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Introduction to the basic knowledge of magnetic antennas and intermediate frequency transformers

This article analyzes the important components in the radio - magnetic antennas and intermediate frequency transformers.
First, the intermediate frequency transformer
The intermediate frequency transformer (commonly known as the middle week) is a unique transformer with a fixed resonant circuit in the superheterodyne transistor radio, but the resonant circuit can be fine-tuned within a certain range so that a stable resonant frequency can be achieved after the circuit is connected ( 465kHz). Fine tuning is accomplished by means of a change in the relative position of the core.
Most of the intermediate frequency transformers in the radio are single-tuned, with a simple structure and a small footprint. Since the input and output impedance of the transistor are low, in order to make the intermediate frequency transformer match the input and output impedance of the transistor, the primary has a tap and has a secondary coupling coil with a small number of turns. The advantage of the dual tuning type is better selectivity and wider passband, which is often used in high performance radios.
Two-stage IF amplifiers are commonly used in transistor radios, so three mid-weeks are required to couple and transmit the signals from the front to the back. In the middle of the actual circuit, the BZ1, BZ2, and BZ3 symbols are commonly used. They are not free to change their position in the circuit during use.
The shape of the oscillating coil (medium wave) is similar to that of the middle circumference, and it and the corresponding components constitute the inverter stage of the transistor radio. The isometric double connection (270pF×2) is used to adjust the resonant frequency of the input tuning loop and the local oscillator frequency of the local oscillator circuit to ensure that the f-vibration-f letter = 465kHz is available in the entire receiving band. Commonly used models are LTF-2-1 (primary 144+8.5匝, secondary 11.5匝) and LTF-2-3 (primary 4.5+82匝, secondary 8匝).
In addition: the tuning should be adjusted as much as possible without tuning. Do not oversize the magnetic cap of the IF transformer or the oscillating coil each time. Pay attention to the force to prevent the magnetic cap from rupturing.
Second, the magnetic antenna
A magnetic antenna is a pair of coils that are not connected to each other on a magnetic bar, and functions to receive electromagnetic waves in a space. The magnetic antenna has good directivity, so that when the radio rotates in a certain direction, the sound is loudest and the noise is reduced.
In the process of assembling the transistor radio, a magnetic antenna can be fabricated.
1. The Q value of the magnetic antenna is related to the material of the magnetic bar and the frequency of use. The medium wave signal is a manganese-zinc ferrite magnetic bar; the short-wave signal is a nickel-zinc ferrite magnet bar. For magnets of the same diameter or cross section, the longest length should be preferred to improve the sensitivity of the radio.
2. The Q value of the magnetic antenna is significantly affected by the difference in coil winding position. It can be known from the experiment that the optimal 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 inductance change, directionality. Yeah.
3. The current flowing through the coil of the magnet is high in frequency. Since the high-frequency current has a skin effect, the resistance exhibited by the conductor to the high-frequency current increases as the current frequency increases, and as a result, the power loss of the wire to the high-frequency signal increases. In order to overcome this, in the middle wave range, the magnetic antenna coil is usually considered to increase the cross-sectional area of the conductor, that is, the 7-element winding of the enameled wire with a wire diameter of ∮0.1Mm; the primary coil of the short-wave magnetic antenna is often ∮1～1. 5mm silver-plated copper wire winding, the distance between turns is generally 2 ~ 3mm.
4. Since the tuning loop impedance is high and the triode input impedance is low, the impedance matching problem is considered to maximize the signal output. The ratio of L1 to L2 turns is generally about 10:1, and L1 is usually 60-80匝.
5. When moving the magnetic antenna device, keep away from magnetic fields (such as speakers, transformers, etc.). The bracket is made of a non-metallic material. The magnetic rod is easy to break, so be careful during use.

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