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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Do-it-yourself GSM antenna. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering /Mobile telephony Recently, the coverage area of GSM 900 networks has significantly increased in Russia. Nevertheless, the situation is far from ideal. If in European countries the problem of uncertain reception is practically absent, then most domestic users often encounter it - in country houses, in summer cottages, etc. And how wonderful it would be to go fishing, hunting, to the village without losing touch with the outside world ! The key to solving this problem is external directional or non-directional antennas. True, buying an antenna is not enough - you need to install and configure it correctly. Readers familiar with radio electronics will be able to independently manufacture and configure an antenna operating in the GSM standard. Today we will talk about the types of antennas and how to install and configure them. Help To put it quite simply, a mobile phone is a two-way radio that broadcasts on different frequencies. In total, there are 900 frequencies in the GSM 124 standard. The phone, like the base station, can operate on any frequency determined by the operator. The base station (BS) transmits and the telephone (MS) receives on frequencies 935,2 - 959,8 MHz. The mobile phone transmits and the base station receives at frequencies of 890,2 - 914,8 MHz. The channel from BS to MS is called Down Link, from MS to BS - Up Link. Most operators use a limitation on the range of a mobile phone from a base station - 35 km, which is due to the peculiarities of the standard. Let us explain that in a standard configuration network, 8 time intervals (time slots) are formed in one frequency channel: one is service, and seven are conversational. It is in this case that the maximum communication range on each channel is 35 km. However, GSM also provides a non-standard cell configuration, in which the communication range is increased by 70 - 100 km (Extended Cell configuration). Unfortunately, with this configuration, the number of spoken channels is reduced to 2 - 3, which reduces the capacity of the network. Using this mode in the city and near the operator is not profitable. This mode is sometimes used on the coast to create coastal coverage. Thus, if you have a GSM 900 phone, do not try to establish a connection more than 35 km away from the nearest base station. The maximum communication range achieved by me is 34 km. Communication range The following factors affect the radio range:
Typically, base stations have a power of 20 - 30 watts. Antennas are either whip or directional. The sensitivity of base stations is -100 dB - 115 dB. Of course, the user cannot change or influence all these parameters. The output power of the phone is 0,3 - 2 W, the sensitivity is 90 - 105 dB. The sensitivity of a phone is mainly determined by the technologies used to create low-noise input devices. If in the areas of reliable reception the difference in sensitivity and power between the models is almost imperceptible, then in the area of uncertain reception it can become critical. Often, the handset shows a signal level from the base station of 1 - 2 cubes (on a scale), but cannot establish a connection: there is not enough power. Although ETSI standardizes the standard output power for each class of phone, the actual value may fluctuate slightly. SAGEM, Alcatel, Motorola tubes are distinguished by good sensitivity. And in terms of power, all old phones pass, especially Motorola. All phase 2 phones are about the same power. As for the relief, waves spread better on flat terrain and along the river. The higher you are (within reason), the better the signal. The forest sometimes "extinguishes" the waves more than urban development. Signal Search So, you got out of the city and want to provide yourself with a connection. Climb to the roof, attic, or to the highest point near the house or the place you need. If the phone catches the network, but at the limit (or unstable), you have every chance to fix things by using an external antenna. If the distance to the station is less than 30 km, and the network is not caught, also try using an antenna. In the latter case, try to negotiate with the sellers about the return of the antenna if the experiment fails. If the signal, although very weak, was still caught, dial any city number - to check. If during a conversation the audibility is normal and the interlocutor does not complain about the loss of pieces of your speech, then the energy of both lines (UL and DL) is balanced and you can use a standard antenna tuned to the middle frequency between the receive and transmit frequencies. If the interlocutor disappears from time to time, it is necessary to "strengthen" the downward direction from the base station to your phone. It might be worth looking for a phone with better sensitivity. But if it's hard to hear you, you should amplify the direction from your phone to the base station. Already at this stage, it is clear that when choosing the type of antenna and its parameters, it is desirable to take into account the operator's data and reception conditions. In areas of poor reception, interference (interference) is often observed between channels with the same and adjacent frequencies. Unfortunately, the frequency resource allocated to GSM 900 operators in Russia is limited, which is why frequencies from different base stations with the same or neighboring signal levels are often "visible" in the zone of uncertain reception. Such frequencies create mutual interference that interferes with communication, and at certain signal levels, communication becomes completely impossible. If a strong signal from the base station is fixed on the phone screen, and the connection cannot be established or it is possible, but the speech disappears all the time, then you are faced with "outsiders". It is far from always possible to "force" the phone to select a different frequency, but a special function of Nokia phones - Netmonitor - allows you to do this. I encountered a similar problem in a dacha, which is separated from a large city by a rather open space. The phone received adjacent frequency channels with levels of 70 dB, 73 dB and 72 dB, but the call quality left much to be desired. The only thing that saved me was the forced selection of another frequency channel with a lower level - 80 dB. However, with a limited frequency resource, even frequency rescheduling of the network does not always help. However, if your phone does not have Netmonitor, you can use an external directional antenna with a good radiation pattern. It remains only to make the right choice. Antenna selection As already mentioned, it is external antennas that allow you to increase the range and quality of communication. For telephones, external whip and log-periodic antennas, as well as wave channel antennas, are mainly used. We repeat that in Western Europe there are no zones of uncertain reception. Therefore, directional antennas for GSM 900 terminals are practically not produced. If the manufacturer offers a directional proprietary GSM 900 antenna, it is usually designed to work with repeaters (repeaters) of operators. In the CIS and Eastern Europe, directional antennas are made in a handicraft way. In addition, there are small factories in the Czech Republic, Poland and the Baltic States. A simple car magnetic antenna has a gain of 1 - 3 dB (1 - 2 cubes on the phone scale), a wave channel - 7 - 15 dB (depending on the number of elements, build quality and antenna settings), which is already 2 - 3 cubes on the scale , and log-periodic antenna - 7 - 12 dB. A car antenna is a vertical 3/4, 1/2, or 5/8 wavelength pole. Even a simple car antenna, raised higher, can correct the situation with an uncertain connection. The fact is that when talking on the phone, about 10 - 20% of the energy is absorbed by the user's body, therefore, by raising the whip antenna up, you reduce the influence of surrounding objects on it. I myself have observed how homemade, half-wave and vertical dipoles, raised 5 m, solved the problem of communication. Let us explain how such a dipole is made.
We clean from one end the white television cable RG6U. We get the central conductor and the cable braid. We solder a copper wire of any thickness about 8,2 cm long (for the 900 MHz band) to the central conductor and fasten it vertically upwards. We solder a second piece of wire of the same length to the braid and fasten it vertically down (Fig. 1). We get something similar to the letter "T", laid on its side. (Television uses horizontal polarization, while GSM uses vertical polarization, so just such a dipole is required.) We connect the other end of the cable through an adapter to the phone. Be careful not to short the braided center wire, otherwise the phone may burn out. Well, in 5 minutes we built a primitive antenna, which is not inferior in parameters to Chinese automobile ones. Let me give you a story from my life. Arriving last summer on vacation in the Crimea, I found that there is no reliable coverage on the basis of the promised operator. This meant that I would miss a very important call. A sure signal was found on the roof of the cottage, but the prospect of sitting there for two weeks did not inspire me. In 5 minutes I found a piece of the right wire (I used aluminum instead of copper). It took another 10 minutes to find a 5-meter piece of television cable from the neighbors. The Nokia 7110 has a mechanical switch for an external antenna, so I just plugged the cable's center conductor directly into the phone jack and connected the cable sheath to the metal rim of the phone's external jack. He strengthened the antenna on the roof of the house and after 10 minutes, to the delight of the family and to the envy of the neighbors, he freely spoke on the phone. As they say, back to our sheep. First, we will use a car whip antenna ($ 5 - 10). The main thing here is not to run into too bad quality. With Netmonitor, checking the antenna is much easier. With a car antenna connected to the phone, the signal level should increase by 1 to 3 dB (for example, from -60 dB to -57 dB). As a last resort, the signal should remain the same. If, when connecting the antenna, the signal decreases by 5 dB or more, it is better to refuse to buy. I want to note that a good branded car antenna costs $40 and up. However, even among the Asian 10-dollar products, sometimes there are good little things. Installed on the roof of your "house in the country", a car antenna can solve a communication problem. If the gain of the car antenna is not enough, you can turn to others - for example, to a common directional antenna - a wave channel. It looks like an ordinary television antenna mounted on the roof of a house. The wave channel allows you to get a real gain of up to 7 - 15 dB with an optimal directivity pattern. But it has a drawback - narrowband. The difference between the receive and transmit frequency in GSM 900 is 45 MHz, and the entire operating range is 890 - 960 MHz (70 MHz bandwidth). It is difficult to achieve a linear or close to linear response over such a wide range. Therefore, it is desirable to make the wave channel depending on the frequency of the operator and the specific location, and depending on the situation, shift the resonance to the frequency of the ascending or descending directions. For greater bandwidth, only a loop vibrator should be used, matching it with the cable - for example, with a balancing loop; you should also limit yourself to a small number of elements, say 3 - 12, since if there are more of them, it will be difficult to tune the antenna without equipment, in addition, the operating range of the antenna narrows. I had to deal with a lot of handicraft wave channels. I state: in most antennas, the gain was less than 7 dB, some had a resonance at frequencies of 700 - 800 MHz instead of the GSM band and a standing wave ratio greater than 3 (during transmission, this can easily damage the phone's output stage). Professionally made and tuned homemade antennas were rare. Now next in line are log-periodic antennas (they can also be found on the radio market). Compared to the wave channel, they have a wider operating range. Therefore, such antennas are less critical to the accuracy of manufacturing and tuning. The real gain here reaches 10 - 14 dB. Theoretically, if necessary, you can connect 2 wave channels, one tuned to the receive frequency, the other to the transmit frequency, but this is already too complicated a system. Cable and antenna adapters In the 900 MHz band, the choice of cable becomes paramount. Domestic television coaxial cables can only be used to a limited extent (attenuation of more than 30 dB at 100 m is too high). Of the available imported samples, RG6 is a double-braided coaxial cable. You can find it in any store. Attenuation is 20 - 24 dB per 100 m (tested experimentally). Industrial whip antennas typically include RG59 cable with an attenuation of 28dB at 100m. A 12dB wave channel antenna and 10m of RG6U cable give a total gain of 9,6dB and at 20m 7dB. Most phones have a connector for an external antenna. In addition, for each type of phone there is a so-called antenna adapter (about $ 5), it connects to the indicated connector and is a short piece of cable, on one side of which there is a specific high-frequency telephone connector, and on the other - a standard RF connector. Typically, the attenuation in the antenna adapter does not exceed 1 dB. When buying an antenna adapter, make sure it works. When the adapter is plugged into the phone, the antenna built into the phone is turned off and the output stage switches to the adapter. In other words, if you just connect the adapter to the phone, the signal on the phone scale should drop a little. Then you connect an external antenna to the adapter, and the signal increases. If everything goes that way, then the adapter is working. Setting up the antenna on the ground So, you bought an antenna and connected it to the cable and to the phone. We climbed to a high point and proceeded to tune the antenna. Position the phone so that the screen is visible. As already mentioned, when setting up the antenna with Nokia devices, it is best to use the Netmonitor function. In most other phones, you can enter a special code and open a service menu that allows you to see the reception level of 6 - 8 frequencies received by the phone in descending order, frequency numbers, distance to the base station, percentage of errors in the channel, etc. (For a description of many service menus, see . at: 3ton.com/gsm.) If there is a Netmonitor, we will focus on the signal level in decibels (recall that the signal is stronger when the level value in decibels is lower). If it is absent, we will adjust according to the standard signal scale. Since the GSM 900 base station antennas are vertically polarized, the wave channel should be placed vertically. When aligning the antennas, please note that the receiving signal level displayed on the phone changes with a delay of up to several seconds, so the antenna should be rotated slowly and discretely. If you know the direction to the nearest city, start there. Slowly turn the antenna horizontally. If the signal is found, your task is to find the direction where the signal comes from with the maximum level. If there is no signal, slowly tilt the antenna horizontally until it appears. Remember that every meter of antenna installation height can be decisive. If no signal is found, try moving a few meters to the side and looking again. Maybe you'll get lucky. It is undesirable to use a cable between the antenna and the phone with a length of more than 30 m: in this case, almost the entire signal is lost in the cable. Finally, we give the dimensions of a home-made log-periodic antenna for the range of 850 - 950 MHz (Fig. 2). Dimensions are taken from the site atnn.ru. The program for calculating the antenna-wave channel can again be found at: 3ton.com/gsm.
Parameters: gain - 8,3 dB, impedance - 60 ohms.
Publication: cxem.net
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