Comparison between anechoic chamber and open area test site through analyses with loop antennas

To feed the transmitting antenna, a Tektronix AFG 3101 wave generator was linked to it by a BNC-type cable. The receiving loop was directly connected to a HP 8494A adjustable attenuator to adapt the 50 characteristic impedance of the wire. The wire was a N-Type about 5 meters long and led the signal to an Agilent E4402B spectrum analyzer. The current flowing in the transmitting loop was calculated from the impedance of the antennas and the impedance of the generator. Indeed the load was not adapted while the coaxial cable used was 50 adapted.
The OATS was made of a 6.5 x 6 meters reflecting surface. Since scanning the receiving antenna height is necessary in order to take account of the interference between direct and reflected waves, we built a mast with wood, ropes and plastic supports without iron or ferromagnetic parts. This type of construction, with a sliding support came out to be very useful to maintain the cable as stable as possible. The whole site and a detail of the mast are shown; it is also evident that the surrounding environment is not ideal, because many ferromagnetic objects are quite close to the site and could interfere with our measurements. The distance between the mast and the transmitting loop was 3 meters while the height of the receiving loop swept from 1 to 3 meters with 20 centimeters steps. The mast was closer to the reflecting ground edge to avoid as much as possible the cable interferences.
The anechoic chamber used has inner dimensions 7.94 x 3.74 x 3.34 meters (l x w x h). On ceiling and walls are screwed ECCOSORB FT-4 and glued TDK 512 25001 absorbers. On the floor the absorbers can be removed to set up the instruments. Inside the chamber the mast used was a Diesel one with an height range of 1-2 meters. This chamber has been calibrated in the 30-200 MHz frequency range referring to Seibersdorf OATS. The normalized site attenuation (NSA) for an antennas distance of 3 meters and, as required by specifications, the deviation is in within ±4 dB for the electric field.
The configuration used was exactly the same used for the OATS: the same distance, same instruments, same orientations. The only handicap is that we could not scan the whole height as outdoor, because the maximum height inside was just two meters.