The FM antenna for the music center to a large extent determines both the number of received programs and their quality. The reception mode (monaural or stereo) depends on the signal strength at the input of the FM receiver of the music center. Manufacturers of music centers in their developments are guided to a greater extent by the high-quality sound of the output amplifier of sound frequencies. The signal source for it are CD players and USB flash drives. Receiver circuits are not given due attention. Based on this, the quality of FM receiving antennas plays a significant role. This must be taken into account by fans of listening to the programs of the meter FM band.
Some theoretical background
The FM radio antenna for the music center is designed to receive phase-modulated signals in the frequency range 87.5-108.0 MHz. This corresponds to wavelengths of about 3 meters.
Why know this information? The best reception is provided by antenna devices, the geometric dimensions of the vibrators which correspond to ½ and ¼ the lengths of the waves it receives. The best performance is provided by FM antennas for the music center with radiation patterns approaching a cone with a small angle of aperture. The longitudinal axis of the main lobe should be directed towards the antenna of the transmitting center.
Such devices should be used at a great distance from the broadcast center from the location of the antenna of the radio of the music center. The power of transmitters located in large populated centers allows for high-quality reception, and antennas with a circular radiation pattern, the vibrator sizes of which satisfy the requirements indicated above.
If the sensitivity of the receivers (the ability to receive a radio signal with the required electromagnetic field strength) is determined by the manufacturer, then the quality of the radio transmission can be ensured by the competent choice and use of the antenna device. It can be either industrial production or made by the user independently.
The main types of receiving antennas meter wavelength range
When choosing a receiving antenna for the meter wavelength range, it is necessary to determine the distance at which the transmitting center is located from the receiving location. Often you may encounter a situation in which the desired program is broadcast by another (not near) radio center. In large settlements, as a receiving antenna, you can use the usual length of a stranded wire 75 cm long. Such dimensions comply with the condition of the best reception - ¼ wavelength (middle of the FM range). When buying a music center, it is such antennas that are included in the set of additional accessories. They are equipped with an antenna plug, which is inserted into the corresponding socket on the rear panel of the music center.
With unsatisfactory transmission quality or the absence of the "Stereo" mode in urban conditions, good performance can be achieved by using a half-wave dipole, which is often used for a meter wavelength range (channels 1-12) of TVs. To achieve a satisfactory reception quality, it is possible to manually adjust the length of the vibrators made in the telescopic design, the angle of their relative position.
The best quality is achieved by the "wave channel". This type of antenna is designed for use in a limited frequency range. Its use is advisable in cases where the transmitting antenna of the broadcast center is not in the line of sight and the use of antennas of the types discussed above does not lead to the desired quality indicator.
To obtain stable reception, the antenna has to be raised above the earth's surface. This is due to the fact that meter-wave radio waves propagate in space in a straight line and are not able to envelope the Earth's surface.
Active and passive devices
Pie antennas are not capable of amplifying a signal. They receive radio waves from any direction. The signal strength at the input of the receiver, which came from the FM antenna for the music center, depends on the geometric dimensions of the structure of the latter.
Devices with a beam pattern in the form of a one-sided cone have their own gain. It is due to the ability to concentrate energy received from a certain direction. In this case, signals coming from other directions are evaluated as interference and attenuated. The value of the coefficient, expressed in decibels (dB), is determined in relation to an isotropic emitter with a spherical radiation pattern.
The presence of an additional electronic amplifier in the antenna makes it possible to call it active. The active FM antenna for the music center finds application with a significant length of the coaxial cable connecting it to the antenna socket of the receiver.
The amplifier must be designed to operate in the frequency band of FM radio stations and have a low level of intrinsic noise. Otherwise, the useful signal of the radio station allocated by the antenna will not prevail over the noise level created by the amplifier.
The main purpose of the built-in antenna or a stand-alone additional antenna amplifier is to compensate for the attenuation of the useful signal in an extended cable. It is estimated by the amount of linear attenuation. Depending on the type of cable used, its value varies between (0.15-0.8) dB / m. Based on this, the gain should be selected taking into account the specific attenuation and the length of the coaxial cable. It will be useful to know that in the documentation for an active antenna the manufacturer always indicates the total signal gain, which does not at all determine the selective properties of the structure.
Industrial products
Today you can choose an FM antenna for a music center from a wide range of products. Most of them are intended for placement in a room (room).
Products most often are half-wave dipoles, ergonomic rectangular frames, vertical telescopic pins with the ability to manually change the length of the vibrator. There are active FM antennas for the music center. Their use in residential premises most often leads to the amplification of numerous reflected signals.
For receiving FM radio stations in rural areas or in summer cottages, preference should be given to narrowly directed multi-element active antennas of the "wave channel" type. They are not so widely represented and their price can reach several thousand rubles.
Self made antennas
How to make an FM antenna for the music center? The simplest device for receiving broadcasting stations of the meter range is a piece of copper stranded wire that does not have a shielding braid. The presence of polyethylene insulation does not affect the quality of the received radio signal. Its surface should be clean and dry.
Another kind of FM antenna for the music center (made by yourself) is a half-wave dipole or split vibrator. It consists of two segments of a hollow aluminum tube fixed on a base of dielectric material in a horizontal plane. The gap between their ends is (2-4) cm. The total length of the structure is selected from the condition ½ the length of the received wave. The impedance of this type of antenna is 75 ohms, which allows you to get rid of matching devices and connect a standard television coaxial cable directly. The main core and braid are attached to the ends of the vibrators.
The radiation pattern of a split vibrator lies in a horizontal plane and resembles the figure "8" in shape. The longitudinal axis of any lobe should be aligned with the direction of the transmitting antenna of the radio center. Its gain (relative to the isotropic emitter) is 2.14 dB. It can be increased by using a metal grating (reflector) located behind the receiving dipole.
Conclusion
After reading the article, the reader should have confidence in a conversation with the seller of the goods. This is especially true for the gain of the active antenna. Often, its high value can degrade the quality of reception. It should be noted that when using indoor antennas, it is necessary to select their location in the apartment empirically. Most often, it is located near a window aimed at the radio center.