near 1.0:1.

The ideal antenna doesn’t exist, but its hypothetical description illustrates the important aspects of product selection. As a result, you must decide which characteristics have the highest priority and what compromises are acceptable.

Two Definitions

In dealing with manufacturers of EMC antennas, the specifications refer quite frequently to E-field and H-field when defining performance characteristics. The terms near field and far field also are tossed about quite often. The simplified definitions you learned in an RF fundamentals class relate to transmitting antennas and should suffice for dealing with EMC antenna manufacturers.

E-field refers to the electrical component of the radiated signal, the signal of primary concern in EMC tests. H-field deals with the magnetic component of the antenna’s radiation, which is perpendicular to the E-field. This is important in the lower end of the EMC spectrum but seldom used above 50 MHz.

Near field is an area near the antenna where the E-field characterization is quite complex and the angular distribution of radiated signal changes with the distance from the antenna. Generally, a near field exists for transmitting antennas below 300 MHz.

Far field relates to points at which the E-field signal decreases in a well-defined manner as you move farther away and angular distribution of the signal is independent of the distance.

There is no universal agreement about the location of the boundary that separates the near and far fields. For horns and dishes, one set of commonly used definitions says the boundary is as far away as twice the square of the largest dimension of the radiating aperture divided by the wavelength (measured in the same units). For log-periodic antennas and dipoles, the boundary is located one wavelength from the antenna’s active region or focal point, which may be inboard from the physical end of the assembly.

Antenna Types by Frequency Coverage

Here is a listing of the most popular generic antenna types for EMC testing, ranked in ascending order by the highest EMC test frequency covered. These are general definitions, and specific antennas from different companies may be outside these ranges.

• Loop—20 kHz to 0.1 GHz. A given loop antenna covers a very narrow spectrum. When used for EMC work, it comes with a Faraday shield and aluminum tubing open at the top to permit measuring only the H-field.
• Biconical—20 MHz to 1 GHz. This antenna has a wide spectral coverage, possibly 10:1.
• Dipole—25 MHz to 1 GHz. With a narrow spectral coverage, this antenna has a well-defined gain, pattern, and impedance at its operating frequency and can be used as a standard.
• Log Periodic—25 MHz to 5 GHz. This type and its derivatives are the most popular for EMC work in their frequency range because a single antenna can cover a very broad spectrum, typically 10:1.
• Conical Log Spiral—200 MHz to 10 GHz. This antenna also has a wide frequency coverage, but its circular polarization prompts many users to abandon it in favor of the log-periodic antenna.
• Log-Periodic Dipole—20 MHz to 18 GHz. As the name implies, this antenna type is an adaptation of the dipole for broadband coverage.
• Reflector with a set of replaceable log-periodic feeds—1 GHz to 18 GHz. This configuration offers the best of two worlds. A reflector gives directivity and gain while changeable log-periodic feeds cover a broad portion of the spectrum.
• Horn—200 MHz to 40 GHz. To get those extremely high frequencies, you can’t beat the horn. Frequency coverage, typically 2:1, isn’t the greatest, but you can expand it with a ridged configuration.¹

For example, the AT4004 and AT4004A Horn Antennas from Amplifier Research cover the 0.2-GHz to 1.0-GHz frequency range. Gain is 10 decibels over an isotropic (dBi) minimum, increasing to 18 dBi at the top of the band. The AT4004A power input can be as great as 5 kW from 0.2 GHz to 0.5 GHz or 3 kW at the upper frequencies for immunity testing. Each assembly weighs about 100 lb.

Hybrid Antennas

Since coverage by a generic antenna type is limited, some manufacturers have developed hybrid combinations to cover broader bands. For example, the BiConLog™ Antenna from ETS-Lindgren goes from 26 MHz to 2 GHz by combining the biconical technology with a log-periodic design to span the 80:1 spectrum without changing antennas. Used for susceptibility testing, the antenna handles up to 1 kW average or 1.3 kW peak. The assembly weighs less than 15 lb.

“The antenna is a little more complex than a set of generic antennas,” noted Glen Watkins, director of marketing, “but it does cover an extremely wide band. The alternative is a biconical antenna at the low end, a log-periodic unit to 1 GHz, and a horn at the upper end,” he concluded.

Another example of hybrid technology is the Rohde & Schwarz ULTRALOG HL 562 Antenna that combines a biconical dipole with a log-periodic antenna to obtain a frequency range of 30 MHz to 3 GHz. It can be used for emissions tests or to measure immunity to interference. The radiators are V-shaped to achieve gain and rotationally symmetrical patterns at frequencies above 200 MHz. This gives uniform illumination in the field of coverage. To comply with CISPR 16-1, the antenna has >20-dB polarization isolation.

Antennas by Function

Two aspects are involved in testing for unwanted radiated RF. You need to detect and evaluate emissions from your product or to measure a product’s susceptibility to high-powered radiation from a nearby antenna.

Transmitting antennas for radiated susceptibility testing must handle the large signal strengths associated with the application; receiving antennas for emissions tests are low-powered devices with broad frequency coverage and higher gain. Generally, separate antennas can be used for the two functions, especially if you have a fairly large number of units to test.

“It is difficult to evaluate RF susceptibility in the 10-kHz to 80-MHz band using radiating antennas,” according to Pat Malloy, senior sales applications engineer at Amplifier Research. “To solve the problem that this creates, we developed a series of  E-field generators to work in this band. The goal in immunity tests is to generate a given E-field level. Rather than try to measure antenna output and calculate received field strength, we measure the level at the DUT with an isotropic probe.”

The typical coverage for an E-field radiator is 10 kHz to 30 MHz, but the AT3100 Generator from Amplifier Research uses two sets of radiating elements to extend the spectrum coverage to 100 MHz. The smaller set consists of 14² × 14² folded dipoles spaced 18² apart. With them, the equipment generates >300 V/meter between the elements when driven by a 500-W source.

Accessories: Precertification Test Cells

If your product is relatively small, investing in a test cell and the related instruments could provide you with precompliance testing capability at your company, saving you trips to a test laboratory. Isolation chambers come in various sizes, some even small enough to fit on a laboratory bench. These chambers give you an excellent indication of the capability of a product to pass the official certification tests.

Several companies offer test cells that help make precompliance testing flow smoothly. For example, Amplifier Research manufacturers two transverse electromagnetic (TEM) cells for pre-compliance testing. Going a step further, the company provides turnkey immunity and emissions test capabilies in its ARcell Systems. The equipment includes a TEM cell, a signal generator, an RF power amplifier, and a directional coupler. Additionally, cables and a preamplifier, a power meter, and a field probe with a fiber-optic interface are provided. Each system comes with a computer, a printer, and operating software.

The anechoic chamber, the semi-anechoic chamber, the TEM, or the Gigahertz TEM (GTEM) offers some advantages over the open-area test site (OATS). It gives operators independence from the weather and isolation from ambient interferences, and a user can test both emissions and immunity of small products up to 5 GHz.

The OATS, on the other hand, accommodates products of any size. It only covers emissions, but the frequency range extends to 25 GHz or higher.

Trends

The direction of EMC antenna technology is well-defined. “We see the frequency coverage for products moving from 1 GHz to 2 GHz to cover the newer PCs,” Mr. Malloy of Amplifier Research commented. “The top spectrum will continue to rise because of advances in PCs and telecommunications.”

Reference

1. White, D., The EMC, Telecom, and Computer Encyclopedia Handbook, Third Edition, emi-rfi control, 1999.

Acknowledgements

The following companies contributed to this article:

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Published by EE-Evaluation Engineering
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November 2000