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I am also a member of FM enthusiasts. I have been fond of FM reception for more than ten years and have summarized some experiences. In the "Electronic News" a few years ago, some articles about FM reception were published. Especially after the introduction of the digital tuning radio for American Dodge cars, it caused a nationwide FM reception fever. At that time, it was due to the weather, geographical conditions, people, and the frequency of FM broadcasts across the country. Of Shaoyou joined the ranks of receiving. However, things are not as simple as we thought, there are problems of one kind or another. Today, there are still ringing phone calls and there are many letters. Ask me questions such as material selection, reception, maintenance, etc. About, to mention the pen again, the system introduces the relevant knowledge of FM high-fidelity reception and material selection, so that the majority of FM reception enthusiasts can get their wish and realize the dream of FM fever.
A complete set of FM broadcast receiving system consists of two parts, as shown in Figure 1, including outdoor and indoor. The outdoor part includes antenna rack and antenna amplifier; the indoor part includes FM tuner, power amplifier and speaker. The FM receiver is composed of FM high frequency head and mid-range stereo decoding circuit. Each is explained in detail below.
1. Antenna
The antenna is the gateway of the receiving system. It is the first stop. Without an antenna, no matter how good the receiver cannot receive signals, without a good antenna, remote reception is impossible. In order to facilitate everyone's understanding, we compare with TV reception, because everyone is quite familiar with TV reception, and they are both FM transmission. The design of the FM antenna is completely equivalent to the TV antenna. If the requirements are not high, you can directly replace the FM antenna with a 5-channel TV outdoor antenna. The antenna has the following three indicators:
1. Gain The gain of an antenna is an important indicator used to illustrate the ability of the antenna to receive signals. It is determined by the direction angle of the antenna and the front-to-back radiation ratio. The smaller the directional angle, the stronger the receiving capability of the antenna, and the less interference from other directions, which is why the antenna must be aligned in the direction to receive clear images and sound. The front-to-back radiation ratio of the antenna refers to the ratio of the received signal strength before and after the antenna. The larger the value, the better the antenna performance. The basic unit vibrator antenna has two maximum receiving surfaces, that is, the front-to-back ratio is 1: 1. How to increase the front-to-back ratio to make the directivity more sharp, the best way is to add a director and a reverser, which is why A single vibrator is used only when the signal is strong, and the reason for using a multi-element antenna when the signal is weak. Theoretically, the more directors, the higher the gain, and only one inverter is enough, but too many directors will affect the bandwidth, the mechanical strength is also poor, and the installation size is huge, so antennas with frequency bands are generally used Three-element antennas, but no more than five elements.
2. The directional diagram represents the antenna's ability to receive signals from different directions. It has a different meaning from the direction angle. In principle, the signal direction of the half-wave dipole antenna should be perpendicular to the signal of the radio transmitting station, but in fact, the reception of remote signals may have deviations in reception due to changes in propagation. The actual receiving effect should be the best standard. Of course, we require not only one direction, but 360 ° omnidirectional reception, which results in multidirectional and omnidirectional antennas. In fact, the previous single vibrator is a multidirectional antenna. If the two pairs of vibrators are vertically combined, it becomes a cross omnidirectional antenna. This kind of antenna was originally introduced by the author, but because of its low efficiency and low gain, it is used in signals. Weak areas are not very suitable. The only way is to use a steering antenna, which has both high gain and good pattern. It can receive signals from any direction vertically and obtain maximum gain. It can improve the signal-to-noise ratio and eliminate adjacent frequencies Interference or even co-channel interference is the best choice for our FM receivers.
3. Bandwidth It is an indicator of the frequency of the received FM signal. The wider the passband, the more radio stations it receives. Our country stipulates that the frequency range of FM broadcasting is 87.5 ~ 108MHz, with a total bandwidth of 20MHz, which is still a narrowband antenna
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The heart frequency is = √λ1? Λ2 = 97.4MHz, generally 98MHz, if it is biased to a certain frequency band, such as the low band (88 ~ 98MHz) or the high frequency band (98 ~ 108MHz), when designing the antenna, you can intentionally lean towards a certain end Be flexible.
In short, the requirements for receiving an FM broadcast on an outdoor antenna are high gain, omnidirectional reception (or directional reception if there is no radio in other directions), and it is convenient to use. Generally, the gain of a three-element antenna is 7 dB, and that of a five-element antenna can reach 11 dB. The FM9901A antenna used by the author last year was the first three-unit dedicated antenna designed for FM broadcast reception in China. The power box can be used to control the antenna to rotate in all directions and receive good results. Due to the complex calculation of the design antenna size, only the production size of the five-unit FM antenna is given below, as shown in Figure 2, for the reference of enthusiasts.
Material: Use ∮10mm aluminum tube, made according to the picture, copper tube is better, the larger the diameter, the better, but when bending the vibrator, first fill it with sand and then bend it, so it is not easy to break and the bend is smooth. It can be connected with the same material in the middle, but it must be at the midpoint of the director and the inverter. It can also be connected with insulating materials without affecting the effect.
High-fidelity reception of ultra-long-range FM radio
The reception of FM broadcasts has been a hot topic in recent years, especially last year. This is because FM broadcasts have better sound quality, higher fidelity, stronger anti-interference, and richer programs than AM broadcasts. The most critical It is low cost, although digital audio sources are everywhere, such as CD, VCD, DVD, etc., but the cost is high, music software is an endless expenditure, and the reception of FM radio programs is free, so it is not rich For many audiophiles who want to enjoy music, listening to FM stereo (FM ST.) Radio programs is undoubtedly the best choice. But the pursuit of audiophiles is endless. The more stations you receive, the greater the selectivity, but local radio stations are limited after all. This is the origin of FM remote reception.