Make an Informed Decision on Which Antennas Fit Your Unique Application

There are many different types of antennas used for EMC/EMI testing and, with every type, there are variations on performance and use. How do you decide which ones are correct, or which one works best for the application? What are the compromises? Can one antenna cover the whole frequency range? And for both emissions and immunity? These are all great questions, which I hope to answer in this article as they pertain to IEC 61000-4-3 radiated immunity testing. You’ll find that there are many types of antennas out there that can do the job. By understanding all the requirements, enables you to make more informed decisions to narrow down your selection.

Antenna Requirement

Annex B in IEC 61000-4-3 addresses the various types of antennas and antenna systems that can be used to evaluate frequencies consistent with the requirements of the standard. The Annex gives four examples of the common antennas and leaves the selection open to any “linearly polarized antenna system.” Antenna type or quantities are not limited to just one for the whole frequency range, and multiple antennas could be used. From just this requirement, it would seem that almost any antenna could be used, but there are other requirements in the standard as well as possible setup constraints that need to be considered in antenna selection.

Here is a list of general questions that need to be investigated:

• There is a frequency range that needs to be covered. What is this range?
• The antenna must produce an RF field. What level (V/m) field is required?
• A test area must be illuminated. What is the illumination area and requirements?
• The antenna needs to sit at a distance from the test. What is that distance?
• The antenna must handle an input signal from a high-power amplifier source. What is this power?
• The antenna must fit within the test area or chamber. What are the area constraints?

With this list, it is important to always keep in mind that, throughout the frequency range, there may be different answers for these questions, and which may require different antennas or setups. Let’s break this down for the IEC 61000-4-3 requirements. Other standards may have their own specific requirements, but the application will follow a similar logic.

Frequency Range

The antenna/s will need to work over the frequency range 80 MHz to 6 GHz in order to properly assess both general-purpose equipment as well as digital radiotelephones. We will have a better understanding of whether one or multiple antennas are needed when we know all the other parameters.

Field Level

Table 1 is from the basic standard and the CW calibration levels are for understanding maximum power/field that needs to be delivered from an 80% amplitude modulated signal. Product standards define this further to levels and frequency ranges. They do not always carry the same test level throughout the whole frequency range.

Table 2 provides some examples of different product standard field level requirements.

Do your homework to understand your requirements and required levels and frequency ranges. Since they are not the same for all frequencies it is best to make a table for your requirements. This field level will come in to play to estimate the required power later in the article. The antenna will have a specification for power handling.

*Includes 80% AM

Table 1: IEC 61000-4-3 test levels including modulation

*modulations are Pulsed and FM therefore not requiring a 1.8x factor to be applied for calibration

Table 2: Examples of field levels from various product standards

Test Area/Illumination Area

In IEC 61000-4-3, this is referred to as the uniform field area (UFA), which is defined as a minimum size of 0.5m x 0.5m so long as the whole Equipment Under Test (EUT) and cabling can be fit within the UFA. UFA can be increased in sized in 0.5m increments. The preferred UFA size is 1.5m x 1.5m. This covers a 16-point area as shown in Figure 1. EUTs requiring an area greater than 1.5m x 1.5m can be tested by a method of partial illumination if full illumination cannot be achieved. This allows the EUT area to be covered by moving the antenna (or moving the EUT) into multiple positions, increasing test time. This method is also allowed for smaller EUTs when testing at frequencies higher than 1 GHz. Table 3 outlines the requirements for UFA.

Table 3: IEC 61000-4-3 requirements for UFA

The UFA that is selected will be needed to find a minimum beam-width of the antenna. Beam-width is usually given as an angle in degrees for each direction E (in direction of polarity) and H (in direction perpendicular to polarity), as shown in Figure 2. To find the width in each direction, however, there is one term still missing, that is, test distance.

Test Distance

IEC 61000-44-3 allows for any test distance greater than 1 meter (>1m) so long as the UFA requirements are met. The standard also states that a distance of 3 meters is preferred. The test distance selected for testing can have a significant influence on the testing outcome. Some key arguments on distance and the effect on UFA include:

• A longer distance takes more power to reach a desired field (for example, moving from 1 to 3 meters with the same antenna takes about 9x more power); and
• Moving closer reduces the window size; see Figure 3
• Some antennas have a very wide beam-width, which means that closer testing may be possible to meet the UFA. It also means that the antenna has a lower gain; see Figure 4
• Antenna beam-width is directly related to antenna efficiency (or gain). An antenna with a wide beam-width will have less gain, while an antenna with a narrow beam-width will have higher gain;
• It is possible for an antenna beam-width to be too small to meet the requirements. Moving the antenna further away may help to compensate for this;
• Antenna beam-width is dependent on frequency. Check the frequency and frequency range at which the antenna is useable. Also, remember that using an antenna that has an acceptable frequency range does not guarantee it can be used for each application through the range.

Using trigonometry with our known UFA and test distance, it is possible to identify a suitable beam-width with the preferred size of 1.5m x 1.5m at a test distance at 3m. The beam-width needs to be larger than 30° in both E&H planes.

Required Power

A lot goes into figuring out what power is required to reach the intended field. The first thing that you need to know is what antenna is being used. But, since we don’t yet know the antenna to use, how do we pick the right power level? Here, it is best to make some assumptions based on experience to get us close to the right answer. Table 4 gives power ranges for different distances and field levels based on our own experience.

Table 4 can be used as a starting point to search for an antenna. After selecting an antenna and have run all of the required calculations, you may need to go back and find a different antenna that more closely matches your needs (hopefully, you estimated correctly the first time!). Calculating power is another larger topic, not discussed here.

Table 4: Examples of required power ranges to meet IEC 610000-4-3

Antenna Dimensions

IEC 61000-4-3 does not specify the actual antenna size, though there are some things that need to be considered that are related to the setup. Once the antenna is in position, it should not be “close” to any other objects. Close is a relative term, but a good rule is >0.5m clearance from the absorber. But the important requirement is that it meets the UFA requirement, and more spacing is better for this. You also want to be able to easily rotate the antenna for polarity changes, which may be an issue in some smaller chambers. For testing at 80 MHz, antenna size is a factor, and a larger antenna will normally outperform a smaller one. Size does matter!

Summary of Requirements

To meet the preferred requirements an antenna is needed to test a 1.5m square UFA at a 3-meter test distance. Ideally, it should also be as small as possible for ease of use and offer a high gain to reduce the required power from the RF power amplifier. But small size and high gain are contradictory requirements, so a compromise is needed to reconcile these objectives. Let’s look at what antennas are available for the application.

Table 5: Summary of requirements and preferred requirements from IEC 61000-4-3

Antenna Types

Biconical antennas will cover the lower frequency range 80 MHz up to about 200 MHz. Because this frequency range is not a large portion of the test, it is not the most efficient antenna to use but you may already have one available which is reason enough to consider using it. A biconical antenna might have a slightly better gain performance over a combination antenna in this range. This antenna does cover down to 30 MHz which makes it usable for emissions testing however, high power biconical antennas cannot be used for emissions testing in some cases as they do not meet the required balance of CISPR.

Log-Periodic(LP) antennas cover the frequency range from 80 MHz to 6 GHz. A dipole array can provide good performance, having about 6dB of gain in the far field. It can also handle high power, enabling it to cover the whole frequency range. However, to reach the 80 MHz of the lower end in frequency, the elements can get long and more unwieldy. Antennas on the market feature bent elements, which maintains a good match and only loses minimal gain to achieve a much easier handling. At 80 MHz and testing at 3meters distance, the antenna is in the nearfield, so gain does drop off. This antenna can be used for emissions testing also but since it would become extremely large to cover down to 30 MHz it is normally paired with a low power biconical antenna

Stacked LP antennas are a variation on the log periodic antenna, in which two LP antennas are combined into an array, resulting in a net improvement of 3 dB over an LP (roughly 9dB of gain). This antenna has quickly become the go-to solution for testing to the requirements of IEC 61000-4-3 and other standards, as it has great UFA performance and can handle high power. But its size can make it large and a little unwieldy. However, it is the best solution if it can fit in your test setup. One antenna can be used to cover the whole band (80 MHz – 6 GHz), or it can be split into two bands (80 MHz – 1 GHz, 1 GHz 6 GHz). This antenna is not ideal for emissions testing since it does not cover down to 30 MHz.

Combination antennas are often known by their proprietary names, but are essentially a combination of a biconical antenna and a log periodic antenna. When combination antennas were introduced, they quickly took the market as it greatly reduced the need for antenna changes during emissions testing. Some versions having higher power handling and can also be used for immunity testing, providing an ideal solution. However, the release of new emissions standards means that high power combination antennas do not meet the required antenna balance and therefore cannot be used for emissions testing. For lower level testing (>3V/m), however, these antennas can still be used for dual purposes, resulting in a “thumbs-up” for use in radiated immunity testing when less than 100 Watts of power is required.

Horn antennas offer a great solution for testing at higher frequencies. They come in many frequency ranges and power levels and also offer a large variety of gain offerings. One drawback to horn antennas is that they would need to be extremely large to test at low frequencies, making them a poor choice to testing across the whole band. However, for testing at frequencies greater than 1 GHz, they are used quite regularly. Some high gain horn antennas are too efficient to meet the requirements of IEC 61000-4-3 (the beam width is too narrow), and it is important to keep this limitation in mind during the selection process. The standard 1-18 GHz double ridge horn antenna is commonly suggested and used in the 1-6 GHz range.

Additional Concerns

A concern with testing is setup time, effort, and transition time between frequency bands. If an antenna change is required, how much effort and time does it take? If one antenna can perform the whole test, can the different amplifiers be automatically switched, or must they be manually switched by moving the RF coax? Is changing the polarity easy to do, or can it be automated? These considerations are beyond the scope of this article, but they need to be included in any system evaluation.

Conclusion

Ideally, the best solution is the simplest solution, that is, a single antenna to cover the whole range. With the best performance, a stacked LP is the clear winner to meet this standard. It has great gain, great field uniformity and can handle high powers. Though it could be calibrated and used for emission testing it does not cover down to 30 MHz and would not be able to be used for emissions. It would be ideal to have an emissions antenna and an immunity antenna in one, but this is not available today for higher power requirements. So the use of the stacked LP requires the use of a second emissions antenna, possibly one to cover the whole emissions range as well.

Table 6: Some antenna suggestions

However, if you’re not testing to high field levels, an antenna can be used to cover both emissions and immunity, and a combination antenna would be a good choice. Although the gain is not nearly as good as the stacked LP, not much power is required at 3V/m, even with a poor performing antenna. This makes the setup much simpler with no antenna changes to switch from emissions to immunity.

Of course, if you have existing antennas that work with the power you have, purchasing new antennas might not be desired. However, a single antenna solution does save on setup time and changes. Less cable and equipment switching also means less wear and tear on the test engineer and equipment. Over a period of time, a single antenna solution is likely to represent a sound investment.