What Every Electronics Engineer Needs to Know About: Antennas

To the uninitiated, knowing what antenna to use for EMC radiated emissions and radiated immunity testing can be a bit tricky. At first, determining which EMC antenna is the most appropriate may take a considerable amount of time and effort, with the high probability of the researcher developing some grey hairs before it is all finally sorted out. Fortunately, it doesn’t have to be this way. There are several basic antenna characteristics to take into consideration when selecting an EMC antenna. Knowing what these characteristics are before diving into your own antenna research will hopefully point you in the right direction and ultimately save some time, money, and a lot of grey hairs. At a minimum, you will be equipped to ask some great questions when contacting antenna suppliers to determine if their products will fully meet your EMC testing needs. Don’t be afraid to ask the antenna experts for help.

Let the Standards Guide You

Before getting too far into some of the more technical details surrounding EMC antennas, and trying to decide which antennas will suffice, one of the first things to determine is what EMC standard to apply. Look in the applicable standard for any information that will help identify which antenna to select in order to meet the requirements of the standard. Does the standard call out the required antenna specifications? For instance, MIL-STD-461G calls out by bandwidth, the antenna requirements for that particular frequency range. Specifically, from 10 kHz to 30 MHz, MIL-STD-461G says the 104 cm rod with impedance matching network is required. From 30 MHz to 200 MHz it states the 137 cm (from tip-to-tip) biconical antenna is required. The double-ridge horn, with a 69.0 by 94.5 cm opening, is required from 200 MHz to 1 GHz. Finally, the much smaller double-ridge horn, with a 24.2 by 13.6 cm opening, is required for frequencies between 1 GHz to 18 GHz. Notice that no log-periodic antenna is mentioned and that if you want to state you can perform EMC testing in accordance with MIL-STD-461G then you cannot use the log-periodic antenna or any other antenna not specifically described in the standard. The point of all this is that by referring to the standard, in this case MIl-STD-461G, you can gather lots of useful information quickly, saving needless head-scratching because it clearly defines which antennas to use and specifically excludes (does not mention) antennas we cannot use. Now that this piece of information is out of the way, on to some of the basic EMC antenna characteristics.

Antenna Factor

For EMC radiated emissions testing, Antenna Factor (AF) is the most important characteristics to consider. AF is defined simply as the ratio of the incident electromagnetic field (E) to the output voltage (V) from the antenna. When you purchase a calibrated antenna, it is provided with a table and plot of its AF in dB/m versus frequency for both vertical and horizontal antenna polarities. This data is input into your EMC emissions testing software and is used to calculate actual field strength (E – in dB micro-volts per meter) that is emanating from the equipment under test (EUT).

The conversion of field strength from measured voltage using AF is a simple formula as shown below:

E (dBµV/m) = V (dBµV) + AF (dB/m) + A (dB)

where A in dB is cable and connector loss present in the measurement system.

There is not much you can do about what the AF is for any particular antenna you choose. Just know what it is, how it is used in making emissions measurements, that it will be roughly the same for similar sized antennas (especially above 300 MHz), and that if your antenna gets damaged, then its AF may change and may need to be re-calibrated after repair.

VSWR and Gain

The second most important characteristic to consider with an EMC antenna is the Voltage Standing Wave Ratio (VSWR). This characteristic is more of a concern during RF immunity tests as the amplifier has to be able to withstand a certain amount of power reflected back into it from the antenna. As an electric wave travels through different parts of an ideal 50Ω RF system, it will encounter impedances other than 50Ω. At each interface, some fraction of the wave’s energy will reflect back to the source (RF amplifier), forming a standing wave in the feed line. The ratio of maximum power to minimum power in the wave can be measured and is called the voltage standing wave ratio (VSWR). A VSWR of 1:1 is ideal as no incident power is returned back to the source. A VSWR of 2:1 results in only 10% reflected power and is considered about normal. A VSWR as high as 6:1, which equates to 50% incident power returned to source, may still be usable with the right RF power amplifier. Minimizing impedance differences at each interface and selecting an antenna with as low of a VSWR as possible will help in maximizing power transfer through each part of the RF immunity test system. In RF immunity testing we are more concerned with the field strength that can be achieved, and as VSWR increases, the level of RF amplifier output power required to achieve a desired level of field strength will also need to increase. This is particularly a troublesome issue with biconical antennas used in their lower frequency range (~25 to 50 MHz or so) and is something to keep in mind when trying to effectively match amplifier with antenna.

Along with VSWR, numerical antenna gain is something else to considered in determining the correct antenna and power amplifier pair. Depending on these factors, which vary with frequency, you may require a power amplifier with more output power.

Other Considerations

Some other things to think about regarding the use of EMC antennas is less technical and falls more into the category of laboratory management. Each laboratory has its own requirements and you have to decide for yourself the best options to go with. For instance, will you use an antenna for emissions or immunity only, optimized for each purpose, or use an antenna specifically designed to perform both? If for both, then you may have to make some adjustments, like adding a high power balun (a device that provides a low impedance to differential current, and high impedance to common mode current) to your biconical antenna. Is there one antenna available that will improve test continuity and save in test setup time? One such antenna is the biconical/log hybrid (a.k.a. biconilog or bilog), that can be used over a larger frequency span than is covered by two separate standard biconical or log-periodic antennas. There are several manufacturers who produce these. Will the antenna (no matter what type) be used indoors or outdoors? If used outdoors, will the antenna and its positioning mechanisms have to withstand rain, dust, ultraviolet exposure, etc.? You may have several test sites and need to share equipment so how portable does the antenna have to be and how suitable is its packaging for this purpose? How large is the antenna and will it fit through the EMC chamber door? In order to avoid unnecessary damage from occurring, it’s probably a good idea to get the antenna up off the floor and out of the way of other work when it’s not being used for actual EMC testing. You may want to think about how the antenna will be stored when it’s not in use. How will the antenna be transported back and forth to the calibration lab? It’s probably a good idea to keep its original shipping crate and to budget for calibration and shipping charges as well. Finally, for radiated emissions measurements, have you determined your system’s sensitivity and the minimum measurable level as determined by the noise floor of the receiver? You may have to use a preamplifier or preselector place near the output port of the antenna (prior to any long cable runs) in order to lower the effective system noise floor. This will ensure the test systems properly capture all signals emanating from the EUT because they won’t be buried in the noise floor.

References

MIL-STD-461G, Requirements for the Control of Electromagnetic Interference, Characteristics of Subsystems and Equipment, Department of Defense Interface Standard, 11 December 2015. Williams, EMC For Product Designers, Fifth Edition, Newness, 2017 Denisowski, An Introduction to EMC Amplifiers White Paper, Rohde & Schwarz, 2016 Mullineaux, The 10 Factors You Need to Consider When Selecting an EMC Antenna, A.H. Systems, EMC Live Test Boot Camp, 2015

Don MacArthur is the Principal EMC Consultant at MacArthur Compliance Services, LLC. He can be reached at don.macarthur@mcs-emc.com.