Higher Testing Frequencies Impact EMC Antennas

We all want our test systems to have the smallest possible number of instruments and components. When you�re conducting EMC studies across a wide range of frequencies, from the low megahertz region well into the gigahertz range, how many different antennas are needed?

Ideally, you�d have one that covers the entire range of interest and then scan results in one uninterrupted sweep. So, are we achieving this goal?

Antennas covering an extremely wide frequency range are available. But in the real world, there�s rarely a one-size-fits-all solution, and you must be aware of trade-offs when working with a broadband EMC antenna.

Credence Technologies/3M: CTS054 Receive-Only Unit/Antenna

It�s also important to distinguish the two sides to the EMC equation:
� Source equipment whose controllable emissions must be limited, and you can measure their emissions with receive-only antennas.
� Equipment that must have adequate immunity to distur�bances in its environment, requiring transmitting antennas that create the necessary electrical field.

EMC test antennas are designed differently than communications antennas, which are optimized for a certain frequency band. Because of their narrower bandwidth, they typically have a higher gain.

In contrast, EMC antennas, with their emphasis on wide bandwidth, blend a number of parameters to allow engineers to conduct a test with minimal time-consuming changes to the setup. To assure usability over a wide spectrum, EMC antennas even may sacrifice other parameters such as gain, efficiency, and good impedance matching.

Rising Frequency Requirements
Before moving into product highlights, let�s consider how requirements in terms of frequency coverage change how you approach selecting an antenna. In 2006, The International Special Committee on Radio Interference (CISPR) updated standards such as CISPR 22 that give radiated emissions limits up to 6 GHz. It is likely that 6 GHz will remain the top mandated frequency for the foreseeable future, noted Martin Alexander, a principal research scientist at the National Physical Laboratory (NPL), the U.K. national standards laboratory.

But depending on the industry and applicable standards, the frequency range can be very different. Current EMC test standards are not designed to evaluate issues that arise in today�s increasingly complex wireless products.

According to Dr. Michael Foegelle, ETS-Lindgren�s director of technology development, �Many wireless devices today have multiple radios in a single product such as cell phones with Bluetooth, and coming soon are converged devices with even more radios�cellular, WiFi, WiMAX, Bluetooth, GPS, Digital Video Broadcasting�Hand-Held (DVB-H), and proprietary schemes such as Qualcomm�s MediaFLO.�

Many cases are not covered by standards, agreed Vladimir Kraz, president of Credence Technologies/3M. In EMI diagnostics, every piece of equipment in a setting might be compliant, but they don�t work properly when you combine them in their operating environment.

For instance, in a semiconductor fab, there might be a lot of ground noise, and when you put in a sensitive photolithography tool, you get errors. Engineers need test systems to help uncover these problems.

Similar problems are increasingly cropping up on the consumer side as well. The International Electrotechnical Commission, of which CISPR is part, stated on its website that the convergence of certain newer technologies is making it difficult to decide whether some products should be designed to television or computer EMC standards.

This forces some manufacturers to test their multimedia products to both, which is costly and time-consuming. A new subcommittee, CISPR SC1, draws on the combined expertise of the former SC E and SC G groups and will produce new EMC standards for these multimedia products.

How high are frequencies of interest getting? While most present applications use spectra in the 1-MHz to 10-GHz range, systems such as automotive collision avoidance radars that operate up to nearly 100 GHz are being developed. These frequencies will challenge all antenna designers who want broadband units.

Mix and Match
Looking for a single antenna that spans megahertz into the high gigahertz range with uniform good performance will be fruitless. In fact, most suppliers agree with Oren Shri, sales and marketing manager at Dynamic Sciences, who recommends that customers pick the optimal antenna for each range and a controller that can select among the antennas during a test.

The Dynamic Sciences R-1150-10A Portable Antenna Kit starts with an antenna base unit with whips and ferrite rods for electric and magnetic field tests between 100 Hz and 30 MHz. It has a collapsible biconical antenna for 20 MHz to 200 MHz and a log periodic for 200 MHz to 1 GHz.

AR RF/Microwave Instrumentation: Model AT5080 Log Periodic Antenna

The kit includes the two types of antennas that have historically been used for emission measurements: the log-periodic and biconical. The log periodic is an excellent antenna for radiated immunity testing because it exhibits virtually constant gain and beam width vs. frequency.

One drawback is its size, especially at lower frequencies, where it can become unwieldy in a testing chamber. Further, the radiating phase center moves with frequency along the antenna�s length. The highest frequencies radiate from the smallest elements at the front of the antenna and the lowest frequencies from the larger back elements.

The range of log periodics have been increasing as exemplified by the Model AT5080 from AR RF/Microwave Instrumentation. It uses a bent-element approach and measures 52″ x 8″ x 38.5″ for a size reduction of approximately 60% over conventional log periodics without sacrificing key electrical performance such as gain and bandwidth. The size reduction minimizes field loss resulting from room loading, which is especially troublesome when using conventional log periodics in small enclosures. It specs a maximum VSWR of 10:1 from 26 to 80 MHz and 3.0:1 from 80 MHz to 5 GHz.

An Antenna Milestone
For frequencies below 300 MHz, engineers traditionally turned to a biconical antenna, which generally looks like a wire cage. Chase EMC (later Schaffner, and now Teseq due to a management buyout of Schaffner�s EMC division) collaborated with York EMC Services to create the BiLog antenna. It combines the best features of a biconical and a log periodic and that, according to NPL�s Mr. Alexander, was a milestone in broadbanding antennas.

A good example from Teseq is the CBL 6144, an X-wing BiLog for 25 MHz to 3 GHz. It measures roughly 63″ x 60″ x 26″. As is typical for a hybrid-type antenna, the CBL 6144 has a poor match into 50 ? at the lower band edge, as much as 10:1. Some vendors spec the VSWR only for the log-periodic portion and not for the lower frequencies, and receive-only antennas tend to not spec VSWR at all.

As a result, a 500-W amplifier is needed to generate the required field strength of 10 V/m with 80% amplitude modulation. In this regard, explained Richard Fink, EMC project manager, you need an amplifier designed to withstand the high amounts of reflected power at low frequencies. It also is advantageous to have a matched amplifier that can automatically adjust down its output power as the VSWR improves to maintain a constant E-field.

Teseq: CBL 6144 X-Wing BiLog Hybrid Antenna

BiLog is a trademarked name, but other companies make variations under the generic title hybrid broadband antenna or biconical-log hybrid antenna. And manufacturers continue to find ways for incremental improvements.

BiConiLog is the term ETS-Lindgren uses for its hybrids. The Model 3140B, intended for radiated immunity measurements but not radiated emissions measurements, works from 26 MHz to 3 GHz. A patented T-Bow-Tie element design improves low-frequency performance. It specs an average VSWR of 2:1 across 26 to 150 MHz; above 30 MHz, it requires 300 W to generate 10 V/m with 80% AM at 3 m.

Another choice is the Model JB6 from Sunol Sciences, which is suitable for both radiated emissions and immunity testing up to 10 V/m. Its low-frequency wings are swept forward and turned in at the ends to reduce ground-plane coupling while maintaining performance. It specs a VSWR of <2:1 above 200 MHz and rates maximum input power at the low frequencies at roughly 1,150 W.

The big trade-off with wide bandwidth is gain, noted Stuart Kron, an engineer at Sunol. While this antenna works up to 6 GHz, a 1- to 18-GHz horn will have better gain and a higher power rating above 22 GHz. Depending on the type of test, engineers need to know what equipment will make their testing more efficient and cost-effective.

Taking a different approach to hybrid antennas is Electro-Metric�s EM-6104, which covers 20 MHz to 18 GHz. The receive-only antenna consists of a discone and a biconical horn. It is vertically polarized and omnidirectional.

Small at 12″ long and 10″ in diameter, it provides two connectors, 20 MHz to 1 GHz and 1 GHz to 18 GHz, and specs an average VSWR of < 2.0:1. The antenna requires an external power source of 12 VDC at 60 mA to drive its amplifier, but that amp might make it subject to saturation when exposed to a spectrum containing very high-level signals in the antenna�s band.

Jason Smith, supervisor of applications engineering at AR RF/Microwave Instrumentation, agreed that, while combination antennas can offer a broad frequency range, in most cases, this is not the best solution. For emissions testing, a single antenna can work well, but immunity testing is quite different.

To use an antenna that covers the 3-MHz to 6-GHz range, there will be sacrifices in antenna gain, which makes a big difference in the power needed to produce a field and consequently a difference in system cost. Above 2 GHz, the log periodic part of the antenna will never have the gain that a horn antenna can and requires an amplifier. He suggested using different antennas for both immunity and emissions. This way, equipment can be optimized for the best value.

Horn antennas are most common for testing above 1 GHz, and one antenna typically cannot cover a wide range, said Travise Miranda, manager at Com-Power. His company supplies four products that cover the 1- to 40-GHz range.

Electro-Metrics: EM-6104 Antenna

He noted that the limitation when testing above 1 GHz is determined by the entire measurement system. This includes the test site, cables, receiver, and preamplifier as well as the antenna used.

Com-Power�s AHA-118 Active Horn Antenna measures from 1 to 18 GHz. It has a built-in preamplifier and reduces the effect of cable loss above 1 GHz by amplifying the signal at the antenna terminal. This allows test engineers to detect low-level signals that might not be seen otherwise.

Looking at the Near Field
The next frontier in EMC is near-field measurements according to Mr. Kraz of Credence Technologies/3M. At a sufficient distance from an antenna, there exists only a radiated electric field that can be approximated by a plane. This region is known as the far field. The region nearer to the antenna, the near field, has both electric and magnetic field components that must be measured.

The concept of field regions is important in EMC testing because of the wide frequency range over which devices are tested, and different regions require different types of antennas. At the low end of the range, standard test distances such as 1 m and 3 m sometimes dictate that the DUT be placed in the near-field regions of the test antennas. If using a log periodic or hybrid antenna, you could measure the E-field. But for the H-field, you also would need to work with another antenna such as a magnetic loop.

The near field comes into play in real-life applications when the co-location of equipment creates issues with interoperability. The SEMI EMC Standards Task Force, which Mr. Kraz co-chairs, is dealing with such issues, and he noted that far-field tests are not sufficient in understanding how equipment affects its neighbors. Such concerns go beyond a production environment.

For example, consider an airplane cockpit where a lot of equipment is close together. You need a small antenna that is usable in such a small space, where the environment won�t distort the readings, and that senses the near field to help isolate exactly where the radiation is coming from.

Credence Technologies/3M specializes in active miniature broadband antennas that avoid errors associated with size. For example, the CTS011 Active Dipole Receiving Antenna measures 3.75″ x 3″ x 0.75″ and specs a range of 30 MHz to 2 GHz. A low-noise preamplifier built into the antenna assembly provides equalization of frequency response to assure high sensitivity at low frequencies.

While even large log periodic antennas have a drop of sensitivity at the lower end of the spectrum, the CTS011 provides good sensitivity at this range. The metal element in the antenna is a dipole, and the proprietary electronics allow it to perform over such a wide range.

Another interesting antenna from Credence is the CTS054, also a receive-only unit that measures roughly 2″ x 2″ x 0.5″ in a rectangular package. It likewise includes a preamp to achieve a frequency coverage of 30 MHz to 3 GHz. To allow it to measure both near and far fields, it actually contains four antennas: two dipoles to measure the E-field from 30 MHz to 1 GHz and 1 to 3 GHz and two shielded loops to measure the H-field across those same ranges.

When it comes to small antennas, one interesting technology is the fractal antenna, which Mr. Kraz claimed has opened up a whole new frontier in the field of antenna design. High performance is possible in antennas with a very small footprint compared to conventional designs. Although they have been used in applications such as RFID, no EMC antennas with this technology are yet commercially available. Meanwhile, designers are tweaking traditional designs to get even more gain and bandwidth from them to meet the needs of equipment operating at or transmitting at ever higher frequencies.

October 2007