Can I tap the radio signal? History and the present question

Practically in all forums devoted to radio, periodically a question arises - is it possible to recruit a working radio. The most "advanced" who still remember the basics of physics, answer that you can because in the receiver there is a local oscillator, which emits, others say that it is impossible. The question was relevant about 30 years ago, when people secretly listened to the “voices”, but even now you can hear horror stories that if you set up the receiver for a “police wave”, then the owner will immediately arrive.

Is it possible or not? Let's try to figure it out and conduct an experiment.

Details under the cut.

In order to understand whether it is possible to assign a receiver, refer to their creation history.
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Regenerative receiver

In the 30-50s, the first years of the emergence of mass radio, the regeneration receiver was the simplest and most popular then:



This receiver works due to positive feedback in the antenna circuit, due to which the circuit is introduced into the generation mode. Thanks to the positive feedback, the reception quality is improved (details can be found in Wikipedia ). The receiver has a simple design from just one lamp, while providing very high-quality reception (some hams collect them so far ), but the circuit connected to the antenna really emits. These receivers really radiated so much that their users interfered with each other, and this became a problem in the cities. According to some sources, during the Second World War it allowed the Germans to find and catch the owners of the receivers in the occupied territories.

Direct gain receiver

Receivers of this type do not have internal generators, so they emit nothing. Due to the simplicity of the design were popular with the pioneers in the radio circles. However, their parameters leave much to be desired, and with time they were replaced by more advanced superheterodyne ones.

Superheterodyne receiver

Receivers of this type have good characteristics, they are used now.



As can be seen from the flowchart, the input signal is multiplied with the local oscillator signal, which allows (recall the school cosine multiplication formula) to transfer the signal to the desired frequency. You can find out more on Wikipedia , but the essence is important to us, that this receiver contains elements that create spurious radiation. The local oscillator of the receiver is not connected directly to the antenna, but its signal can be re-emitted through adjacent tracks on the board, passing conductors nearby, etc. The efficiency of such a process is small, but it exists.

So, is it possible to locate the radiation of the local oscillator? Yes and no. The first receivers of this type were imperfect, no one was particularly worried about spurious emissions. As Wikipedia says about the power of the local oscillator, "in 1940-1960 it reached 300 mW, in the 1970s with the transition to transistor circuits was reduced to 20-30 MW, with the transition to integrated circuits in the 1980s - reduced to units milliwatts, and in modern digital tuners does not exceed tens of microwatts ".

Much or not 300mW for radio? For old shortwave receivers, large outdoor antennas were often made. Suppose that only 1% of the power emitted by the local oscillator reaches the antenna. A 3mW signal with an outdoor antenna can actually be received at a distance of 50-100 meters. However, already for transistor receivers the power of the local oscillator is 10 times less, and the reception of a signal with a power of a fraction of milliwatts can no longer be considered seriously. At best, it would be heard at a distance of several meters.

In general, theoretically, everything is already clear. Is it possible to recruit a working radio? The problem really was with the regenerative receivers (and, according to reviews, they were really bearing on the war) could be with the old tube radioli (but we couldn’t find any evidence of this), and later with the decrease in size and element base, the radiated power became so small that it could already be neglected. Moreover, it is not even known whether attempts of such direction finding existed at all during the Soviet era, there are no reliable references. Perhaps, experiments in some research institutes were carried out, but the matter probably did not go further. And, as you know, there was no criminal liability in the USSR for listening to the “voices of the west”, and, according to reviews, practically everyone who wanted to listened to them.

Experiment

Finally, the promised experience. Unfortunately, I don’t have the old “Rigonda” or something similar (and it would be interesting to check), so let's check this myth in the modern version. The subject is a broadband Icom IC-R6 receiver (often called a “scanner” in everyday life) with reception frequencies from 0 to 1.3 GHz (it was bought quite legally in a Moscow store, with a check and a guarantee).



If you believe the horror stories, then immediately after turning on they should come to me. Let us verify that this “scanner” radiates on the air. There will be two tasks - to catch the signal of the local oscillator, and find out what the user is listening to for the station.

First of all, we find a service manual on the network and look at the radio part scheme:



As you can see, the input signal must “fall” into the band-pass filter, whose center frequency is 266.7 MHz. VFO (Variable Frequency Oscillator), which is also a local oscillator with which the frequency is tuned, is capable of operating in the range of 266-1100 MHz.

From here it is clear what to look for. For example, let's tune the receiver to an FM radio station at 89 MHz. The station signal must fall into a band-pass filter at a frequency of 266 MHz; for this, the local oscillator must be at a frequency of 89 + 266 = 355 MHz. For the search, we use a control SDR receiver capable of displaying a wide bandwidth of several MHz.

First we look for the local oscillator signal, tune to 355 MHz. We see a lot of things:



Where is the desired signal?



As you can see, the signal of the local oscillator is lower in the level of other third-party interference, and without knowing it, it is impossible to distinguish it. I specially rebuilt the receiver so that the line “moved”, otherwise it would be completely incomprehensible where to look for it. The maximum reception range was 5 meters , then the signal completely disappeared below the noise level.

Now let's try to “catch” the station that the user is listening to. We adjust to the input frequency of the filter 266 MHz. Turn on ... and nothing. No sound present. We turn to the plan "b": we wind the receiver with a wire, connect it instead of the antenna, so that the signal emitted by it is guaranteed to be received. There is a contact: the signal of our FM station at 266.8 MHz is rather weakly visible on the spectrum.



You can even listen to the signal and make sure that this is the same radio station.

findings

You can briefly answer the questions posed by the topic.
- Is it possible from a long distance to locate the local oscillator operating receiver? You can, if you place it in a clean field, where there is no interference, and aim at it the antenna of remote space communications. In all other cases, the millivolts of the heterodyne signal will be drowned in the general noise of city noise.
- Is it possible to quietly know which station the user is listening to? You can, if you quietly wind the receiver with a wire, and connect the wire to another receiver.
- Is it possible to unnoticeably find out from a signal of a local oscillator, what frequency is the user's receiver tuned to? No, because There is no single standard for conversions in modern superheterodyns; you need to look for a description of a specific model in the service manual.

I hope this educational program was useful to someone.