The locked room mystery by Jakey G3JKY

Strictly speaking, the door wasn’t locked, it was shut tight, but first some background.

The affair occurred in the late 1970s during my time as a laboratory manager at Woolwich Arsenal. Our part of the site was QAD(FVE), Quality Assurance Directorate, Fighting Vehicles and Equipment. My little bit of the QAD was devoted to radio frequency interference tests on their products.  We had two screened rooms of our own, one about the size of a Portacabin, for testing components, and another one to accommodate a whole vehicle. The latter was big enough not just to swing a cat but to swing the gun turret of a Chieftain tank!

Such screened rooms are designed to exclude any external RF signals so that measurements can be made without outside interference.  Even under-floor screening is necessary. Older rooms were made from two wire mesh cages one inside the other, while later models are solid metal boxes.  All power and telephone lines have to be heavily filtered to prevent any signals from by-passing the screening.  Doors close tightly and have “spring fingers” to prevent any “slot aerial” effects.

To get back to the locked room, we were asked to test a mobile screened room for use by REME units working away from their base workshops.  It was a solid metal job on the back of a truck, with just one door.

The usual procedure was followed, with measuring receivers and aerials inside the room and a series of transmitters and aerials outside.  Signal level  was measured with the door open and again with it closed, the difference giving a measure of the attenuation. As was usual with a solid room, there was barely any discernible signal with the door closed, but somewhere in the region of 700 MHz there was a sudden peak in signal level.

Most of the team were vehicle electricians who had learned enough radio to conduct specified tests, so as far as they were concerned this was a puzzle worthy of  Sherlock Holmes or Miss Marple!

After a certain amount of  brain-racking, I realised that the metal door-handles were about a quarter wavelength long at the offending frequency. So, the outside handle received the transmitted signal which was then conducted  through the lock to the inside handle which promptly re-radiated it to the receiver.

I can’t remember whether there was electrical continuity between the lock spindle and the door itself but I assume there was not.

I never heard the end of the story because the room was returned to the makers and it had not re-appeared by the time I left the establishment.

Funny stuff RF!

Return to the index of Vital Spark articles.

Jakey – (G3JKY). From VS April 2009.

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