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Military Microwaves Supplement
Near-field Focusing of a High Field EMC Test Antenna
This article describes a novel antenna for high field strength electromagnetic compatibility (EMC) susceptibility testing. The antenna system includes a small parabolic antenna and a set of five interchangeable feeds located laterally and axially off the reflector axis. The antenna system's bandwidth is 400 MHz to 18 GHz.
John M. Seavey
Seavey Engineering Associates Inc.
The world is populated increasingly by wireless, radar, satellite uplink and other microwave radiating systems. Regulatory bodies now require equipment designers to test electronic equipment for electromagnetic susceptibility at progressively higher field strengths. NEXRAD Doppler radars are an example of sources in the environment that are driving this requirement.
EMC susceptibility testing demands high field strengths to be generated over wide frequency ranges. However, limited power is available from off-the-shelf transmitters. Innovative antennas can provide a solution for the generation of these high field strengths.
Parabolic Reflector Solutions
Parabolic reflectors operate effectively above 400 MHz. At 400 MHz, a 1.8 m reflector's aperture is approximately 2.5 wavelengths. Investigation shows that even with this small electrical size, a parabolic reflector has useful focusing properties.
Parabolic reflectors are inherently frequency independent. Above 400 MHz, the 1.8 m reflector can operate effectively up to a very high frequency and is limited only by the accuracy of its reflecting surface. For instance, it is possible to work up to 18 GHz easily with a low cost reflector. In addition, octave-bandwidth feeds that can handle high power levels are readily available. Off-the-shelf transmitters can generate 1000 W CW in the 400 to 1000 MHz range and 200 W CW up to 18 GHz. Transmitters with significantly higher peak powers are also available.
Innovative Antenna Design
A standard parabolic reflector's feed is located at its focal point and energy is focused at infinity. In this case, field strengths close to the aperture are relatively low. If the feed moves away from the reflector, focusing occurs on the axis of symmetry and at a closer distance. Good near-field focusing can be achieved as close as 1 m from the reflector aperture.
However, with the feed located on the reflector's focal axis, the feed itself presents significant aperture blockage. This effect degrades the field strength seriously at this near-field focus. The solution to this problem is to move the feed laterally off the axis of symmetry while also moving it away from the reflector. This technique yields good near-field focusing at a distance of 1 m from the aperture. Figure 1 shows the antenna geometry for this type of configuration. The near-field focus is approximately 0.46 m below the axis of symmetry for this 1.8 m reflector.
This approach offers many benefits: The entire antenna system is very compact. The complete reflector, feed and reflector mount can be installed in a small laboratory or even on a benchtop. Out of the way of the device under test (DUT), the feed is located above the reflector's axis of symmetry, as shown in Figure 2 , resulting in a clear space surrounding the near-field focus. This space may envelop a benchtop, allowing for easy access to the DUT. Since the near-field focal zone is large (in particular, the focal zone's depth of field is quite large), useful testing at high field strengths can be performed on a wide range of devices. In addition, the transmitter can be connected to the feed with very short cable assemblies, thus minimizing power losses.
Interchangeable feeds are pre-aligned to ensure they are at the correct location. The optimum feed location has been investigated. Analysis using the numerical electromagnetic codes shows that a single offset feed position for the 1.8 m reflector is satisfactory for all frequencies above 400 MHz. Figure 3 shows a 4 to 8 GHz feed structure; Figure 4 shows details of other typical linear polarized feeds. The antenna's polarization can be adjusted to any angle by rotating the feed about its axis.
The reflector analysis software predicts the measured near-field focusing very well, ensuring that the focal zone is well defined. The antenna can be mounted permanently so that the DUT test space is delineated accurately. In addition, the extension of this technique to other aperture sizes and frequencies is straightforward.
Measurements were made on an antenna system composed of a 1.8 m axisysmmetric parabolic reflector with a universal feed support structure. The antenna system utilizes a 400 to 1000 MHz corner-reflector feed with a 7/8" 50 W connector, horn feeds with N-jack connectors for the 1 to 2, 2 to 4, 4 to 8 and 8 to 12 GHz frequency bands and a horn feed with an SMA connector for the 12 to 18 GHz band. Optional 0.15 m extensions were available to increase the reflector width where required.
Figure 5 shows a plot of peak field strength at the near-field focus vs. frequency. The near-field focus is located 1 m in front of the 1.8 m reflector. (The transmitter's CW power level is appropriately noted.) The field strength measurements used calibrated probes with known gain and transfer characteristics. (Note the higher field strengths at higher frequencies with a falloff at the lowest frequencies.) These measurements correlate well with expected results.
The graph shows that field strengths of 1 to 3 kV/m can be readily achieved above 1 GHz. More than 500 V/m can be achieved from 400 to 1000 MHz. Optional reflector width extensions increase this low band performance by 20 percent. This performance is accomplished with moderate power transmitters.
Figures 6 , 7 and 8 show the field strength at 1, 2 and 4 GHz vs. the x and y (lateral) and z (axial) near-field zone coordinates, respectively. (Note that the zone size for useful field strengths typically is 0.25 to 0.50 m.)
A novel antenna structure that provides high field strengths for EMC susceptibility testing purposes has been demonstrated. The 1.8 m parabolic antenna covers the 400 MHz to 18 GHz frequency range using five interchangeable offset feeds. The antenna system enables high power EMC tests to be performed with moderate power transmitters in a relatively compact environment.
The author wishes to extend special thanks to Eric Borgstrom of TUV Product Service Inc., New Brighton, MN, for his validation of this unique antenna system.
1. T. MacNamara, Handbook of Antennas for EMC, Artech House, Norwood, MA, 1995.
2. T. Williams, EMC for Product Designers, Butterworth Heinemann, 1996.
3. V.P. Kodali, Engineering Electromagnetic Compatibility, IEEE Press, 1996.
4. R. Perez (editor), Handbook of Electromagnetic Compatibility, Academic Press, 1995.
John M. Seavey received his BS degree in physics from MIT in 1958. For more than 40 years, he has been responsible for a wide range of antenna development programs. In 1981, he established Seavey Engineering Associates Inc., Cohasset, MA, which is engaged in commercial antenna development and manufacturing. Seavey holds six patents and has published many technical articles.
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