Antennas used for EMC testing possess several characteristics which make them ideal for use in a fast-paced, production-like EMC test environment. This article will briefly describe what these characteristics are, starting with the most important parameter – antenna factor.
The EMI receivers, spectrum analyzers, cables, attenuators, and other paraphernalia used in EMC testing are specified with a characteristic impedance of 50 Ω. Therefore, an antenna used to measure emissions must be calibrated in terms of volts output into 50 Ω in order to supply a given field strength at each test frequency. This characteristic is called antenna factor.
Antenna factor units are dB/m for E-field antennas. Make sure the antenna you choose is provided with a table of the antenna factor versus frequency because in order to convert (in software) the measured voltage at the antenna terminals into the actual field strength at the antenna, you have to add the antenna factor and cable attenuation (also a function of frequency). Here is an important equation to remember in case you need to troubleshoot a problem with your measurement system not measuring correctly:
E (dBµV/m) = V (dBµV) + Antenna Factor (dB/m) + Cable Attenuation (dB) – Preamp gain (dB).
Note 1: Ignore preamp gain if a preamp is not used.
The most accurate reference antenna for emissions measurements is the tuned dipole. The problem with using this type of antenna in an EMC test facility is the need to re-tune the dipole for each frequency to be measured. In order to speed up the measurement process, broadband antennas that cover multiple of frequency ranges are used. Their use eliminates the need to re-tune at each measured frequency. Simple examples of broadband antennas are the biconical (30-300 MHz typical), log periodic (300 to 1000 MHz typical), or the combined BiLog (30 to 3 GHz typical).
Note 2: Military standards do not allow use of the BiLog and stick to using other broadband antenna types like the horn antenna.
Note 3: Log-periodic antenna designs with the same dimensions will provide largely matching performance.
CISPR requires use of “plane” polarized antennas such as the biconical, log periodic, or BiLog that can be positioned either in the vertical or horizontal polarity for testing. Circularly polarized, log-spiral-type antennas are not permitted.
Power Handling Capability
Although not always recommended, the same antennas that are used for emissions can also be used for immunity testing. However, they must be able to handle the amount of power they will need to deliver to generate the required field strength. This capability is achieved by using a properly rated wide-band ferrite core 1:1 transformer placed at the antenna’s feed point. Because this item converts the balanced feed of the dipole to the unbalanced connection of the coaxial cable, it’s been given the unique name balun. See a related article for more information on baluns.
Since the balun could get fried by supplying it too much power during immunity testing, many have separate antennas devoted to just emissions and/or just immunity testing.
Low Voltage Standing Wave Ratio (VSWR)
Antennas such as the biconical have high VSWR ratings at their lower frequency range. High VSWR means more power is required to generate any given field strength, so a lower VSWR is desirable. Please refer to this article for more information on VSWR.
For an antenna used for immunity testing, low gain means needing a bigger amplifier to generate the required field strength. Be sure to check the antenna gain over the entire frequency range of interest to ensure there is enough power available in the amplifier chosen. Please refer to this article for more information on antenna gain.
To a lesser extent, some other characteristics that are important in selecting antennas for EMC testing include:
- Quality – Does the antenna have a rugged design and is it able to withstand the normal wear and tear of daily use and survive shipping in a protective box back to the manufacturer for repair or to a calibration facility for calibration?
- Cost – Will the lab go broke outfitting the entire facility?
- Customer support – If I have a question, can I get a hold of someone, and can they help me with my technical problem?
- Availability – If it breaks, can I get another one just like it? I want to buy a new one now, is it available?
References and Further Reading
- Williams, T., EMC For Product Designers, Fifth Edition, Newnes, 2017.