Receiving antenna, described by the author in the article "Active indoor antenna LW band" ("Radio", 2009, vol. 7, pp. 16-18), has good features, but not protected from the so-called common mode noise (they penetrate to the input of the RF amplifier and along with the useful signal are input to the receiver). To combat them using differential amplifiers. Replacing such amplifier used in the amplifier design for a MOSFET, it is possible to considerably reduce its sensitivity to common-mode interference and thereby improve the reception quality.
Fig. 1
Diagram of the proposed active antenna shown in Fig. 1. The differential amplifier is built on dvuhstvornyh field-effect transistors VT1 and VT2, the load of which is RF matching transformer T1. With its secondary winding signal is sent to the socket XS1, and with him on the shielded cable to the external antenna Jack of the radio. Magnetic antenna WA1 - shielded single-turn frame. Its construction is similar to that described in the article mentioned above, but connected it differently: one output frame through the contacts section of the switch SA1.1 is connected to the input of the upper (the scheme) of the shoulder of the amplifier, the other (via the contacts section SA1.2) from the lower entrance. To improve the symmetry of the "lengthening" of the coils L1, L2 and L3, L4, used for coarse adjustment of the antenna in frequency, included both the inputs of the amplifier, and the smooth setting is dual capacitor variable capacitor C2, the sections of which, unlike the above construction, used separately.
The signal frequency to which the tuned antenna, are fed to the inputs of the amplifier in opposite phase, so in the RF transformer T1 they add up in-phase and the amplitude increases.
The same signals with frequencies different from the frequency settings, as well as the induced interference signals from the surrounding household equipment arrives at the amplifier's common-mode, so the transformer are added in phase, and their amplitude decreases. The gain of the stage is controlled by varying the voltage on the second gate transistors coming from the engine, variable resistor R3. In a differential amplifier using resistors and capacitors of the same type as that of the amplifier on a single transistor. RF transformer T1 is wound wire sew-2 0.1 na annular magnetic core with a diameter of 8...10 mm ferrite magnetic permeability 600... 1000. Winding I contains 30 turns with a branch of the middle coil 11-10 turns. To the best of symmetry of the primary winding should be wound folded in half wire (15 turns), and then connect the end of one wire to another and thus to obtain the withdrawal. Coils L1, L3 (16 turns) and L2. L4 (50 turns) wound wire sew-2 0,2 directly on the threaded trimmers with a diameter of 4 and a length of 11.5 mm from carbonyl iron (used in armored cores SB-12A). Field-effect transistors it is desirable to select for the same current drain for several values of the voltage of the gate.
Fig. 2
Details of the amplifier are mounted on the side of the printed circuit Board conductors from bilateral foiled fiberglass thickness 1...1,5 mm made in accordance with Fig. 2. The foil opposite side is used as the common wire. For the passage of the conclusions to be with him parts in the circuit Board drill 14 holes. Sizes fees are the same as those of the amplifier on a single transistor that can be replaced without substantial revision of the design of the active antenna.
The establishment of the differential amplifier RF is reduced to setting the same current through the transistors. For this purpose, the primary winding of the transformer T1 temporarily replace the same constant resistors 200...300 Ohms, set the slider to the resistor R3 to the middle position and by connecting a DC voltmeter to the drains of the transistors, a trimming resistor R5 sets the zero voltage If necessary, the boundaries of the sub-bands displace the same by changing the number of turns of the inductors L1, L3, L2, L4 (if the border must be shifted towards higher frequencies, the number of turns is reduced, and if, on the contrary, in the lower, - increase).
Author: I. Nechaev, Moscow; Publication: www.radioradar.net