Evaluation of applicability for semi-anechoic chamber as test site above 1 GHz

Free space condition is required of test site for EMI measurement above 1 GHz. The users of semi-anechoic chambers currently used below 1 GHz take notice of the applicability to use them above 1 GHz. We characterized conventional semi-anechoic chambers of 3 m and 10 m methods and investigated the requirements to realize free space condition, for example the requirement of covering the floor with electromagnetic absorbers in the range of 1 and 6 GHz. We covered at most a 3 m square of the floor of a semi-anechoic chamber with the electromagnetic absorbers. The electromagnetic absorbers used have return loss more than -30 dB in case of vertical incidence and more than -20 dB in case of oblique incidence at the applicable frequencies. We measured the height patterns of the electromagnetic field strength, compared them with the deviation of free space and evaluated them. In the result, it was found that free space condition is satisfied within an accuracy of 2 dB for horizontal polarization and 2.5 dB for vertical polarization in a semi-anechoic chamber of 10 m method. It was also found that when the frequency is over 3 GHz, the deviation from free space condition showed a marked increase in the semi-anechoic chamber of 3 m method, compared to the chamber of 10 m method.