Application Design of Magnetic Antenna and Intermediate Frequency Transformer in Radio
This article analyzes the important components in the radio-magnetic antenna and intermediate frequency transformer.
1. Magnetic antenna
A magnetic antenna is a coil of two sets of coils that are not connected to each other on a magnetic bar, and its role is to receive electromagnetic waves in space. The magnetic antenna has good directivity, so that when the radio rotates in a certain direction, the sound is the loudest, and the noise is reduced.
In the process of assembling the transistor radio, a magnetic antenna can be made.
1. The Q value of the magnetic antenna is related to the magnetic rod material and the frequency of use. Receiving medium-wave signals uses manganese-zinc ferrite magnets; receiving short-wave signals uses nickel-zinc ferrite magnets. For the magnetic rods with the same diameter or cross section, the longest length should be preferred to improve the radio sensitivity.
2. The Q value of the magnetic antenna is significantly affected by the position of the coil winding. It can be seen from the experiment that the best position of the coil on the magnetic bar is 1/5 to 1/3 of the distance between the center of the coil winding length and the midpoint of the length of the magnetic bar. At this time, the movement of the coil has a large change in inductance and directivity. Okay.
3. The current flowing through the magnet bar coil is higher in frequency. Because the high-frequency current has a skin effect, the resistance of the conductor to the high-frequency current increases with the increase of the current frequency, and as a result, the power loss of the wire to the high-frequency signal also increases. In order to overcome this, in the mid-wave range, magnetic antenna coils usually consider increasing the cross-sectional area of ​​the conductor, that is, using 7 strands of enameled wire with a diameter of ∮0.1Mm; short-wave magnetic antenna primary coils often use ∮1-1. The 5mm silver-plated copper wire is wound, and the distance between turns is generally 2 to 3mm.
4. Since the impedance of the tuning loop is high and the input impedance of the triode is low, the impedance matching problem is considered to maximize the signal output. The turns ratio of L1 and L2 is generally about 10: 1, and L1 is usually 60 to 80 turns.
5. When using magnetic antenna devices, keep them away from magnetic fields (such as speakers, transformers, etc.). The bracket should be made of non-metallic materials. The magnet bar is easy to break, so be careful during use.
Second, the intermediate frequency transformer
Intermediate frequency transformer (commonly known as mid-week) is a transformer with a fixed resonant circuit unique to superheterodyne transistor radios, but the resonant circuit can be fine-tuned within a certain range so that it can reach a stable resonant frequency after being connected to the circuit 465kHz). Fine-tuning is done by means of changes in the relative position of the magnetic core.
The intermediate frequency transformers in radios are mostly single-tuned, with a simple structure and a small footprint. Due to the low input and output impedance of the transistor, in order to make the intermediate frequency transformer match the input and output impedance of the transistor, the primary has taps and has a secondary coupling coil with few turns. The advantage of the dual-tuning type is better selectivity and a wider passband, which are mostly used in high-performance radios.
Transistor radios usually use two stages of intermediate frequency amplifiers, so three mid-periods are needed to couple and transmit the front and back stage signals. The BZ1, BZ2, and BZ3 symbols are commonly used in the middle circuit in the actual circuit. It is not possible to change their positions in the circuit at will during use.
The shape of the oscillating coil (medium wave) is similar to that of the mid-periphery, and it and the corresponding components form the frequency conversion stage of the transistor radio. Adopt equal volume double connection (270pF × 2), adjust the resonant frequency of the input tuning loop and the local oscillation frequency of the local oscillator circuit at the same time to ensure that there are: f vibration-f letter = 465kHz in the entire receiving band. Commonly used models are LTF-2-1 (primary 144 + 8.5 turns, secondary 11.5 turns) and LTF-2-3 (primary 4.5 + 82 turns, secondary 8 turns).
Finally, one point should be mentioned: the tuning should be as far as possible without tuning. Each time the range of the magnetic cap of the intermediate frequency transformer or the oscillating coil is adjusted not to be too large, care must be taken to prevent the magnetic cap from breaking.
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