Transmitter without a battery.

[Pommie]

In the recent thread about implanted devices, someone stated that a transmitter needs a battery. I was surprised to find out this was not true.
In 1945 a listening device was installed in a wooden seal installed in the USA's Russian Embassy.
It has no battery, no electronics and was only detected when someone (A radio operator) picked up the transmission from it.
A fascinating device - read all about it here.

Mike.

 

[rjenkinsgb]

If I remember that incident correctly, it had RF beamed at it, as a power source.
So, "Coupled" power from an external source, which I did mention.

No magic.

 

[Pommie]

In the documentary I saw about it, they likened it to a half full wine glass that can be made to resonate by rubbing a finger around the edge.

If I remember that incident correctly,

Can you really remember that far back?

Mike.

 

[Nigel Goodwin]

I certainly remember of it - although it was long before my time.

As I recall the Americans suspected the Russians were beaming high power RF at them, in order to make them ill - eventually it was realised that they were either powering, or recharging, bugs inside the building.

I also seem to recall that one of the bugs was a copy of the American 'seal' (whatever that was - but a big round wooden thing), which had been presented to the Americans and had a passive externally powered bug inside.

But basically it's all just RFID

 

[rjenkinsgb]

Can you really remember that far back?

I remember seeing an article on it, when I was a teenager.

 

[dougy83]

In the documentary I saw about it, they likened it to a half full wine glass that can be made to resonate by rubbing a finger around the edge.

Sounds like an analogy for a resonant LC tank circuit. There'd be another "wine glass" for transmitting the audio on a different frequency.

 

[nsaspook]

https://conferences.sigcomm.org/sigcomm/2013/papers/sigcomm/p39.pdf

The design of our ambient backscattering transmitter buildson conventional backscatter communication techniques. At a highlevel, backscattering is achieved by changing the impedance of anantenna in the presence of an incident signal. Intuitively, when awave encounters a boundary between two media that have dif-ferent impedances/densities, the wave is reflected back [18]. Theamount of reflection is typically determined by the difference inthe impedance/density values. This holds whether the wave is amechanical wave that travels through a rope fixed to a point on awall or an electromagnetic wave encountering an antenna. By mod-ulating the electrical impedance at the port of the antenna one canmodulate the amount of incident RF energy that is scattered, henceenabling information to be transmitted.To achieve this, the backscatter transmitter includes a switch thatmodulates the impedance of the antenna and causes a change in theamount of energy reflected by the antenna. The switch consists of atransistor connected across the two branches of the dipole antenna.The input signal of the switch is a sequence of one and zero bits.When the input is zero, the transistor is off and the impedences arematched, with very little of the signal reflected. When the switch in-put signal is one, the transistor is in a conducting stage which shortsthe two branches of the antenna and results in a larger scatteredsignal amplitude. Thus, the switch toggles between the backscatter(reflective) and non-backscatter (absorptive) states to convey bits tothe receiver

 

[nsaspook]

Hiding stuff.