More about the eight-story microphone.
More about the «eight-story microphone».
The article «The story of the «eight-story microphone». Fiction or reality» contains a number of controversial points, which may have been a consequence of the limited volume of the publication and the authors' attempt to popularize the material presented.
Thus, the article considers the possibility of imposing a high-frequency (HF) signal on a telephone handset. In this case, the handset is represented by some equivalent oscillatory circuit with a resonant frequency w0. However, in our opinion, the physics of the modulation and re-radiation processes is presented not quite correctly. In particular, the statement that the current arising under the action of HF radiation and flowing through the microphone is a consequence of the EMF induced only in the telephone handset seems to be an error. In fact, the EMF is mainly induced in the supply line, and the resonant re-radiating system is a circuit caused not by the equivalent capacitance and inductance of the telephone handset, but by the total reactance of the entire device together with the subscriber wiring.
A change in the microphone resistance under the influence of an acoustic signal causes a change in the circuit quality factor and, as a result, leads to modulation of the HF signal. In this case, the modulated signal is re-radiated and can be detected.
From the above it follows that shunting the microphone with a capacitor is a reliable method of protection against the resonant method of information retrieval, which is unreasonably rejected in the mentioned article.
In addition, in our opinion, the article also somewhat overstates the range of possible listening by this method. The authors claim that if the source of the RF signal with a frequency of Fи = 370 MHz (wavelength l = 0.81 m) is the G4-107 generator with an output power of Ри = 40 μW, and the receiver is the R-375 device, then confident reception is ensured at a range of R = 100 m. Simple calculations can show the erroneousness of this statement. Thus, the level of the induced signal in the telephone handset at the specified range is determined by the expression:
where Gi = 1.6 is the directivity of the antenna, similar to the antenna of the R-375 receiver (Gprm = 1.6)
Gtlf = 1 is the directivity of the telephone handset,
h = 0.5 is the coefficient of mismatch in the polarization of the signal and the received elements of the handset,
V = 1 is a multiplier that takes into account the attenuation of radio waves in the propagation medium.
If we assume that the telephone set is an ideal re-radiator that does not introduce losses, then the signal power Pvx at the receiver input can be found using a similar relationship:
This power level in the antenna corresponds to the signal voltage at the receiver input Uvx = 1*10-7 V, which is an order of magnitude lower than the sensitivity of the P-375 (Uvx0 = 1.5*10-6 V).
It is necessary to pay attention to the fact that the modulation index, as well as losses during propagation (V = 1) and re-radiation, were not taken into account in the calculations, therefore the real value of the signal level at the receiver input Uin will be 10-8…10-9 V. Consequently, the possible range of HF imposition under these conditions will hardly exceed a distance of several meters. A review of the available literature shows that a range of close to 100 m can be achieved, but only with an electrical connection of the HF generator and receiver directly to the lines and their full matching.
The authors' assertion about the possibility of modulating the RF signal with a video pulse in linear elements (wires, lines) «with sharp bends» is questionable. It is known that modulation is a purely nonlinear process, and bending of wires does not cause nonlinearity. In fact, modulation is possible only in semiconductor and vacuum tube elements (diodes, transistors, integrated circuits, etc.) that are part of terminal devices, and the presence of bends and twists only leads to a change in the parameters of long lines as antenna systems.
From our point of view, the explanation of the possibility of using a picture in a metal frame to intercept speech signals can be challenged. Thus, the authors claim that under the influence of acoustic waves, the capacitance of the «picture-substrate-frame» system changes and this leads to modulation of the probing RF signal.
Of course, any physical body has reactive parameters and these parameters can really change under external influence. However, the capacity of the specified system is very small, and its relative change is so insignificant that it does not allow us to talk about registered modulation in this case, especially since the authors assume that «the radiation levels are very small».
It is known that since about the beginning of the 40s some special services actually used resonant systems that changed their parameters under the influence of acoustic signals to modulate probing high-frequency radiation, but these were specially designed devices matched to the parameters of acoustic and electromagnetic waves. However, even in this case, to achieve an acceptable range (several tens of meters), such significant power levels were required that the service personnel had to work in special protective clothing. It was the high level of irradiating radiation that was the unmasking factor that significantly limited the possibility of using such systems.
In the case under consideration, the physical basis for the ability to intercept speech signals is not a change in the capacitance of the pattern, but the Doppler effect. The fact is that the oscillation of the pattern causes the appearance in the spectrum of the reflected signal of components shifted relative to the frequency of the probing radiation by an amount proportional to the oscillation speed.
Using a receiver with a frequency detector allows you to isolate the original speech signal, which has been practically confirmed in laboratory conditions.
Despite the physical feasibility of this method of obtaining confidential information, the same energy problems stand in the way of its practical use. The main ways to solve them are to reduce the wavelength of the probing signal and narrow the directional patterns of the transmitting and receiving antennas.
These circumstances forced the developers of special equipment to switch to laser acoustic reconnaissance systems (approximately in the 60s), and to use window panes or other «membranes», such as paintings, as oscillating elements. Nevertheless, laser systems have not found wide application either due to their high cost and complexity of operation.
Thus, the problem of protecting commercial secrets from HF imposition via a radio channel is not as urgent as the authors of the publication under discussion assume, so it would be more appropriate to direct the main efforts at present to closing other technical channels of information leakage…