Encyclopedia of radio electronics and electrical engineering / Civil radio communications
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It is known that the sensitivity and selectivity of simple single-loop receivers are low. Beginning radio amateurs use them to receive powerful stations in the LW and MW bands. However, if positive feedback is applied, i.e., the receiver is made regenerative, its sensitivity and selectivity will increase many times over. This allows you to receive radio signals on a short antenna, even in the KB range.
A diagram of a simple KB range regenerator is shown in the figure.
The input circuit is formed by the coil L1, the capacitance of the antenna WA1, the capacitors C1, C2 and the input capacitance of the transistor. The regenerative detector is assembled on a field-effect transistor with a pn junction. Part of the high-frequency energy amplified by this transistor is returned to the circuit through a capacitive divider formed by capacitors C1, C2. This compensates for the losses in the loop, increasing its quality factor and, accordingly, narrowing the bandwidth. The depth of feedback is regulated by a variable resistor R1, which allows you to come close to the threshold of self-excitation.
Signal detection is obtained due to the quadratic non-linearity of the transistor characteristic. The detected and amplified signal is filtered from high-frequency components by capacitor C3 and reproduced by high-resistance telephones BF1. The receiver is powered by any battery ("Krona", "Korund") with a voltage of 9 V. The current consumption is very small and is about 1 mA.
Coil L1 is wound turn to turn with PEL wire 0,4 ... 0,6 on a frame with a diameter of 9 ... 15 mm. The frame is equipped with a magnetodielectric core with a screw axis, at the free end of which a smooth adjustment knob is fixed. The number of turns depends on the desired range - for reception in the 12 MHz (25 m) range, it is 10 ... 12. You can also use a brass core, then the number of turns will increase slightly. It is desirable to use ceramic or mica capacitors, a variable resistor is suitable for any type (except wire), preferably with a smooth moving contact.
The receiver is mounted on a panel of any metal or foil material - this weakens the influence of hands on tuning. On the front side of the panel are displayed tuning and feedback knobs, phone jacks, antennas and grounding. On the reverse side of the panel there is a coil with a threaded bushing for the adjustment screw and all other details.
Setting up the receiver begins with checking the transistor mode - the drain voltage should be 3 ... 5 V with a DC resistance of 3,6 ... 4,4 kOhm. If necessary, select the resistor R2. Then, by connecting the antenna (a piece of wire about 1 m long) and adjusting the tuning frequency and the depth of the feedback, they try to receive radio stations. The tuning frequency range is adjusted by changing the number of turns and the winding pitch of the coil, the feedback depth is adjusted by selecting capacitor C2. It is desirable that the generation threshold be approximately in the middle of the stroke of the feedback adjustment resistor R1.
It should be noted that the passage of radio waves from distant radio stations in the KB range strongly depends on the time of year, day and solar activity. As a rule, in the daytime the passage is better on the high-frequency bands (25, 19 and 16 m), in the evening and at night - on the low-frequency bands (31, 41 and 49 m).
During the tests, the receiver with sufficient loudness received all the radio stations that, under the same conditions, were audible on the superheterodyne of the second class. The highest sensitivity is obtained near the self-excitation threshold. If, however, we go slightly beyond the threshold, in the autodyne mode (on beats) it is possible to listen to the telegraph and single-sideband telephone signals of amateur radio stations.
Author: S. Kovalenko, Kstovo, Nizhny Novgorod Region
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