Failure of the Faraday Canopy

Our FE2B Faraday enclosure provides a minimum of 20 dB (99%) rejection of interference power. As we described earlier, we hoped to duplicate this performance on a larger scale, so as to enclose an entire IVC (Individually Ventilated Cage) rack. The photograph below shows our faraday canopy with three towers of AN-77 absorbers in a triangle. Each AN-77 absorber is 61 cm square.


Figure: Triangle of Absorber Walls in the Faraday Canopy. Kirsten is standing still so as to allow us to obtain a stable interference measurement with the antenna resting upon the stool.
With no absorbers in the canopy, we usually obtained 10 dB isolation. With six AN-77 absorbers standing on the floor, we sometimes obtained 20 dB isolation, but often as little as 6 dB. With nine AN-77 absorbers in a wall 1.8-m square, we usually obtained 20 dB isolation, but sometimes as little as 6 dB. In one experiment, we observed the isolation to changed from 7 dB to 20 dB as the experimenter inside the enclosure changed where she was standing. (You will find a complete account of our experiments here.)

If we are to enclose an IVC rack, we must accommodate the two ventilation pipes. Catherine tried running the ventilation through steel mesh fabric, in the hope that we could simply pass the air through the sealed enclosure, but the result was dirt building up in the mesh, which degraded the performance of the ventilation. Thus we would have to insert two breaks in the enclosure of around 10 cm diameter each. This diameter is one third of our 900-MHz radio-frequency data wavelength, and so would allow interference to enter the cage, and once inside, the interference would reflect off the canopy walls until it struck an absorber. A sock of reflecting mesh around the ventilation pipe will not stop the penetration of interference, because once inside the sock, the radio waves will continue bouncing along its length until they enter the enclosure.

Even if we could improve the isolation of such an enclosure to 20 dB in the presence of ventilation holes, there remain many practical difficulties in moving the IVC rack into and out of the enclosure, and in supporting a sealed enclosure around the rack. Thus we have abandoned our plan to make a faraday canopy for the IVC. But we do have an alternative, and we will describe this alternative in our next post.

(NOTE: Our continuing work on radio-frequency isolation for IVC racks is funded by a budget separate from our ISL development budget, but we present the work here for want of a better venue, and because it is relevant to the ISL implementation.)

UPDATE: [01-JAN-14] We suspect that the poor performance of our faraday canopy is due to the canopy itself acting as a resonator in combination with the shield of our pick-up antenna cable or with the antenna itself. Any break in the canopy, such as an imperfect seam, can turn the canopy from an isolation chamber into a resonant cavity, as described here. Our cable sock will act as such a break, and cables passing through the sock can act as one half of a dipole antenna with the canopy, picking up energy incident upon the outside of the canopy. Solving these problems would be impractical for a canopy used in animal experiments, in which the experimenter should be able to enter and leave easily, and through which we must be able to pass ventilation pipes.

UPDATE: [28-FEB-14] We set up the faraday canopy in our new office. Ambient 902-930 MHz interference peaks at −43 dBm on our work bench. It enters via the roof and windows only, because we are in a basement. We place three antennas inside the cage, one on the floor and two on a box. We tape the shields of all three cables to the copper fabric floor of the canopy. Interference peaks at −61 dB, −52 dB, and −64 dB on the floor, box, and box.