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Bad Reflections by David Mawdsley, Laplace Instruments
In an ideal world, a test site would comprise the EUT suspended in open space by a nonmetallic string from some imaginary crane, with the antenna similarly suspended at the appropriate distance from the EUT. Then we would measure the free-space emissions level. Unfortunately, because of gravity, we are stuck on the ground. The presence of this ground means that the EUT cannot exist in a free-space environment. The ground will act as a reflector and a variable reflector at that. In general, any EUT will emit some radiation in all directions. There may be variations with EUT orientation, consequently it must be rotated to find the worst-case direction. Some radiation will impinge on the ground, which will partially reflect this radiation. When measuring emissions in the far field, the signal received by the antenna will comprise a direct signal and a signal that has been reflected from the ground (Figure 2). The amount of this reflected signal depends on ground conditions and may vary considerably in amplitude. Figure 2.
Ground Plane Reflection On soft ground such as soil, the reflection will vary as conditions change. This means that the integrity and consistency of the results will vary. To overcome this problem, the standards require a test site with a metal ground plane consisting of a continuous metal sheet (or equivalent) under the EUT and between the EUT and the antenna. This gives a consistent reflection. In one sense, this is worst case because the effect of the reflection will be maximized, but at least it will be consistent. The effect of the reflection will depend on frequency and the difference in the length between the direct path and the reflected path. If this difference is equal to half a wavelength at the frequency of interest, the two signals will be 180° out of phase and cancel each other, producing up to 20-dB reduction in signal strength. To overcome this effect, the standards call for the antenna to be mounted on a mast so that it can be varied in height over a range of 1 to 4 meters. For each frequency, there will be a height at which the two signals are in phase and additive. This is the height at which that frequency is measured. Figure 3 shows the relationship among frequency, antenna height, and signal gain/attenuation. The light areas correspond to gain, and the dark areas represent attenuation. If using an OATS on dry soil, the reflection will be small and the gain/attenuation effect will be minimal. However, if any metal surface is in the vicinity, Figure 3 gives some idea of the consequences. All these problems relate to just one reflection. Imagine the situation with several reflections as would be the case indoors with filing cabinets, equipment, wiring, metal partitions, and metal-framed furniture all around. The effect on any RF signal radiating from the EUT would be complex in the extreme.
About the Author David Mawdsley is the managing director and founder of Laplace Instruments. Mr. Mawdsley originally trained with Rolls Royce (Aero Engines) as a mechanical engineer and later was a manufacturing manager for Data Acquisition. He received a B.Sc (Hons) London in electronic engineering. Laplace Instruments, Tudor House, Grammar School Rd., North Walsham, Norfolk NR28 9JH U.K., 011 44 1692 500 777. Return to EE Home Page Published by EE-Evaluation
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