Radio Engineering has given birth to a great number of structures, even without delving into the fact that an FM-range antenna, it is possible to distinguish three types: linear, aperture, traveling surface waves. The second type is familiar to everyone; traditional satellite dishes, of course, do not participate in radio reception. Aperture is called opening the antenna. As for the designs of the traveling wave, wave channels occupy a prominent place. The antennas of Udo-Yagi saw everyone studying the roof: there is a mast, a horizontal rod is attached to the vertical, studded with pieces of wire. Wave channel, catching television. The difference in designs for radio is limited by direction. The polarization is vertical, the directors stick across.
We consider as a distinctive feature good directional properties, decent gain, suitable for stationary objects, not suitable for moving. Do-it-yourself FM antenna is made taking into account the peculiarities of the future location - in one case one solution will be suitable, otherwise it will be different. ..
FM antenna design
The purpose of the FM antenna is to convert the aerial waves into electrical current reaching the receiver. In radio broadcasting, linear polarization is applied, which leaves an imprint on the type of antenna. The simplest is the famous half-wave vibrator. The disadvantage is small: the directional properties are completely absent, a large number of interference from all directions. However, for mobile devices do not need anything else. The object makes variations in space, the FM antenna continues to perform functions. For stationary devices, something like a wave channel is more suitable. It is worth once and for all to find the beam of reception, no longer commit actions. As for the half-wave vibrator, is called, because the length of the structure is half the wavelength.
One or two channels will ideally be caught. For broadcasting is not critical, the active band fits a segment of about 10 MHz. However, it happens that they want to expand the reception range. Several vibrators are connected in parallel vertically, soldered on round forming cylinders. Below is a drawing of the recalculation of the lengths of one type of devices by others.
n - the number of vibrators lined up in a row;L is the total height of the lattice( (n - 1) x distance between the vibrators), D is the diameter of the cylinder from the generators. There is a linear antenna array made up of vibrators, it is permissible to modify to a cylindrical. Why do? If you try to create a solid, even cylindrical vibrator of large diameter, you get a massive device that does not stand the mast. The scientific name of the cylindrical design is the dipole Nadenenko.
To calculate the impedance of the device for matching, you need to know the equivalent diameter Deq, calculated by the formula: Deq = Dn √nd / D.Then the required value is calculated according to the following expression: Ro = 120( ln( l / d) - 1).
Clearly, getting the required 50 ohm does not always work, you will need matching devices. Performing the goals( bandwidth expansion), the vibrators are often attached along the guide cone, fan diverging from the center to the edges. Often the form of the receiver is formed by steel, tin, the length along the conductor is more than half a wave. It is noteworthy that the wave resistance increases dramatically, in some cases it will allow to increase the value to 300 Ohm, to power it with the appropriate cable( you will find it on sale, the issue was discussed with a review on designing radio frequency antennas).
Often we meet the design of a wave-type antenna channel using a loop vibrator, considered by the reviews of the VashTehnik portal. Thanks to tricks, it is possible to catch more channels, the reception is better. Below is a table where trace the value of the working band of the FM antenna relative to the diameter of the conductor d, the wavelength. Based on the information, let us understand what effect the one who placed the vibrators on cylindrical surfaces will achieve, the recalculation formula to linear led. In the table, the letter B means the expression 0.71 f cut / Q.Where Q is the quality factor, the resonant frequency coincides with the channel. If you want to expand the range of reception, it is necessary to construct an antenna array.
Installation Process:
- In the case of parallel vibrators, they are connected by tires at both ends. Each passes perpendicular, combining elements.
- As for the cylindrical design, the ends are bent into one point( the ends resemble a cone with a generatrix at an angle of 45 degrees).Solder together.
If you make your own FM antenna of the above sense, high-quality reception is provided.
Connecting an FM antenna
It is believed that we connect an FM antenna using three methods:
- For direct, the design is an integral part of the device. Used by cell phones. Often in the transmitting devices the antenna is performed by a quarter-wave dipole, the earth will serve as the body of the device. The resistance is 30 ohms, the reactive is reduced by rebuilding the resonant circuit. The antenna is asymmetric, the equivalent current flowing to the cascade flows to the ground. Generates losses, plus alignment creates interference. In the case of stationary devices, good grounding helps, if the length of a bare wire is comparable to a tenth of a wave, the ground loop will emit, generating additional interference.
- In the case of nonresonant power, a standard feeder is used, the characteristic impedance of which is equal to the characteristic impedance of the antenna. Provides no loss, maximum signal level. In the line there is a running wave, the length of the feeder does not play a role, the linear loss is weighty. A cable meter to the first amplifier stage reduces the sensitivity of the receiver. Avoid folding coils on the floor. The excess cable between the receiver and the FM antenna is cut off. The problem of a long feeder cuts off the active amplifier. We'll have to get a cable of 12 volts on the roof, let's stop worrying about the length. Decisions are appropriate, where there is nothing more to do, except to listen to the radio. No internet, television, other communications. If most purchased products are tuned to 50 ohms, self-made products need to be properly matched. It is done with the help of a meter CWS; a special screw has been found near the car structures for the operation. Will significantly improve the reception. And changing the parameters of the antenna, length. Ideally, the CWS is equal to one, in practice it is allowed below 2.
- Resonant antenna power supply lines are the most curious phenomenon. There is one problem: the conditions of signal transmission depend strongly on the wavelength. The feeder must be a multiple of half. The radio station broadcasts in a certain spectrum, the frequency to which the receiver is tuned, the FM antenna, is the carrier. Harmonics swarm around, which are actually useful. A carrier without benefit is of no value. For television, resonant communication lines lose their meaning, frequencies can be tripled. To maintain the conditions of multiplicity half the wavelength is impossible. The beauty that the line impedance stops playing a role. An antenna in such conditions will be agreed. A curious feature is cheerfully used to measure resistance. For example, with the help of a line with a length of 5λ, the antenna is investigated with the exception of errors and interference. To measure the antenna resistance, the characteristics of the feeder do not play any role, as mentioned above. However, a number of practical amateur radio antennas are used at the resonant frequency, the frequency of harmonics. Eating with the help of a resonance line will be a good way out. It remains to match the receiver input with the resistance of the FM antenna.
The importance of alignment is clear to readers. It is not enough to implement the device FM-antenna, it is recommended to reconcile. Otherwise, the impatient can borrow a method from Wi-Fi: a piece of a quarter-wave feeder( solid wire) is taken, cleared from the screen, soldered onto the connector, stuck into the receiver so that the cable is positioned vertically. A primitive FM antenna, according to experienced people, shows itself well when there are waves on the air, therefore, it will suit impatient readers who do not want to coordinate the project. The simplest FM antenna made with any kind of trimming coaxial cable is capable of receiving. Feel free to take a television, power will be lost.
We hope that we have let our readers understand the basic principles for designing FM antennas. By the way, do not throw out broken gadgets. Inside MP3 players, phones, modems often have the necessary. Even a tiny factory FM antenna is capable of a hundred points odds to give a homemade design. Remove the required part, study the electronics. It will be useful. The time is not far off - we will program home controllers, create your own digital information processing devices.
