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I was just trying to make a passive cavity resonator based listening device just the same way as Leon Theremin did (the great seal bug). I know that I am to use metallized mylar film as the diaphargm, and I am aware that the cavity dimensions are frequency based.

So what I do not know is how to calculate the cavity dimensions at 400MHz and what is to be the size of the Tuning post stationed in the center of the cavity, and the monopole antenna would be transmitting the harmonics.

The cavity resonator is energized remotely by an RF transmitter @ 400MHz, 17dB.

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    \$\begingroup\$ Please provide a link to your design source. \$\endgroup\$ – Andy aka Dec 11 '13 at 11:01
  • \$\begingroup\$ "Spycatcher" has some details - not many. Easier would be lumped constant TX and RX circuits. \$\endgroup\$ – Russell McMahon Dec 11 '13 at 15:05
  • \$\begingroup\$ Not to be too picky, but this doesn't have anything to do with RFID, just plain ol' RF. :-) \$\endgroup\$ – TimH - GoFundMonica Dec 11 '13 at 22:09
  • \$\begingroup\$ @TimH You are being too picky :-). The similarity with RFID is strong enough for the name to be usefully used - and changing to plain "RF" greatly detracts from the title. RFID typically (not invariably) uses a master TX to supply power to operate a remote TX transponder. The remote may be close to fully passive In the 'Great Seal bug" the device is powered remotely and reradiates an audio signal rather than a telemetry signal. Arguably it was tasking the pulse of the US ambassador :-) \$\endgroup\$ – Russell McMahon Dec 12 '13 at 6:05
  • \$\begingroup\$ Perhaps a more accurate title would have been "Passive RF Listening Device", since RFID is not always passive, and this circuit has no method for electronically storing identification information... But RFID gets the point across, I guess. \$\endgroup\$ – TimH - GoFundMonica Dec 12 '13 at 17:02
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It seems likely from inspection of diagrams and photos of the device that the cavity was resonated in conjunction with a coupled external 1/4 wave whip - although that is not what is said in the text accompanying the diagram below. Text and photo from here

They say

  • It was simply a resonate chamber, with a flexible front wall that acted as a diaphragm, changing the dimensions of the chamber when sound waves struck it. It had no power pack of its own, no wires that could be discovered, no batteries to wear out. An ultra-high frequency signal beamed to it from a van parked near the building was reflected from the bug, after being modulated by sound waves from conversations striking the bug's diaphragm.

Photo of "The Thing" here from this website

enter image description here

This gives a clearer picture. The cavity is a high Q resonant cavity tuned to the frequency of the whip. Details of design of such cavities can be had in the various ARRL and RSGB amateur handbooks or via various on web papers and calculators

enter image description here

Two photos below from "Spybusters" here

enter image description here

enter image description here


Cavity resonator design:

Useful paper. Bell Labs 1946 !!!
High Q Resonant cavity for microwave testing

Masters thesis. 2010
Design of a Cylindrical Cavity Resonator for Measurements of Electrical Properties of Dielectric Materials

CERN? 1995?
Resonant Cavity Design Using the Finite Element Method

Wikipedia - "Thing"

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  • \$\begingroup\$ i wanted to ask about the dimensions of the cavity resonator @ 400mhz..... \$\endgroup\$ – user33984 Dec 12 '13 at 3:19
  • \$\begingroup\$ can anyone help mme out... \$\endgroup\$ – user33984 Dec 12 '13 at 3:20
  • \$\begingroup\$ and yes it is not rfid based its rf :) \$\endgroup\$ – user33984 Dec 12 '13 at 3:20
  • \$\begingroup\$ @user33984 Nobody can help you out if you do not read the materil that they provide. The diagrams above show dimensions and the papers that I posted links to provide resonator design information. Have you looked at the articles cited? \$\endgroup\$ – Russell McMahon Dec 12 '13 at 6:07
  • \$\begingroup\$ @ Russell McMahon; Thanks Russel for your kind and generous help. I have been able to find the dimensions of the cavity resonator quite successfully. And there is 1 more thing that i want to know, How the dimensions tuning post- which will make the device a capacitive resonant cavity- are calculated? \$\endgroup\$ – user33984 Dec 12 '13 at 7:05

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