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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING External cell phone antennas. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Mobile telephony Each mobile phone has an internal small-sized antenna. Sometimes it protrudes from the body of the "mobile phone" with a small pin or "growth". In most cases, the standard antenna provides a stable connection. But there are situations when you need to "squeeze" everything that is possible out of the phone. After all, even in the service area of almost every base station there are areas with non-guaranteed coverage, i.e. "dead zones", where in order to lose or restore communication, it is enough to take 2-3 steps to the side. To ensure good communication in such places, the internal antenna will need help: an additional replacement antenna. There are several cases when it is advisable to use additional external antennas: type="disc">An additional antenna is especially necessary for a phone in a car, since the metal body is a screen that prevents the radio signal from passing through. Receiving an insufficiently strong signal, the device receives a command from the base station to increase the output power, therefore, the radiation level increases and the power consumption increases. In addition, inside the car, radiation from the phone's own antenna is repeatedly reflected, as a result of which all passengers find themselves sitting inside a closed circuit with internal radiation, like food in a microwave oven. Another point that needs attention: the radio signal emitted by the phone can affect the operation of some electronic devices in the car. An external car antenna not only helps to avoid the above troubles, but also improves the quality of communication. This is achieved mainly due to the removal of the signal outside the car and a more efficient redistribution of the antenna pattern. How to properly position the car antenna? There is a rule that a vertical (whip) antenna should be located above the largest possible metal surface. From this point of view, the middle of the car roof is the best place to install a vertical antenna. In addition to the roof, rear or front fenders are also suitable for mounting such an antenna. But a metal bumper is far from an ideal place. Among the wide variety of designs, the so-called "cut-in whip antennas" deserve attention. Their main advantage is direct contact with the body ("ground") of the car, which has a positive effect on performance and, as a result, on the quality of communication. To install a mortise antenna, it is necessary to drill the roof of the car, isolate the antenna installation site from moisture and carefully fix the connecting cable so that it does not knock on the skin inside. Antennas on a magnetic base (Fig. 1) are extremely popular among motorists due to their manufacturability and ease of installation. Still, if necessary, such an antenna can be easily removed into the cabin, and the magnet will not let it fall off even at high speeds. But you have to pay something for convenience, and payback is somewhat worse performance.
When choosing a "magnetic" antenna, it must be taken into account that changing the length of the cable and installing it without a rubber gasket between the magnet and the body adversely affects the performance of the antenna. There are "two-component" antennas, consisting of two parts. The base part with a cable leading to the phone is mounted on the glass inside the cabin, and the other part is outside, opposite it. Capacitive coupling is provided between them. Unfortunately, this is not the most efficient way to transmit a signal, because passing through the glass, it weakens. In addition, it is undesirable to install such an antenna near various metal coatings, for example, tinting or heater strips on glass. There are also adhesive antennas on the market. They are made in the form of a tape or a small box (Fig. 2) and are glued to the inside of the glass.
The cable is often included with the antenna as it is permanently connected to the antenna. The original length of the cable is usually 3 m. If it is cut off during installation of the antenna, you have to install a connector at the end. This operation must be done very carefully. An incorrectly installed connector or poor contact in it can disrupt the operation of the entire system. Many cell phone models have a jack for connecting an external antenna. Then the cable is connected to it. When the connector is plugged into the phone, its antenna path automatically switches to an external antenna. In case of difficulties with "getting" the right connectors, you can use commercially available antenna adapters. They are a short piece of cable, on one side of which there is a telephone high-frequency connector, and on the other - a standard cable high-frequency connector (Fig. 3). Typically, the attenuation in the antenna adapter does not exceed 1 dB.
Some phone models do not have a special socket for a changeable antenna. They can be connected via a speakerphone. The cable from the antenna is connected to the device with a phonophone, and the phone is inserted into the socket of this device. In principle, you can disconnect the phone's stock antenna and connect an external antenna cable instead. but this. of course it's inconvenient. When communicating with stationary objects (apartments, offices, cottages), in case of uncertain reception, it is advisable to use external directional antennas. Of course, in this case, the phone becomes less "mobile", as it is connected to an antenna cable, but the quality of communication in many cases becomes very good. The main varieties of directional antennas are "wave channel" and log-periodic antennas. The first ones are the most popular. They have great reinforcement and are easy to manufacture. Log-periodic antennas are more complex and expensive, but they have a large frequency band and do not require additional tuning. The "wave channel" antenna (Fig. 4) is similar to the television antenna of the same name. It consists of a number of elements located in the same plane: a half-wave (linear or loop) active vibrator, to which a drop cable is connected, a reflector and directors (passive vibrators). The length of the reflector and its distance from the active vibrator are chosen in such a way that the reflector attenuates the radiation of the active vibrator in the reverse direction and amplifies it in the forward direction, i.e. the reflector is a kind of reflector that provides the formation of a unidirectional characteristic of radiation (reception). Often, a system of vibrators or a grid is used as a reflector. Radiation amplification in the forward direction is facilitated by directors, which are excited, like a reflector, under the influence of radiation from an active vibrator.
The advantages of the "wave channel" antenna include a relatively high gain with a simple design, the disadvantages are the complexity of tuning with more than three directors. Antennas, even assembled according to one drawing, turn out to be tuned differently. The actual gain of such an antenna is usually lower specified (by an average of 3 ... 4 dB). In addition, a narrow bandwidth leads to a sharp decrease in gain in those communication systems that use duplex frequencies with a large spacing. For example, the DAMPS standard uses frequencies of 824 ... 840 MHz and 869.. 894 MHz, so the use of a "wave channel" antenna tuned to the middle of this range leads to a noticeable deterioration in the performance of the antenna at the edges of the range (that is, at operating frequencies).The same applies to the popular GSM-900, GSM- 1800. The program for calculating the "wave channel" antenna can be found at 3ton.com/gsm. Log-periodic antennas are one of the types of antennas with a constant beam shape and constant gain over a wide frequency range. The log-periodic antenna is formed by a collecting line in the form of two pipes arranged in parallel, to which vibrators are attached in turn (through one). Such an antenna provides good coordination with the feeder over the entire frequency range. The operating frequency band "from below" depends on the dimensions of the longest vibrators, and "from above" - on the dimensions of the shortest ones. Antenna gain is determined by the number of vibrators, each of which is active. Therefore, by setting the frequency band (sizes of the maximum and minimum vibrators), it is possible to obtain a sufficiently high gain over the entire range by increasing the number of vibrators. Log-periodic antennas work well in broadband communication systems, but are more complex in design than wave channel antennas. A stationary antenna is mounted on a bracket or mast and directed towards the base station, the signal of which is received steadily. The "base" does not have to be the nearest one, as it all depends on the location of the antenna and the terrain. It is possible that the base with the strongest signal level "does not want" to work with the phone, as it is at a distance that is greater than the theoretical range limit for the standard operation mode (35 km in GSM-900). Antenna orientation must be done carefully, slowly rotating the antenna "along the horizon" and watching the signal strength indicator on the mobile phone display or using the Netmonitor engineering menu function (for Nokia devices). In most other phones, you can enter a special code and open the service menu, which allows you to see the level of 6 ... 8 frequencies received by the phone in descending order, channel numbers, distance to the base station, percentage of errors in the channel, etc. Descriptions of many service menus are available at www.3ton.com/gsm. If there is a Netmonitor, it is better to navigate by the signal level in decibels (the signal is stronger when the level value in decibels is lower). If it is absent, the setting is carried out according to the standard signal scale. Since the change in field strength is not displayed immediately, but after 5 ... 10 s, it is necessary to change the position of the antenna taking this time into account so as not to slip the direction to the strongest signal. In the cell phone menu, you must enable Continuous network search. For example, the GSM-900 range contains 124 channels with a step of 0,2 MHz and a separation between the receive and transmit frequencies of 45 MHz. For a mobile phone, receive frequencies start at 890 MHz and transmit frequencies start at 935 MHz. The average mid-band frequency is 922.5 MHz (a quarter wavelength is 81.3 mm). To increase the efficiency of an external antenna, it is recommended, when manufacturing and configuring it, to take as an operating frequency not the average between the frequencies of reception (Rx) and transmission (Tx), but the transmission frequency (Tx), since the maximum power of a mobile phone transmitter is naturally less than at the base station, and under the worst conditions, the phone simply will not be heard. The gain of the entire antenna system connected to the cell phone depends on the length and quality of the connecting cable. Passport data of external antennas do not take into account the length of the cable (this also applies to car antennas with a circular radiation pattern). If it is possible to have a conversation at least sometimes without an external antenna, then you can get by with a directional antenna with a gain of 9..11 dB with a cable length of up to 10 m. If installing the antenna in a convenient place requires 15 ... 20 m of cable, then you need to either take it with improved quality, or use an antenna with a gain of 13 ... 16dB, which is usually cheaper. Domestic television coaxial cables can only be used to a limited extent (their attenuation - more than 30 dB at 100 m - is too high). Of the available imported samples, RG6 is suitable - a double-braided coaxial cable (attenuation is 20..24 dB at 100 m). Industrial whip car antennas are usually equipped with an RG59 cable (attenuation - 28 dB at 100 m). Wave channel antennas with a gain of 12 dB and a 10-meter RG6U cable give a total gain of about 10 dB, and with a 20-meter cable - 7 dB. When the communication quality is at a "critical" level, even a simple whip antenna, but raised higher, can improve the situation. Depot in that. that when talking on a mobile phone, about 10 ... 20% of the energy is absorbed by the subscriber's body, therefore, when the antenna is raised up, the influence of surrounding objects on it decreases. Well, the simplest dipole antenna can be made in just 15 minutes. We take a white television cable RG6U, cut off the top insulation from one end and "cut" it. We get the central conductor and cable braid We solder a copper wire with a diameter of 1 .. 2.5 mm and a length of 82 mm (for the 900 MHz range) to the central conductor. We solder a second piece of wire of the same length to the braid (Fig. 5).
We connect the other end of the cable to the phone (via a connector or adapter). We arrange the * horns "vertically (one up, the other down) and we get something similar to the letter "T" laid on its side (GSM uses vertical polarization, so this dipole arrangement is required) When manufacturing, be careful not to short the central core with a braid, Otherwise, the phone may burn out. Information sources
Author: A.Kashkarov, St. Petersburg
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