EMC Question of the Week: July 31, 2023
Two planes in a circuit board form a patch antenna producing radiated emissions at 1.5 GHz. The circuit board sits on a 1-meter wooden table, and the emissions are measured in a semi-anechoic chamber at a 1-meter height. The test distance is 3 meters. The received field has two components: one radiated directly from the board and one reflected off the chamber floor. Which component is stronger?
- direct component
- reflected component
- both components are approximately equal
- could be either (a.) or (b.)
Answer
The best answer is “d.” If the source were omnidirectional, the difference in the magnitudes of the direct and reflected components (due to the greater distance traveled by the reflected component) would be less than 2 dB. However, patch antennas are directional with maximum radiation generally in the direction perpendicular to the planes. At the lower resonant frequencies, a patch antenna formed by a power plane below a larger ground plane in a circuit board would have maximum emissions straight down and a null in its pattern in the direction of the receiving antenna. In this situation, the reflected component could be much stronger than the direct component. The problem statement did not specify how the circuit board was oriented, so either the direct or the reflected component could be stronger.
It's worth noting that, at a 3-meter test distance in a semi-anechoic environment, the receiving antenna is still in the radiative near-field of this 1.5 GHz source. The phase relationship between the direct and reflected components is highly dependent on the source and antenna positions. The design and orientation of the receiving antenna can significantly impact the measurement (i.e., two antennas with the same nominal antenna factor may detect different field strengths).
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