Radios as separate devices are almost never found in retail outlets. They are widely represented multifunctional products - car radios and music centers. Manufacturers focus on the sound quality that defines an audio amplifier. Using a good FM antenna greatly enhances the efficiency of radio reception. Their broadcast is conducted with high quality and stereo sound.
Some theoretical background
In order for the antenna device to concentrate in its main vibrator the maximum power radiated by the transmitting center, its geometric dimensions are selected from the condition ½ or ¼ of the transmitter wavelength. The antenna for receiving the FM band should work at frequencies 88-108 MHz. The middle of this section corresponds to a wavelength of 3 meters. More often, the size of the vibrators is 0.75 meters (¼ average wavelength).
The wave resistance determines the power loss during the transition of a wave from one medium to another due to the partial reflection of the signal. The optimal case is the equality of the wave impedances of the antenna, cable and input impedance of the receiver at a frequency of 100 MHz (middle of the FM band). Total losses are estimated by the standing wave coefficient (SWR) and are expressed in arbitrary units (times). The normal value is the SWR in the range of 1.1-2.0. A higher value corresponds to a higher level of losses. An antenna for receiving the FM band is used by vertical polarized radio waves. This is due to the vertical arrangement of the transmitting antennas of the broadcasting centers. Therefore, the longitudinal axis of the vibrators must be placed in a vertical plane (in contrast to the horizontal arrangement of television vibrators).
The receiving antenna does not amplify the received signal. The gain determines only its selective properties. It is expressed in decibels (dB) and is the ratio of the power received by the antenna in the direction of the main lobe of the radiation pattern to the power received at the same point by a vibrator with a circular radiation pattern.
Execution of industrial antennas for a room
Active elements of indoor antennas (vibrators) are performed most often in the form of telescopic "whiskers", the length of which and the angle of inclination can be changed manually.
Such operational changes allow you to achieve the best quality stereo reception for different frequency modulated radio stations (FM). Such indoor FM antennas have a circular radiation pattern. Their gain is close to unity.
Narrowly oriented log-periodic devices or wave-channel antennas are highly amplified. But the size of the structures for the broadcast range of FM radio stations allows you to place them only in the immediate vicinity of the window "overlooking" the transmitting center. They are a horizontal traverse, with the main vibrator, directors and reflector fixed to it. The gain of such structures is determined by the number of directors and can reach 12-16 dB.
Active and passive devices
Often manufacturers of FM antennas use the electronic signal amplifiers built into them. Such devices are called active.
The name is due to the presence in the circuits of amplifiers of active elements - transistors. Using an amplifier requires an external power source. The package includes adapters, which are small-sized AC rectifiers in direct current. The output voltage of such devices is 9-12 V. Connection can be made through a special low-frequency connector on the housing or an antenna separator (voltage injector) via a high-frequency cable.
Accordingly, devices without an antenna amplifier are called passive. They provide an increase in the level of the useful signal due to the selective properties of the antenna design. The purpose of antenna amplifiers is to reduce signal attenuation in an extended cable from the antenna to the receiver input. It is due to wave impedance and is determined by the amount of specific attenuation. The latter depends on the brand of cable used and is 0.15-0.75 dB / m.
FM band antennas for car
Separation of devices of this class can be carried out according to their location - external or internal. The method of attaching antennas of an external arrangement can be mortise and using a magnet. FM car antennas are most often located on the roof, on the rear bumper or front wing of the car.
They are available in telescopic design. The vibrator is controlled by a special motor. This engine is controlled by the driver directly from the passenger compartment. Another way is to automatically start when you turn on the radio. The telescopic antenna extends with the full length. The first option is more preferable.
Magnetic mount is notable for its simplicity.
Due to the short length of the pin vibrator, the sensitivity of the antenna is inferior to the telescopic design. There is a need to remove the device overnight from its installation site.
For in-cab antennas, the installation location is the upper right corner of the car windshield.
Most of them (but not all) have a built-in amplifier that requires external power. The use of an amplifier significantly increases their price. In the city limits, satisfactory reception of nearby radio stations is provided, but when driving in rural areas, the unsuccessful (horizontal) arrangement of vibrators affects.
As mentioned above, FM transmitters operate in vertical polarization mode. The same polarization should have a car FM antenna.
Use of connected radio stations
The use of radio communications for the needs of the civilian population is possible only in 3 frequency ranges - PMR (44 600 000 - 44 610 000) MHz, LPD (433, 075 - 434, 775) MHz, CB (10-meter range). In the CB band, the frequency of 27.135 MHz is used without prohibition by all motorists, including truckers. An FM transmit antenna is also a receive antenna. Therefore, nothing needs to be invented. By maintaining the dimensions appropriate to the range used, satisfactory reception and transmission can be ensured. It makes sense to buy connected stations as finished products. They do not require the necessary complex coordination of the transmitting antenna with the output stage of the transmitter.
Homemade Antennas
An acceptable signal reception quality can be obtained if you make an antenna for the FM band with your own hands. From a coaxial antenna cable with a wave impedance of 75 Ohms, it is necessary to carefully remove the protective insulation 75 cm long without damaging the shielding braid. The exposed part of the braid is turned inside out by the “stocking” and put on the remaining external insulation.
The upper part of the cable (without braiding) is used as a vibrator ¼ the wavelength of the middle of the range. The remaining inverted braid acts as an antenna counterweight, significantly improving reception quality. After unsoldering the standard connector, the antenna is ready for use. Its location is chosen empirically.
Conclusion
After reading the article, the reader will be able to make the right choice among the variety of antenna devices offered by the trade. Do not chase the high gain of active antennas. It is suitable, in most cases, for devices located on the street at high altitude. The transmission quality determined by the indoor antenna is largely dependent on the choice of a good location.