Encyclopedia of radio electronics and electrical engineering / radio reception

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In portable, and not only in portable radios, in most cases a ferrite magnetic antenna is used. It is easy to manufacture, has a fairly high sensitivity.

However, it is far from always possible to realize the advantages of this antenna in small-sized structures. In particular, in such designs it is necessary to use very short magnetic cores, which leads to a sharp decrease in the efficiency of the ferrite magnetic antenna. In this case, the use of conventional loop antennas gives a good effect, as is done in a miniature receiver, the circuit of which is shown in Fig. 1.

Rice. 1 (click to enlarge). V1, V2, V5 - KT315B; V3, V4 - D9A

The receiver is designed to receive signals from broadcasting stations in the medium wave range (500...1500 kHz). The input circuit is formed by a loop antenna L1 and a variable capacitor C1. The signal selected by the C1L1 circuit is taken from part of the turns of the coil L1 and fed to the first stage of the RF amplifier on the transistor V1. From the load stage (resistor R2), the amplified signal is fed through the capacitor C3 to the second stage of the RF amplifier (transistor V2). From the load of this stage (resistor R4), the signal is fed to the detector, assembled on diodes V3, V4 according to the voltage doubling scheme. The load of the detector is resistor R5, shunted at high frequency by capacitor C5.

The low-frequency signal from the detector goes further to the low-frequency amplifier, assembled on just one V5 transistor. Its load is a small-sized telephone TM-2, plugged into socket X1. The DC voice coil resistance of the phone is 65 ohms, and the impedance at 1 kHz is 450 ohms. Here you can apply other phones, the parameters of which are close to those given above. The most pleasant sounding timbre is set by selecting the capacitor C7, connected in parallel with the telephone.

The receiver is powered by switch S1. The power source is one galvanic cell 316.

The receiver uses fixed resistors MLT-0.125; capacitor C1 - with a solid dielectric, type KP-180; C2, C5, C7 - KM; C6 - K53-1; transistors - KT315B, but KT306, KT301, KT312 are also suitable. Diodes D9A can be replaced with other diodes of the D9 series. Under these details, a printed circuit board is calculated (Fig. 2), which can be cut out of foil-coated getinax or fiberglass. Element G1 is glued to the board or attached, for example, with a wire loop, a metal bracket, and other methods. Conductors are soldered to the output element and connected to the corresponding contact pads of the printed circuit board.

Fig. 2

For the manufacture of a loop antenna, you will need a plywood blank 5..6 mm thick and 56x56 mm in size. The corners of the blank are slightly rounded, and then 39 turns of PEV-2 0,15 wire are wound on its end surface. The tap is made from the 4th turn, counting from the bottom, according to the scheme, output. It is advisable to check the inductance of the antenna - it should be 350 μH.

It is advisable to place the antenna in a PVC tube with a diameter of 3 and a length of 225 mm. In this case, the tube cut along the length is pre-fixed on the blank, the antenna wire is wound and the seam is welded with a heated soldering iron. Then the antenna is removed from the blank and attached (with threads or glue) to the board with parts.

In this state, the receiver can already be checked in operation by connecting a telephone in parallel with capacitor C7 and applying power (by closing the conductors going from the board to the switch). By rotating the knob of the capacitor C1 and simultaneously orienting the antenna in different planes, they tune in to the radio station. The loop antenna is left in the highest volume position. The loudest and most undistorted sound of the receiver is achieved by selecting resistors R1, R3, R7 (for the time of testing, it is advisable to replace them with variables).

You can change the inductance of the loop antenna only by selecting the number of turns. This is a rather time-consuming operation, therefore, if the range of received frequencies is shifted towards higher frequencies, then a constant capacitor C1 should be connected in parallel with the variable capacitor C1 in the figure in the text). It is soldered directly to the terminals of the variable capacitor C1. However, the introduction of such a capacitor reduces the frequency coverage of the receiver. That is why it is advisable to use a dual capacitor as Ce (from any small-sized receiver), including its sections in parallel. This creates a certain amount of overlap, which makes it easier to set the desired frequency range. If the range of received frequencies is shifted towards lower frequencies, then its installation can be carried out only by selecting the number of loop antenna turns.

The board is placed inside the case (on the tab at the top right) with dimensions of 60x60x25 mm. On the side wall of the case, a power switch S1 (slider type) and a socket X1 for a TM-2 phone connector are fixed. You can do without a switch if you use a connector with a group of contacts that close when the phone plug is inserted.

Author: G.Shulgin, Moscow

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