The traditional upper frequency limit for commercial EMC testing of radiated emissions (ANSI, FCC, VDE, CISPR, and others) is 1 GHz. For many years, this was factored into the design of EMC test systems and accessories such as antennas. One GHz became a commonly encountered upper frequency limit for most commercial testing.
In recent years, the FCC has increased that limit, and other standards-generating organizations are expected to follow suit. As a result, manufacturers of EMC test equipment now must provide efficient testing solutions at higher frequencies.
Two underlying factors are driving this trend to higher frequency emissions testing. First is the proliferation of communications equipment above 1 GHz. One of the most visible new applications is the Personal Communications Service (PCS). Other uses include wireless digital links applied to computer networks. The main purpose of EMC testing in the commercial environment is to prevent interference with communications services such as these.
In this information-dominated age, the public and commercial sectors are hungry for bandwidth. With development of the lower frequency bands at its limit, there is no place to go but upward in frequency. Most likely, this trend will continue.
The second factor prompting an increase in the upper-frequency limit for radiated emissions testing is the ubiquitous appearance of high-speed digital devices such as personal computers (PCs) and their associated peripherals. Desktop computers with internal clocks at 300 MHz soon will be fairly common, and there is no end in sight for this progression to faster processing speeds. Prototypes operating at 400 MHz and above already have been demonstrated.
In multimedia applications, video displays also are being pushed to faster performance. Higher frequencies generated in these digital devices invariably correspond to emissions profiles that feature significant energy at higher frequencies.
What we are seeing, then, is a proliferation of potential interference generators as well as systems and devices that might be susceptible to interference at frequencies above 1 GHz. Experience tells us that the combination of these two trends will result in increased occurrences of interference unless regulation of the spectrum is implemented.
In response to this situation, the FCC has extended the test requirements for unintentional radiators (the equipment category governed by Part 15 that includes most computer hardware) to at least 2 GHz if the equipment internally generates frequencies greater than 108 MHz. Clearly, this includes virtually every PC made today. Uncharacteristically, the European test specifications are lagging behind the FCC requirements in this regard. But upward revisions of the radiated emissions test frequency limit are expected for equipment destined for sale in Europe.
Military EMC testing into the microwave frequency region (to 18 GHz) has beende rigueur for many years. Equipment for this testing is readily available, but the antennas used are not adequate for commercial testing. There are a couple of reasons for this.
Ridged guide horn antennas typically are used for military EMC testing. These are not well-suited for compliance with either the letter or the spirit of commercial regulations. For commercial uses, test results with broadband antennas should correlate well with what would be obtained if tuned dipoles were used.
Although a good technical case could be made for abandoning this principle above 1 GHz, there still are sound reasons for retaining it at the lower frequencies. The log-periodic dipole array does correlate well, and it has been the broadband antenna of choice for commercial testing in the 200-MHz to 1-GHz range.
Military testing also differs from commercial tests in how the antennas are applied. MIL-STD tests use a fixed receiving antenna. As a result, it is relatively easy to automatically switch from one antenna to another when a frequency breakpoint is encountered.
Commercial testing, on the other hand, employs an antenna positioning tower with a movable height setting, making automatic switching more difficult. Because it is impractical to use multiple antenna positioning towers, the ideal solution for the commercial tester would be a single antenna that covers the entire test range.
Log-periodic dipole arrays belong to a class of antennas described as frequency independent. This means that the design may be scaled to arbitrarily wide frequency coverage.
The length of the longest elements determines the lower frequency limit, and the upper limit is set by the accuracy of reproducing the smaller elements. Log dipole arrays can easily be manufactured for use above 18 GHz, but higher frequency operation usually comes at the compromise of power handling when the antenna is used for transmitting.
Transmitting power handling of antennas for commercial EMC testing has become more of interest since the institution of radiated immunity testing for electronic devices sold in Europe. The transmit power-handling capability of the antenna is important if users want to make emissions measurements as well as perform immunity testing.
The desired characteristics for an ideal antenna for commercial EMC testing would include:
Wide bandwidth without frequency switching.
Small size for mounting on an antenna positioning tower.
High power-handling capability for transmitting (immunity testing).
Good sensitivity for receiving (emissions measurements).
All these requirements can be met with a combination of biconical and log-periodic elements. Mating requires careful consideration of impedance and phase matching to ensure compatibility among the various antenna elements. In addition, the antenna array must be isolated from the feed line to prevent undesired feed-line radiation or signal pickup.
The recent popularity of the biconical log-periodic is testament to the need for such an antenna. Early version hybrid antennas, however, were limited to 1 GHz. They also did not address the feed-line radiation problem.
A recently introduced antenna, the Electro-Metrics Model EM-6917A, addresses these concerns. It features a calibrated frequency range of 30 MHz to 3 GHz (Figure 1). The antenna is specified at 1,000 W continuous power at 1 GHz. The same antenna can be used for radiated immunity testing as well as emissions measurements.
The trend to perform commercial radiated emissions testing at higher frequencies is well-established and, no doubt, will continue. Broadband antennas are an important step in maintaining an efficient test environment for this trend.
About the Author
Carl Herzog has been employed by Electro-Metrics for 14 years and currently is the manager of antenna and sensor products. He is a graduate of Rensselaer Polytechnic Institute with a B.S.E.E. degree. Electro-Metrics, 231 Enterprise Road, Johnstown, NY 12095, (518) 762-2600.
Copyright 1998 Nelson Publishing Inc.