FM-antenna for the music center to a large extent determines both the number of received programs and their quality. The reception mode (monophonic or stereophonic) depends on the signal strength at the input of the FM receiver of the music center. Manufacturers of music centers in their developments focus more on the quality sound of the output audio frequency amplifier. 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 receiving FM antennas plays a significant role. This should be taken into account by fans of listening to meter FM broadcasts.
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 around 3 meters.
What to knowthis information? The best reception is provided by antenna devices, the geometric dimensions of the vibrators of which correspond to ½ and ¼ of the wavelengths received by it. The best performance is provided by FM antennas for a music center with radiation patterns approaching a cone with a small opening angle. 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 of the broadcasting center from the location of the radio receiver antenna of the music center. The power of transmitters located in large population centers allows for high-quality reception, and antennas with a circular radiation pattern, the dimensions of the vibrators of which meet the requirements indicated above.
If the sensitivity of the receivers (the ability to receive a radio signal with the required strength of the electromagnetic field) is determined by the manufacturer, then the quality of the radio transmission can be ensured by the correct choice and use of the antenna device. It can be either commercially manufactured or user-made.
Main types of meter-wave receiving antennas
When choosing a receiving antenna for the meter wave range, it is necessary to determine the distance at which the transmitting center is located from the place of reception. Often you may encounter a situation in which the desired program is broadcast by another (not nearby) radio center. In large populationspoints as a receiving antenna, you can use a regular piece of stranded wire 75 cm long. Such dimensions comply with the condition for the best reception - ¼ wavelength (middle of the FM band). When buying a music center, these antennas are included in the kit of additional accessories. They are provided with an antenna plug that plugs into the appropriate jack on the back of the music center.
If the transmission quality is unsatisfactory or there is no "Stereo" mode in urban conditions, good performance can be achieved using a half-wave dipole, which is often used for the meter wave range (1-12th channels) of TVs. To achieve a satisfactory reception quality, it is possible to manually adjust the length of the telescopic vibrators, the angle of their relative position.
The best quality allows you to achieve the "wave channel". This type of antenna is intended for use in a limited frequency range. Its use is advisable in cases where the transmitting antenna of the broadcasting 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 a stable reception, the antenna has to be raised above the earth's surface. This is due to the fact that the radio waves of the meter range propagate in space in a straight line and are not able to bend around the surface of the Earth.
Active and passive devices
Antennas with circularradiation pattern are not able to amplify the signal. They receive radio waves from any direction. The strength of the signal 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.
Device with a one-way cone pattern has its 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 coefficient value, expressed in decibels (dB), is determined in relation to an isotropic radiator with a spherical radiation pattern.
The presence of an additional electronic amplifier in the antenna allows us to call it active. An active FM antenna for a music center is used with a significant length of the coaxial cable connecting it to the receiver's antenna jack.
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, selected by the antenna, will not prevail over the noise level created by the amplifier.
The main purpose of an additional antenna amplifier built into the antenna or a stand-alone one is to compensate for the attenuation of the useful signal in a long cable. It is estimated by the value of per unit attenuation. Depending on the type of cable used, its value varies within (0.15-0.8) dB/m. Based on this, the valuethe gain must be chosen taking into account the attenuation per unit length 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 design.
Industrial products
Today you can choose an FM antenna for your music center from a wide range of products. Most of them are designed to be placed indoors (room).
Products are most often 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 a residential environment most often leads to the amplification of numerous re-reflected signals.
To receive FM radio stations in rural areas or 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.
Home made antennas
How to make an FM antenna for a 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 must be clean and dry.
Another kind of FM antenna forthe music center (made by yourself) is a half-wave dipole or a split vibrator. It consists of two segments of a hollow aluminum tube, fixed on a base of a 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 of ½ of the length of the received wave. The characteristic 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 directional pattern of the split vibrator lies in the horizontal plane and resembles the figure "8" in shape. The longitudinal axis of any lobe should be aligned with the direction to the transmitting antenna of the radio center. Its gain (with respect to an isotropic radiator) 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 when talking with the seller of the goods. This is especially true for the magnitude of the gain of the active antenna. Often, a high value can degrade reception quality. It should be noted that when using indoor antennas, you have to select their location in the apartment empirically. Most often it is located near the window directed to the radio center.
