High-fidelity reception of ultra-long-range FM broadcasts

The reception of FM radio has been a hot topic in recent years, especially last year. This is because FM radio has better sound quality, higher fidelity, stronger anti-interference, richer programs, and the most important thing is that it is low-cost compared to AM radio. Although digital audio sources are now everywhere, such as CDs, VCDs, DVDs, etc., the cost is high, and music software is an endless expense. The reception of FM radio programs is free, so for many audiophiles who have limited income but want to enjoy music, listening to FM stereo (FM ST.) radio programs is undoubtedly the best choice. However, the pursuit of audiophiles is endless. The more stations they receive, the greater the selection, but local radio stations are limited after all. This is the origin of FM remote reception.
I am also a member of the FM audiophile. I have been interested in FM reception for more than ten years and have summarized some experience. In the past few years, I have published some articles on FM reception in the "Electronic News". Especially after the introduction of the American Dodge car digital tuning radio, a nationwide FM reception fever was triggered. At that time, it was also due to the right time, right place, and right people. At that time, there were more and more FM broadcasts across the country, which made many enthusiasts join this reception. However, things are not as simple as we imagined. There are problems of one kind or another. To this day, the phone rings constantly and there are many letters asking me questions such as material selection, reception, and maintenance. In addition, I was invited by the editor to write again to systematically introduce the relevant knowledge of FM high-fidelity reception and material selection, so that the majority of FM reception enthusiasts can get what they want and realize their FM fever dreams.
A complete FM broadcast reception system is shown in Figure 1, including two parts: outdoor and indoor. The outdoor part includes an antenna rack and an antenna amplifier; the indoor part includes an FM tuner, a power amplifier, and a speaker. The FM receiver consists of an FM high-frequency head and an intermediate amplifier stereo decoding circuit. The following will explain them in detail.
1. Antenna
The antenna is the gateway to the receiving system and the first station. Without an antenna, no matter how good the receiver is, it will not receive the signal. Without a good antenna, remote reception is impossible. To make it easier for everyone to understand, we compare it with TV reception, because everyone is quite familiar with TV reception, and they both belong to FM transmission. The design of the FM antenna is completely equivalent to the TV antenna. If the requirements are not high, you can directly use a 5-channel TV outdoor antenna to replace the FM antenna. The antenna has the following three indicators:
1. Gain The gain of the antenna is an important indicator used to illustrate the antenna's ability to receive signals. It is determined by the antenna's directional angle and front-to-back radiation ratio. The smaller the directional angle, the stronger the antenna's receiving ability and the less interference from other directions. This is why the antenna must be aligned in the right direction to receive clear images and accompanying sounds. The front-to-back radiation ratio of the antenna refers to the ratio of the strength of the front and back received signals of the antenna. The larger this value is, the better the antenna performance is. The basic unit dipole antenna has two maximum receiving surfaces, that is, the front-to-back ratio is 1:1. How to increase the front-to-back ratio and make the directivity more acute? The best way is to add directors and deflectors. This is why a single dipole is used only when the signal is strong, and a multi-unit antenna is used when the signal is weak. In theory, the more directors there are, the higher the gain, and only one deflector is needed, but too many directors will affect the bandwidth, the mechanical strength is poor, and the installation size is large, so the antenna of the FM band generally uses a three-unit antenna, and no more than five units.
2. Directional diagram It indicates the antenna's receiving ability for signals in different directions, which is different from the meaning of the directional angle. In principle, the half-wave dipole antenna should be perpendicular to the signal direction of the radio station transmitter, but in fact, when receiving distant signals, due to changes in propagation, there may be some deviations during reception, and the actual receiving effect should be the best standard. Of course, we require not only one direction, but 360° omnidirectional reception, which gives rise to multi-directional and omnidirectional antennas. In fact, the single vibrator in front is a multi-directional antenna. If the two vibrators are combined vertically, it becomes a cross omnidirectional antenna. I have introduced this type of antenna before, but because of its low efficiency and low gain, it is not very suitable for use in areas with weak signals. The only way is to use a steering antenna, which has both high gain and good directional pattern. It can receive signals from any direction vertically and obtain maximum gain. It can improve the signal-to-noise ratio, eliminate adjacent frequency interference and even co-frequency interference, and is the best choice for us FM reception enthusiasts.
3. Bandwidth It is an indicator of the frequency of the received FM signal. The wider the passband, the more radio stations it receives. China stipulates that the frequency range of FM broadcasting is 87.5~108MHz, and the total bandwidth is 20MHz. It is still a narrowband antenna, where the
core
frequency is =√λ1?λ2 =97.4MHz, generally 98MHz. If it is biased towards a certain frequency band, such as the low band (88~98MHz) or the high band (98~108MHz), when designing the antenna, you can intentionally lean towards one end and flexibly control it.
In short, the requirements for outdoor antennas to receive FM broadcasting are high gain, omnidirectional reception (if there is no radio station in other directions, directional reception can also be used), and easy to use. Generally, the gain of a three-unit antenna is 7dB, and that of a five-unit antenna can reach 11dB. The FM9901A antenna used by the author last year is the first three-unit dedicated antenna designed for FM broadcasting reception in China. Using the power box, the antenna can be controlled to rotate in all directions for reception, and the effect is good. Since the design antenna size calculation is complicated, the following only gives the production size of the five-element FM antenna, see Figure 2, for reference by enthusiasts.
Materials: Use a ∮10mm aluminum tube and make it according to the diagram. Copper tubes are better. The larger the diameter, the better. However, when bending and folding the vibrator, fill it with sand first and then bend it. This way it is not easy to break and the bend is smooth. The same material can be used to connect in the middle, but it must be at the midpoint of the director and the reverser. It can also be connected with insulating materials, which will not affect the effect.