Small Isolation Chamber

Open Source Instruments Inc. has moved from 397 Moody St, Waltham, MA, to 5 Pratt Ave, Waltham, MA. Our new office is a 180-m² basement with a concrete floor and plenty of half-windows.


Figure: New OSI Location: 5 Pratt Ave, Waltham, MA.

On Moody St, interference power picked up by a Damped Loop Antenna (A3015C) in the 925-928 MHz band was up to −58 dBm (1.6 nW). Here on Pratt Ave, the same 925-928 MHz power peaks at −45 dBm (32 nW). On a work bench, reception from a subcutaneous transmitter only at ranges less than 10 cm. Interference power in our new office is four times greater than in any other place we have measured (London, Oxford, Edinburgh, Guildford, Waltham, Boston). Our new location is a fine place to test the efficacy of radio-frequency isolation chambers.

We set up a small isolation chamber. It consists of six AN-77 absorbers, gray sides inwards, standing on the concrete floor. We wrap an outer layer of resistive sheet around the absorbers. We put a cover of the same resistive sheet on top. We assume the floor is a perfect absorber, because we are in a basement.

Figure: Small Isolation Chamber on Concrete Floor.

We set up our A3008C spectrometer and measure 925-928 MHz interference power for one minute with the pick-up antenna on the corner of our work bench, on the floor out in the open, in the center of our isolation chamber, and in the corner of our isolation chamber.


Figure: Interference Power in Separate Minutes and in Various Locations.

The peak interference power outside the enclosure is −45 dBm. Inside the chamber, the power never rises above −68 dBm (160 pW). The chamber appears to provide 23 dB of isolation. We place a Subcutaneous Transmitter (A3019D) inside the chamber 50 cm from each of two antennas connected to an Octal Data Receiver (A3027C). Over the course of one minute we obtain 100.0% reception. We remove the enclosure lid and reception drops to 29%.