External antenna for GSM cell phone. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Telephony

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The range of a cell phone can be increased by equipping it with an external remote directional antenna. How to make such an antenna yourself is described in the proposed article. The antenna can be successfully used in the country, in a remote country house, and even, given its small size, in the field.

New and new users of cell phones are convinced of the advantages of cellular telephone communication from their own experience. The ability to contact the desired subscriber by phone at any time and from almost anywhere makes this type of communication perhaps the most popular and attractive.

But in addition to the numerous and undeniable advantages of cellular telephony, as with any technical means, there are also some disadvantages. Recall the need to pay for every second of the conversation. In addition, cellular communication is characterized by such phenomena as the presence of "dead" zones and deterioration in the quality of communication at a significant distance from the antenna of the nearest base station.

Many cell phone users are probably familiar with the situation when they have to look for a long time for a place where the device can establish a stable connection to the network. Moreover, such situations arise not only far from the network base station antenna (which is typical for remote places, for example, in a garden plot), but also in a city where the base station signal can be shielded by buildings, structural elements, etc.

An external antenna connected to the mobile phone can help in these cases. Of course, it limits the subscriber's mobility, "tying" him to this antenna, but this limitation may not be such an expensive price to pay for high-quality communication.

When developing a stationary external antenna for a cell phone, a zigzag antenna design [1, 2] was chosen, which is widely used in professional communications and is popular with radio amateurs and TV viewers.

Antennas of this type in the frequency range used in cellular telephony make it possible to realize good characteristics with small dimensions, are quite simple to manufacture and tune, and have good repeatability of parameters in their production.

The zigzag antenna consists of eight closed conductors of length L, forming two diamond-shaped cells (see figure). The design of the antenna is such that its conductors, excited at points a and b, form a kind of in-phase antenna array of four vibrators. The antinodes (maxima) of the current are located at the feed points and in the corners indicated by the letters P. The antenna has a linear polarization, in the case shown in the figure - vertical.

The antenna pattern is maintained in the frequency range with an overlap fmax/fmin = 2...2,5. The directivity characteristic of the antenna is symmetrical with respect to the plane of location of its conductors.

To increase the directivity of a zigzag antenna, a reflector is used that reflects part of the energy incident on it towards the antenna web. The phase of the field reflected by the reflector in the plane of the antenna web should be close to the phase of the field emitted by the web itself, then the addition of the in-phase fields of the emitted and reflected signals increases the directional coefficient (DPC) of the antenna. The phase of the reflected field depends on the shape and dimensions of the screen, but mainly on the distance between it and the antenna web.

The GSM cellular communication standard provides for the operation of communication systems in the frequency ranges of 890...960 MHz for GSM-900 and 1710...1880 MHz for GSM-1800 [3, p. 102], and at frequencies 935 ... 960 MHz and 1805 ... 1880 MHz, a direct channel is organized, and at frequencies 890 ... 915 MHz and 1710 ... 1785 MHz - a reverse channel. In the frequency range of 890...960 MHz, the antenna has a traveling wave coefficient (TWF) of no worse than 0,77 and an efficiency factor of no worse than 7 dB compared to a half-wave dipole. In the frequency range 1710 ... 1880 MHz, the KBV of the antenna is no worse than 0,5, and the directivity factor is no worse than 6 dB.

The design of the antenna is clear from the figure.

The main design parameter L determines the operating frequency of the antenna. For a zigzag antenna, the directivity factor depends on the ratio L/λ, and the directivity gain is maximum at L/λ - 0,4. The maximum KBV - 0,8 is achieved at L / λ - 0,3, and with ratios L / λ = 0,25 ... 0,5 KBV is at least 0,5. Therefore, for the average frequency of the operating range of the antenna, the value L = 80 mm was chosen, while L / λ is 0,37. In addition to L, the width of the antenna vibrators d and the distance from the antenna web to the reflector affect the KBV value. It is usually recommended to choose d = 0,033λmax, where λmax is the maximum wavelength of the antenna operating range. In our case, d = 10 mm. From the point of view of increasing the directivity of the antenna, it is desirable to reduce the distance to the reflector, and from the point of view of matching - to increase. In this design, it is 45 mm, which provides the above characteristics of the antenna.

Antenna sheet 1 and reflector 2 are made of one-sided foil fiberglass of the SF-1 brand with a thickness of 1 ... 1.5 mm. The antenna sheet is formed by two symmetrical square cells, which are cut out of foil fiberglass from the outside along the contour of the antenna. The inner contour of the zigzag antenna is scratched with a cutter from the side of the foil, after which the foil is removed from the inside of the antenna contour. To facilitate the process of removing the foil, it can be preheated with a powerful soldering iron. If desired, you can remove most of the dielectric inside the antenna circuit.

At points close to the tops of the cells of the antenna sheet, holes are drilled for the M4 or M2 screw. The same holes are drilled in reflector 1 (screen). Antenna sheet 2, on the one hand, and reflector 3, on the other, are screwed to the posts with 2,4 screws. Racks are made of any dielectric material (fluoroplast, getinax, plexiglass, etc.), metal racks can also be installed. Inside them, holes with a diameter of 3,2 or 10 mm are drilled on each side to a depth of about 4 mm for screws with an M50 or MXNUMX thread. The antenna is powered by a coaxial cable with a characteristic impedance of XNUMX ohms. The central conductor of the cable, freed from the shielding braid, is soldered to point b, and the braid (screen) is soldered to point a.

The cable is laid along the vibrators that form one of the sides of the antenna cell, and is led out through the zero potential point of the antenna sheet P. To fix the cable, it can be soldered with tinned wire to the antenna vibrator. Next, the cable is fixed on the stand and led out through the hole in the antenna screen. An FME 740 connector is soldered to the end of the feeder, to which an adapter (antenna adapter, it can be purchased at mobile phone stores) is screwed to a connector for an external cell phone antenna.

It should be noted that the value of KBV strongly depends on the type of coaxial cable used. As a rule, the thinner the cable, the greater its attenuation, which degrades the performance of the antenna. At the same time, a thin cable, when connected to a telephone set, hinders the subscriber's movements less, it is more convenient to connect such a cable to an antenna adapter. In general, if the distance from the point at which the antenna provides an acceptable signal quality to the location of the telephone is no more than 2 ... 4 m (for example, the antenna is located indoors by the window), then you can use a thinner cable with polyethylene dielectric (for example, RK 50-1,5-11).

If, however, to provide an acceptable signal, the antenna needs to be moved to a greater distance, the requirements for the feeder are tightened. Good characteristics for this case have cables PK 50-2-21, PK 50-2-2. In the described design, as a feeder 2,5 m long, a high-frequency coaxial cable RK 50-2-21 was used with an attenuation coefficient at a frequency of 900 MHz of not more than 0,6 dB / m, at a frequency of 2 GHz - 0,8 dB / m.

If it is planned to use the antenna outdoors, to reduce the windage of the structure, the reflector can be made in the form of a lattice of metal bars or tubes located at a distance of 0,05 λmin from each other, here λmin is the minimum wavelength of the operating frequency range. The reflector elements are oriented parallel to the line a- b, i.e., in the plane of polarization. For grounding, the antennas are connected to the P-P points of the antenna sheet with the screen with metal racks, and the screen itself is securely attached to the ground bus (mast, support). Inside the antenna sheet, the fiberglass freed from the foil is removed, and the elements of the vibrators of the antenna sheet are protected by a corrosion-resistant coating with low dielectric losses.

During testing, the antenna made it possible to increase the signal from two gradations of the scale of the Motorola M3788 cell phone indicator to four, which ensured high-quality telephone communication.

If the resulting antenna gain is not enough, it can be increased by constructing an antenna array, for example, from two or four zigzag antennas, or using a "truncated" horn reflector [2, p. 77]. But these solutions significantly complicate the manufacture and tuning of the antenna and, therefore, have not been tested by the author.

Data on frequencies and wavelengths of various cellular communication standards are given in the table. Using them and recalculating the linear dimensions, it is possible to design similar antennas for other ranges.

Literature

1. Kharchenko K. P. VHF antennas. - M.: DOSAAF, 1969.
2. Kharchenko K. P. Antenna of the DTSV range: Sat: "To help the radio amateur", vol. 94, p. 68-79. - M.: DOSAAF, 1986.
3. Ratyneky M.V., Telegin A.V. Phone in your pocket. Cellular guide. - M.: Radio and communication, 2000.

Author: V.Vasilevsky, Dzerzhinsky, Moscow Region

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