The Application of Magnetic Antennas and Intermediate Frequency Transformers in Radios

First, the magnetic antenna The magnetic antenna is a coil that is not connected to each other around two groups of magnetic rods. The role is to receive electromagnetic waves in the space. The magnetic antenna has a good directionality, making the radio the loudest and reducing the noise when the radio is turned in a certain direction.

In the process of assembling transistor radios, magnetic antennas can be made.

1. The Q value of the magnetic antenna is related to the magnetic rod material and the frequency of use. The receiving medium-wave signal adopts a Mn-Zn ferrite magnetic bar; the receiving short-wave signal adopts a nickel-zinc ferrite magnetic bar. For the same diameter or cross-section of the magnetic bar should be the longest choice, in order to improve the radio sensitivity.

2. The Q value of the magnetic antenna is significantly affected by the different coil winding positions. From experiments, it can be known that the best position of the coil on the magnetic rod is 1/5 to 1/3 of the distance between the center of the coil winding length and the midpoint length of the magnetic rod. At this time, the movement of the coil has a large change in inductance, and the directionality Ye Hao.

3. The current flowing through the coil of the magnet coil is of high frequency. Since the high-frequency current has a skin effect, the resistance that the conductor presents to the high-frequency current increases as the frequency of the current increases, 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, the magnetic antenna coil is usually considered to increase the cross-sectional area of ​​the conductor, that is, 7 strands of the enameled wire with a wire diameter of ∮0.1Mm; the primary coil of the short-wave magnetic antenna often uses ∮1-1.5mm. Between silver-plated copper wire, the distance between the turns is generally 2 ~ 3mm.

4. Due to the high impedance of the tuning loop and the low input impedance of the transistor, the impedance matching problem is considered to maximize the signal output. The L1 and L2 turns ratio is usually about 10:1, and L1 is usually 60 to 80 turns.

5. Magnetic antenna devices should be kept away from magnetic fields (such as speakers, transformers, etc.). The stent is made of non-metallic materials. Magnetic sticks are easy to break and care must be taken during use.

Second, the intermediate frequency transformer intermediate frequency transformer (commonly known as the week), is a unique superheterodyne transistor radio with a fixed resonant circuit, but the resonant circuit can be fine-tuned within a certain range, so that access to the circuit can reach stability The resonant frequency (465kHz). Fine-tuning is accomplished by means of a change in the relative position of the magnetic core.

Most of the intermediate frequency transformers in radios are single-tuned and have a simple structure and occupy less space. Since the input and output impedances of the transistors are low, in order to enable the intermediate frequency transformer to match the input and output impedances of the transistors, the primary has taps and a secondary coupling coil with a small number of turns. The double-tuned type has the advantage of better selectivity and wider passband, and is used in high-performance radios.

Transistor radios usually use two-stage IF amplifiers, so three front-to-back signals must be coupled and transmitted. The BZ1, BZ2, and BZ3 symbols are commonly used in the middle of the actual circuit. In use, their position in the circuit cannot be interchanged.

The shape of the oscillating coil (medium wave) is similar to that of the middle lap. It and the corresponding components make up the frequency conversion stage of the transistor radio. Adopt equal capacitance double connection (270pF×2), adjust the resonance frequency of the input tuning circuit and the local oscillation frequency of the local oscillation circuit at the same time, and ensure that there are f-f signal=465kHz in the whole receiving band range. Commonly used models are LTF-2-1 (primary 144+8.5 inch, secondary 11.5 inch) and LTF-2-3 (primary 4.5+82 inch, secondary 8 inch).

One last point to mention: Tuning should be used as much as possible without tuning. Every time you adjust the range of the magnetic cap of the intermediate frequency transformer or the oscillating coil, do not use too much force. Be careful to prevent the magnetic cap from breaking.