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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Is everything a detector? Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur It would seem that such a simple device as a detector receiver has been known to everyone since school. Nevertheless, even today publications about him are not uncommon. Apparently, because the interest in "detectors" is not weakening among new generations of radio amateurs who need to start somewhere. And with the accumulation of experience, the revision of everything old, a warm attitude to the good "retro" is also preserved ... Below is a number of practical schemes developed by the author and tested experimentally almost twenty years ago, but have not lost their value and relevance to this day. SECONDARY MODULATION WORKS Let's take two radio receivers: detector (DP) and conventional broadcasting (RVP), for example, a network lamp (Fig. 1). Let's connect to both by antenna and ground. And then we will set up to receive the same broadcasting (RB) station, preferably the most powerful one received in the area. For example, in the medium wave (MW) range. If now we start talking into the "headphone" BF1 DP, then we will hear our voice in the RVP dynamics.
What is going on? RV station emits electromagnetic oscillations (waves). Spreading in all directions, they cross the antennas of the receivers, inducing EMF there. In each of the input circuits, electrical oscillations will occur. Moreover, the range of the latter depends significantly on the resonant properties of the circuits themselves. And to a large extent - from the so-called quality factor: the higher it is at the input oscillatory circuit, the greater the radio frequency (RF) voltage can be removed from it. This is, so to speak, one side of the matter. And the other is that in the DP we, in fact, are dealing with the output circuit of a low-power "transmitter", which receives (as mentioned above) RF energy from the RF station and re-radiates it (using the same antenna) in the form of secondary radio waves Moreover, this process will proceed the stronger, the closer the antenna dimensions are to resonant ones (that is, the better the antenna is tuned and matched with the DP). In order to detect secondary radio waves (and their parameters, with the exception of the amplitude of oscillations, reduced at the "transmitter" due to inevitable losses, will coincide with what the RV station emits), secondary modulation is necessary. It is easy to implement using the headphone BF1 in the circuit and the germanium diode VD1. Capacitor C2 will then serve as a "decoupling" for RF and audio frequency oscillations (see Fig. 1). The "range" of such an experiment depends both on the magnitude of the signal received by the detector receiver and re-radiated by the improvised "transmitter" (TP), and on the thoroughness of the manufacture of the DP antenna (as mentioned above). The frequency of our IP is rigidly synchronized with the frequency of the RV station. If secondary modulation is not heard in the RVP speaker, then the broadcasting receiver is tuned to the frequency of another transmitter broadcasting the same program. Or the above principles are violated and the range of the IP turned out to be extremely small. Although in practice the "range" of an improvised "transmitter" can even reach up to several hundred meters. Phone BF1 - high-resistance (1600 - 2200 ohms). Loop data L1C1 is not given, as it depends on the wavelength (frequency) of the RF station, which is confidently received in your area. Yes, and the circuit solution for homemade products is offered in such a way that it practically removes the very severity of the problem. After all, the tuning frequency of the L1C1 can be changed quite easily and over a wide range. It is enough just to turn the rotor at the variable capacitor (KPI) C1 by the appropriate angle. The phenomenon of secondary modulation (and emission of radio waves) was applied by the author in practice in a security device, the basis of which was the DP considered above, equipped, however, with a multivibrator. As the latter, a device assembled according to the scheme and recommendations [1] is quite acceptable. Connection - in parallel with the "headphone" BF1, but through a capacitor. And in the power supply circuit - contacts from sensors installed at the protected facility. In standby mode, a regular radio broadcast was listened to at the RVP. The appearance of an additional sound with the frequency of a multivibrator meant the operation of a security device. Moreover, as practice has shown, the sensitivity of such a system can easily be increased during transmission pauses. It is enough, by setting the RVP volume control knob to the maximum volume position, to switch to listening to the protected premises through ... the "headphone" BF1 DP. Of course, such a simple-to-manufacture security system only works effectively when a confidently received RV station is operating. That is, when there is - its carrier. It is also quite acceptable to use a similar home-made device as a kind of demonstration communication system (albeit over short distances), for which it is necessary to have two DPs, two RVPs, resonant antennas and high-quality grounding. NON-CONVENTIONAL POWER SOURCES The next aspect is the use of detector receivers as "unaccustomed" power supplies (PSU) of not too energy-intensive homemade products. On fig. 2 shows a circuit diagram of such a "low-power battery".
This device differs from a conventional DP by the presence of an RF low-pass filter, which eliminates the penetration of RV station signals to the output of an unconventional power supply. It is advisable to use such a PSU near radio transmitting stations, where the field strength is quite high. For example, in Tyumen, within the city, there is a powerful MW RV station, its field strength was enough not only to power the generator [1], but also for a fairly powerful receiver [2], thanks to which the programs were received confidently in the VHF FM band. The PSU capacitor C4 is oxide, with the maximum possible capacitance and low leakage resistance. Diode VD1 - silicon (with maximum reverse and minimum forward resistance). Well, the requirements for the antenna, grounding and quality factor of the circuit are well known. In particular, the antenna used for these homemade products must have a resonant length. Grounding - to be of high quality. As for the quality factor of the oscillatory circuit, the more significant it is, the higher the voltage can be obtained, which, in conjunction with C4, will result in the corresponding power delivered by the PSU to the load. If the antenna has a low-impedance reduction, made, for example, with a coaxial cable, then it should be connected to the L1 coil, as shown in the diagram (dotted line). Moreover, we recommend choosing the number of turns from which the tap is performed experimentally (according to the maximum output voltage). In this case, the L1C1 circuit must be tuned into resonance with the received powerful RV station. When using surrogate antennas (in order to reduce the influence of their parameters on the quality factor of the L1C1 circuit to a minimum), it is advisable to install an isolation capacitor Cp, the capacitance of which is selected according to the maximum output voltage of the PSU. The use of surrogate antennas is justified only at a very high field strength of the received RV stations and, of course, gives worse results compared to resonant ones, which can still be made full-sized (without shortening) in the medium wave range. Coils L1 and L2 - from any RV receiver of the corresponding range. Capacitors C2, C3 - radio frequency (for example, K10-7, KM). And as a C4, the fairly common K50-16 is quite suitable. RECOMMENDED MODIFICATIONS OF THE DETECTOR RECEIVER Do you want to simplify the DP circuit as much as possible, or even in general: to make the detector receiver "superminiature", portable? Of course, all this is possible if there is a high field strength in your area created by RF stations.
It is quite acceptable, in particular, to implement any of the circuit diagrams shown in Fig. 3. Moreover, the modifications "a" and "b" are such that when point A touches the antenna (and sometimes even the central heating battery), a powerful station is received loudest of all. Germanium diodes D2, D9, D18 work well here; silicon ones "work" worse, if not not suitable at all for use as "the simplest amateur detectors". It has also been noted that DCs made according to the schemes (Fig. 3, a and 3, b) have higher performance characteristics if the diodes are placed in the immediate vicinity of point A. An increase in "parasitic" capacitances between DP and "earth", which can be easily seen if, say, take hold of the wires leading to the telephones. The elementary structures discussed above can be safely used as an RF probe when tuning and matching amateur transmitters with antennas (or, for example, "during checking for the presence of vertical and horizontal pulses in television technology). But if these simplest DCs are supplemented with an L1C2 circuit with the selection of decoupling capacitances C3 , C4, we get more advanced devices.The best work in them is not germanium, but silicon diodes. The required nominal value for C3 and C4 is determined by temporarily connecting a graduated KPI block instead of them, followed by replacement (when the output of the DP reaches the macro signal level as the rotor rotates) with the corresponding capacitors of constant capacitance. Is it possible to make DP "talk" louder? Of course. For example, by connecting several detector receivers in parallel when operating on a common load. Each DP here has its own antenna, which can be placed in different ways (phase shifts do not have a significant effect on the SW and especially on the LW due to the large wavelength). The number of simultaneously operating detector receivers is determined by the number of antennas and sections of the KPI block at your disposal. Well, if the "composite" DP operates at a fixed frequency, then the effect will depend only on the antennas themselves. As a "group load" you can use the RT broadcast receiver. The volume level here is already determined by a combination of several factors. The result will undoubtedly be affected by the power of the incoming signals of RT stations, the number of DPs in the group, and the thoroughness of their tuning. And, of course, - the quality of workmanship, debugging of grounding and antennas. Moreover, the latter is covered in sufficient detail in the relevant literature [3]. Group inclusion of detector receivers can be recommended for foresters' lodges, tourist camps, dachas located in the coverage area of powerful RV stations. That is, where there is enough space for large antennas, but there is no power grid. During the operation of the DC (with group inclusion), the voltages obtained in the process of detection are supplied to the common load, significantly increasing the current in it. The detectors in all receivers can be both ordinary half-wave and improved ones (Fig. 4), but they are the same for all DPs in the group.
Literature
Author: V.Besedin (UA9LAQ), Tyumen
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