Radios as separate devices are almost never found in retail outlets. They are widely represented by multifunctional products - car radios and music centers. Manufacturers focus on sound quality, which determines the audio signal amplifier. Using a good FM antenna greatly improves radio reception efficiency. They are broadcast in high quality and stereo sound.
Some theoretical background
In order for the antenna device to concentrate in its main vibrator the maximum power emitted by the transmitting center, its geometric dimensions are selected from the condition of ½ or ¼ of the transmitter wavelength. The antenna for receiving the FM band must operate at frequencies of 88-108 MHz. The middle of this section corresponds to the lengthwaves 3 meters. The most common vibrator sizes are 0.75 meters (¼ average wavelength).
Impedance determines the power loss during the transition of a wave from one medium to another due to partial reflection of the signal. The optimal case is considered to be the equality of the impedances of the antenna, cable and input impedance of the receiver at a frequency of 100 MHz (the middle of the FM band). The total losses are estimated by the standing wave ratio (SWR) and are expressed in arbitrary units (times). A SWR value within 1, 1-2, 0 is considered normal. A higher value corresponds to a higher level of losses. The antenna for receiving the FM band is used by radio waves with vertical polarization. This is due to the vertical arrangement of the transmitting antennas of the broadcasting centers. Therefore, the longitudinal axes of the vibrators must be placed in a vertical plane (as opposed to the horizontal arrangement of television vibrators).
The receiving antenna does not amplify the received signal. The gain factor 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 beam to the power received at the same point by the omnidirectional vibrator.
Industrial room antenna design
Active elements of indoor antennas (vibrators) are most often made in the form of telescopic "whiskers", the length of which and the angle of inclination can be changed manually.
Such operational changesallow you to achieve the best stereo reception for various frequency modulated (FM) radio stations. These indoor FM antennas are omnidirectional. Their gain is close to unity.
Narrowly directed log-periodic devices or antennas of the "wave channel" type have a high gain. But the dimensions of the structures for the broadcast range of FM radio stations allow them to be placed only in the immediate vicinity of the window "with a view" of the transmitting center. They represent a horizontal traverse, with the main vibrator, directors and a reflector fixed on it. The gain of such structures is determined by the number of directors and can reach 12-16 dB.
Active and passive devices
Manufacturers of FM antennas often use built-in electronic signal amplifiers. Such devices are called active.
The name is due to the presence of active elements in the amplifier circuits - transistors. Using the amplifier requires an external power supply. The package includes adapters, which are small-sized AC-to-DC rectifiers. The output voltage of such devices is 9-12 V. Connection can be made through a special low-frequency connector on the case 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 useful signal level due to the selective properties of the antenna design. The purpose of antenna amplifiers is to reduce signal attenuation in a long cable from the antenna to the receiver input. It is due to wave resistance and is determined by the value of per unit 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 in-cab. The method of fastening antennas of an external location can be mortise and using a magnet. Car FM antennas are most often located on the roof, rear bumper or front fender of a car.
They are available in telescopic version. The length of 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 to its full length. The first option is more preferable.
Magnetic mount is simple.
Due to the small length of the whip vibrator, the sensitivity of the antenna is inferior to the telescopic design. There is a need to remove the device from its installation site at night.
For in-cab antennas, the installation location is the upper right corner of the car windshield.
Most (but not all) have a built-in amplifier that requires external power. The use of an amplifier significantly increases their price. In urban areas, satisfactory reception of closely spaced radio stations is provided, but when driving in rural areas, the unsuccessful (horizontal) arrangement of vibrators affects.
As mentioned above, FM radio transmitters operate in vertical polarization mode. The car's FM antenna should have the same polarization.
Using communication radios
The use of radio communications for the needs of the civilian population is possible only in 3 frequency bands - 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 prohibitions by all motorists, including "truckers". The FM transmitting antenna is also the receiving antenna. So there is no need to invent anything. By maintaining the dimensions corresponding to the band used, satisfactory reception and transmission can be achieved. It makes sense to buy communication stations as finished products. They do not require the necessary complex matching of the transmitting antenna to the output stage of the transmitter.
Homemade Antennas
Acceptable signal reception quality can be obtained if you make an antenna for the FM band with your own hands. With a coaxial antenna cable with a wave impedance of 75 ohms, it is required carefully,Without damaging the shielding braid, remove the 75 cm protective insulation. The exposed part of the braid is turned inside out with a "stocking" and put on the remaining outer insulation.
The upper section of the cable (without braid) is used as a ¼ wavelength midrange vibrator. The remaining inverted braid acts as an antenna counterweight, which significantly improves the 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 outdoors, at high altitudes. The transmission quality of an indoor antenna depends to a large extent on choosing a good location for it.
