How to make a cellular antenna with your own hands. How to make a cell phone signal booster

IN lately In Russia, the coverage area of GSM 900 networks has significantly increased. However, the situation is far from ideal. If in European countries the problem of unreliable reception is practically absent, then most domestic users often encounter it - in country houses, dachas, etc. And how wonderful it would be to go fishing, hunting, to the countryside, without losing contact with the outside world !

The key to solving this problem is external directional or omnidirectional 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.

Reference

Simply put, a mobile phone is a duplex radio station that communicates on different frequencies. There are a total of 124 frequencies in the GSM 900 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 at frequencies 935.2 - 959.8 MHz. The mobile phone transmits and the base station receives at frequencies 890.2 - 914.8 MHz. The channel from BS to MS is called Down Link, from MS to BS - Up Link. Most operators use range limitation mobile phone from the base station - 35 km, which is due to the features 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 increases by 70 - 100 km (Extended Cell configuration). Unfortunately, with this configuration the number of conversation channels is reduced to 2 - 3, which reduces the network capacity. It is not profitable for the operator to use this mode in and around the city. Sometimes this mode is used on the sea coast to create a coastal coverage area.

Thus, if you have a GSM 900 phone, do not try to establish a connection more than 35 km from the nearest base station. The maximum communication range achieved by me is 34 km.

Communication range

The following factors influence the radio communication range:

Location of BS and MS and terrain.

MS power and sensitivity.

Power and sensitivity BS.

Antennas used on MS and BS.

The will of the Lord God (experienced signalmen joke that this is the main thing).

Typically base stations have a power of 20 - 30 W. Antennas are used either whip or directional. The sensitivity of base stations is -100 dB - 115 dB. The user, of course, cannot change or influence all these parameters. Output power phone is 0.3 - 2 W, sensitivity - 90 - 105 dB. The sensitivity of a phone is mainly determined by the technologies used to create low noise input devices. If in areas of reliable reception the difference in sensitivity and power between models is almost unnoticeable, then in an area of uncertain reception it can become critical. Often the handset shows the signal level from the base station as 1 - 2 cubes (on the scale), but cannot establish a connection: there is not enough power. And although the ETSI standard regulates the standard output powers for each class of phone, the actual value may vary slightly. Tubes from SAGEM, Alcatel, and Motorola have good sensitivity. And all old phones pass in terms of power, especially Motorola. All phase 2 phones have approximately the same power.

As for the terrain, waves travel better on flat terrain and along the river. The higher you are (within reason), the better the signal. The forest sometimes dampens the waves more than urban buildings.

Signal search

So, you are out of town and want to provide yourself with communications. Climb to the roof, attic, or highest point near the house or location you want. If your phone can catch the network, but at the limit (or unstable), you have every chance to improve the situation by using an external antenna. If the distance to the station is less than 30 km, and the network is not available, also try using an antenna. In the latter case, try to negotiate with the sellers to return 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, audibility is normal and the interlocutor does not complain about the loss of parts of your speech, it means that the energy of both lines (UL and DL) is balanced and you can use a standard antenna tuned to the middle frequency between the receiving and transmitting 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 is difficult to hear you, you should strengthen 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 advisable to take into account the operator’s data and reception conditions.

In areas of poor reception, interference (noise) 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 strengths are often “visible” in areas of poor 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 recorded on the phone screen, but it is not possible to establish a connection, or it is possible, but speech disappears all the time, then you are faced with “strangers”. It is not always possible to “force” the phone to select a different frequency, however special function Nokia phones- Netmonitor allows you to do this.

WITH similar problem I came across a dacha, which is separated from a large city by a fairly open space. The phone received adjacent frequency channels with levels of 70 dB, 73 dB and 72 dB, but the quality of communication 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. All that remains is to make the right choice.

Antenna selection

As already mentioned, external antennas 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. Let us repeat that in Western Europe there are no areas of uncertain reception. Therefore, directional antennas for GSM 900 terminals are practically not produced. If a manufacturer offers a directional proprietary GSM 900 antenna, it is usually designed to work with operator repeaters.

In the CIS and Eastern Europe, directional antennas are made in a handicraft manner. In addition, there is not big 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 telephone 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 the log-periodic antenna is 7 - 12 dB. A car antenna is a vertical rod of 3/4, 1/2 or 5/8 wavelength. Even a simple car antenna, raised higher, can improve the situation with uncertain communications. 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 to 5 m, solved the communication problem. Let us explain how such a dipole is made.

Rice. 1. Vertical dipole

We strip the white RG6U television cable from one end. We get the central conductor and cable braid. We solder a copper wire of any thickness about 8.2 cm long (for the 900 MHz range) to the central conductor and fasten it vertically upward. We solder a second piece of wire of the same length to the braid and fasten it vertically downwards (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 central core with the braid, otherwise the phone may burn out. Well, in 5 minutes we built a primitive antenna, not inferior in parameters to Chinese automobile ones.

Let me give you a story from my life. Arriving last summer on vacation in Crimea, I discovered that there was no reliable coverage on the base promised by the operator. This meant that I would miss a very important call. A reliable 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 the right wire(I used aluminum instead of copper). It took another 10 minutes to find a 5-meter piece from the neighbors TV cable. The Nokia 7110 has a mechanical switch for the external antenna, so I simply plugged the center conductor of the cable directly into the phone connector, and connected the cable braid to the metal rim of the phone's external connector. I mounted the antenna on the roof of the house and within 10 minutes, to the delight of my family and the envy of my neighbors, I was freely communicating on the phone.

As they say, let's go back to our sheep. First, let's use a car whip antenna ($5 - 10).

The main thing here is not to run into too bad quality. Having Netmonitor, it is much easier to check the antenna. When the car antenna is connected to the phone, the signal level should increase by 1 - 3 dB (for example, from -60 dB to -57 dB). As a last resort, the signal should remain the same. If, when connecting an antenna, the signal decreases by 5 dB or more, it is better to refuse the purchase. I would like to note that a good branded car antenna costs $40 and above. However, even among the Asian 10-dollar products there are sometimes some good things.

Installed on the roof of your “village”, a car antenna can solve your communication problem. If the gain of the car antenna is not enough, you can turn to others - for example, a common directional antenna - a wave channel. It looks like an ordinary television antenna installed on the roof of a house.

The wave channel allows you to get real gain up to 7 - 15 dB with an optimal radiation pattern. But it has a drawback - narrowband. The difference between the reception frequency and the transmission frequency in GSM 900 is 45 MHz, and the entire operating range is 890 - 960 MHz (70 MHz band). It is difficult to achieve linear or near-linear performance over such a wide range. Therefore, it is desirable to make a wave channel depending on the frequency of the operator and a specific location and, depending on the situation, shift the resonance to the frequency of the ascending or descending directions. For greater broadband, you should use only a loop vibrator, matching it with a cable - for example, with a balun; 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 is narrowed.

I had to deal with many handmade wave channels. I state: in most antennas the gain was less than 7 dB, some had resonance at frequencies of 700 - 800 MHz instead of the GSM range and a standing wave ratio of more than 3 (during transmission, this can easily damage the output stage of the phone). Professionally manufactured and tuned homemade antennas were rare.

Now the next step is 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 actual gain here reaches 10 - 14 dB.

Theoretically, if necessary, you can connect 2 wave channels, one tuned to the receiving frequency, the other to the transmitting frequency, but this is already too complex a system.

Cable and antenna adapters

In the 900 MHz range, the issue of cable selection becomes paramount. Domestic television coaxial cables can only be used to a limited extent (the attenuation of more than 30 dB per 100 m is too high). Of the available imported samples, RG6 is a suitable double-braided coaxial cable. You will find it in any store. The attenuation is 20 - 24 dB per 100 m (tested experimentally). Industrial pins car antennas typically include RG59 cable with an attenuation of 28 dB at 100 m. A wave channel antenna with a gain of 12 dB and 10 m of RG6U cable give a total gain of 9.6 dB, and at 20 m - 7 dB.

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 specified connector and is a short piece of cable, on one side of which there is a specific telephone high-frequency connector, and on the other - a standard RF connector. Typically, the attenuation in the antenna adapter does not exceed 1 dB. When purchasing an antenna adapter, make sure it is functional. When you plug the adapter 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 simply connect the adapter to the phone, the signal on the phone's dial should drop slightly. Then you connect an external antenna to the adapter and the signal increases. If everything goes this way, then the adapter is working.

Setting up the antenna on site

So, you bought an antenna and connected it to the cable and to the phone. We climbed to a high point and began setting up the antenna. Place the phone so that the screen is visible. As already mentioned, when setting up an antenna with Nokia devices, it is best to use the Netmonitor function. In most other phones, you can enter a special code and open the service menu, which allows you to see the receiving 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: www.3ton.com/gsm.) If Netmonitor is available, we will focus on the signal level in decibels (remember that the signal is stronger when the level value in decibels is lower). If it is missing, we will tune according to the standard signal scale.

Since the antennas of GSM 900 base stations are vertically polarized, the wave channel should be placed vertically. When adjusting 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 a signal is found, your task is to find the direction from which the signal comes with the maximum level. If there is no signal, slowly turn 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 searching again. Maybe you'll get lucky.

It is not advisable to use a cable between the antenna and the telephone longer than 30 m: in this case, almost the entire signal is lost in the cable.

Finally, we present the dimensions of a homemade log-periodic antenna for the range 850 - 950 MHz (Fig. 2). Dimensions taken from the website www.atnn.ru. The program for calculating the antenna-wave channel can again be found at: www.3ton.com/gsm.

Parameters: gain - 8.3 dB, characteristic impedance - 60 Ohms.

Articles and Lifehacks

A remote directional GSM antenna can significantly increase the range of a mobile phone - this is indispensable, for example, in the country. How to make an antenna cell phone with your own hands?

The best option is the Kharchenko/Bikvadrat zigzag antenna - it is quite simple to manufacture, allows direct connection to the cable and requires almost no setup, and the materials necessary for its manufacture can be bought at any hardware store and are inexpensive

What is needed to make an antenna?

You will need copper wire or a lead with a diameter of 2-3 mm - you can use insulated copper wire intended for electrical wiring (but you will have to remove the insulation from it: cut it off on one side - and after that it can be easily removed).
To connect the cable without soldering, you need F-type antenna connectors and connectors for them. If you intend to use a reflector, you will need a piece of aluminum or tin - 277 by 161 mm.
In addition, you need electrical tape or heat shrink tubing. As a working tool you will need a soldering iron.

Antenna manufacturing process

The prepared copper wire should be carefully and very evenly bent in the form of a zigzag frame consisting of two rhombuses with a side of 80 cm, its length (between the opposite corners of the rhombuses located on the same straight line) will be 226 cm.
Now, to the frame - at the point where the diamonds connect - you need to solder a piece of cable 0.5 cm long, and screw an F-type connector onto its opposite end, to which a cable of the required length will then be connected.
If there is a reflector, the antenna works better. In this case, the lower end of the frame will have to be connected not only to the cable braid, but also to the reflector.
To do this, you need to screw a 50 mm long bolt into the reflector and use a tie to pull the F-type connector to it, to which the frame and cable are pre-soldered at a distance of 43 mm.
You can connect the antenna to a mobile phone using a pigtail - a special wire, which is connected at one end to the cable from the antenna, and at the other end - using a special connector - to the antenna connector of the phone.
True, not every cell phone has an antenna connector; in this case, you need to attach a special adapter to the back wall of the phone, which contains a special antenna that re-radiates radio waves.

Such an antenna does not require special configuration; you just have to find the optimal location for it, focusing on the quality of the signal.

Nowadays it is impossible to imagine life without a mobile phone, but in some cases it becomes useless. No connection or very weak signal happens in basements, in cars, in areas far from the relay antenna or in a city where there are many high-rise buildings that dampen the signal. In these cases, you can use a ready-made additional device or learn how to make and connect an antenna to a mobile phone with your own hands.

Before you start searching and reconstructing external antennas for mobile device It is necessary to understand how a modern cellular device works. And it is a small radio station operating at different frequencies determined mobile operator. So, in the “transmission - reception” ratio: base station (BS) - telephone (MS) operates on 935.2–959.8 MHz, and telephone (MS) - base station (BS) - on 890.2–914, 8 MHz. The link relationship from BS to MS is called Down Link, and MS to BS is called Up Link.

Any providers limit the communication range of a mobile phone from a base station to a distance of up to 35 km. With this configuration, the number of talk channels reaches 8 and is the standard mode used in urban areas. While GSM provides a non-standard configuration with a communication range of 70–100 km (Extended Cell), but there are fewer channels - a maximum of 3. The latter mode is usually used in the coastal coverage area.

Which external antenna is suitable for a smartphone?

The quality of communication and signal strength are also influenced by location, terrain, weather conditions and time of year, sensitivity and power of the cell phone and the main antenna. Automotive or standard is installed outdoors, preferably on the roof of a building or car. Like a television, it must be directed towards the nearest base station. To do this, the device should be moved and rotated until the signal strengthens.

The wave one contains a traverse with perpendicularly located reflectors. Their sizes depend on the frequency, which is directly proportional to the wavelength. It has a clear advantage of signal amplification with rather small dimensions. The disadvantage is the need for precise guidance to the station.
Parabolic consists of a reflector and an irradiator. This design amplifies the signal quite strongly, but installation requires certain skills. The downside of this antenna is zero efficiency with inaccurate tuning.
A flat panel is a flat small square with an irradiator inside a reflector. The advantage of the device is a wide range of directionality, which does not require precise aiming, but at the same time gives average signal amplification.
The pin design is a small vertical rod that is both an irradiator and a reflector. The advantage of the antenna is its circular directionality, which does not require precise installation on the provider’s base distributor. But there is a nuance: such an antenna should be placed so that there are no shielding structures between it and the base. It is installed where no special signal amplification is required.

How to connect an external antenna to a mobile phone

Typically cell phones are tuned to high frequencies. Therefore, the quality of the cable and its connection is of great importance for the efficiency of the external structure. It is best to use coaxial wire (RG-6 from reputable manufacturers) and a high-frequency connector. If there is no jack for attaching an external antenna, you can use a speakerphone. In this case, the antenna wire is connected to the speakerphone device, and the mobile phone is inserted into the socket of this device. You can also make 3-4 turns of the central core of the cable around the mobile phone, near the built-in antenna.

Of course, this method of using the phone is not entirely convenient and is limited by the length of the wire. But these inconveniences can be avoided by making the mobile device stationary.

How to make your own antenna for a cell phone

A quick antenna can be made from what you have in the house

Take a copper wire and bend it into a diamond-shaped figure eight with a side of 8 cm. Connect the antenna to the phone with a cable, having previously stripped its ends. In a horizontal position, attach to the ceiling where there is a stronger signal.
Use culinary foil, make a large “bag” out of it and put a mobile phone in it.
You can use an iron basin of a suitable size, using it as a parabolic antenna with a mobile phone in focus.

A home antenna that is mainly suitable for signal amplification. To do this you will need: pliers, pliers or a knife, a coaxial cable, soldering equipment, copper wire (8 cm) and an adapter for a mobile device (Chinese is possible).

Strip one end of the high-frequency cable so as not to damage the central core; it will be connected to the antenna. The other end is connected to the phone.
Solder the copper wire to the cable core so that the structure is strong and positioned vertically upward. Solder the same wire to the cable braid vertically downwards.
Fasten both copper wires to a small wooden board, insulating them in the soldered places.
Fix this structure on the roof as high as possible, and connect the other end of the antenna cable to the phone using an adapter.

IMPORTANT. The vertical and horizontal connection of the copper wire to the cable should be strictly observed.

A portable antenna is suitable even if there is no network at all.

This is a fairly simple device, but quite effective. This device can be used anywhere. To do this you will need: a radio antenna from an old tape recorder, a thin wire (0.5 m), a screwdriver, a soldering iron and a small washer.

Disassemble the radio antenna at the base.
Solder the wires to the washer.
Assemble the antenna base by attaching the washer with the wire.
Remove the cover from the phone and find a connector for connecting an additional antenna.
Strip the other end of the wire by 1–2 mm (no more) and insert it into the hole in the center of this connector.
Fix the wire on the phone and look at the network scale. The indicator should show 6 out of 6 sticks.
The mobile device cover may not be installed.

Besides homemade antennas You can use purchased repeaters, which allow you to make a stable connection in any room and remote areas. But the cost of such equipment is quite high and comparable to the price of a good mobile phone, and more advanced solutions reach 2–3 thousand dollars. Therefore, at home you can always use cheaper and more readily available means of signal amplification.

The situation was hopeless. The monotonous pine forest swirled around me like a colonnade. Two hours later I again found myself in the same place where I was before. I got lost and panicked. Cell phone service didn't work. I decided to find a higher place in the hope of catching a signal. cellular communication. He rushed into the clearing and ran out into a clearing with a lone pine tree. On the tree there is a bird feeder and a piece of paper sealed in plastic with the text: “To make a call, place the phone in the house with the buttons facing up. Turn on speakerphone or connect a headset." Next, the telephone numbers of the forestry department, police, and administration were listed...

When I was already bringing the mobile phone into the feeding house, I noticed that on the phone, next to the symbol indicating the antenna, the cellular signal strength diamonds flashed. The revived telephone refreshed my mind, and the house turned out to be an antenna, protected by the roof from rain and snow, and a coaxial cable running upward from it at a height of 5 meters was connected to another directional antenna. A homemade GSM signal repeater, as I didn’t immediately guess. It does not need any power supply or amplifier, which is why it is called passive. In the mobile phone itself there is no need to look for a high-frequency connector for connecting an external antenna, since its built-in antenna complements the design of the repeater antenna.

The maximum communication range at this frequency in line-of-sight conditions, when hills, houses and trees do not block the path between the antennas, reaches 35 kilometers. It’s unlikely that I could have gone that far, the thought flashed in my head, because the phone is not directly connected to the antenna, there are probably losses in the passive repeater, and I was glad to see that I was separated from civilization, no matter how 10 km.

- Found! Everyone has already been put on their feet. “The devil’s tract”, you can’t get out of there without help, there are swamps all around, a quagmire. Stay where you are, don’t go anywhere, in 1.5 hours Musya will lead me there. Musya is my horse, she knows how to get there.

I had a lot of time to study the passive repeater design.

StandardGSM 900.

The antenna must work for both reception and transmission, so its range is from 890 (880) to 960 MHz, where in the range of 890 - 915 MHz transmission from a cell phone to the base station is provided, and within 935 - 960 MHz there is signal transmission from the base station to the phone.

Here is the antenna design.

photo 1.

DirectedGSM 900 patch antenna.

A suspended plate patch antenna is similar to a microstrip antenna. Its advantage is its small dimensions and ease of manufacture.

These are two plates and an L - vibrator. Antenna elements can be made of copper, brass, aluminum.

The larger plate is a reflector; it reflects radio waves. The braid of the coaxial cable is connected to the reflector. For the convenience of connecting the coaxial cable, desoldering and attaching the L-shaped vibrator, I used a high-frequency SMA type connector. If the cable is attached without a connector, then there is no need to drill a hole, but it is necessary to provide a dielectric fastening of the L-vibrator.

The thin plate is an active L-shaped vibrator, it is connected to the central core of the coaxial cable. I soldered it to the connector and used an additional fastening in the center using an insulator.

Rice. 2. Sketch of an L-vibrator.

A smaller flat plate, by analogy with a “wave channel” antenna, is called a director, and is not electrically connected to anything, but is mounted on dielectric supports - racks.

Rice. 3. Director.

Support posts can be made of plastic, foam, wood.

Purchased (factory) antennas for this frequency have a characteristic impedance of 50 Ohms, and are accordingly equipped with a cable with a characteristic impedance of 50 Ohms. So, for a homemade passive repeater, it is also advisable to adhere to a wave impedance of 50 Ohms, which is associated with lower losses compared to the 75 Ohm standard.

Options.

Compared to the gain of a quarter-wave rod, the antenna we made provided 5 dB more gain.

At the center frequency SWR = 1.2. At the edges of the range SWR = 1.5

Antenna as a passive repeater.

It is convenient to use such an antenna as a stand for a mobile phone when constructing a passive repeater, in which the cell phone with its built-in antenna will be the director.

Let me remind you that the repeater consists of two antennas connected by a coaxial cable. One antenna is raised above the ground and aimed at base station, and the second antenna is located in close proximity to the mobile phone. At the lower table antenna, I made the director out of plastic, without changing the height of its mounting, and placed the phone on the pedestal.

When I did this experiment outside the city, the mobile phone added four levels to one risk. In addition, I received an SMS that I had left my home region. Typically, such messages come after 15 km from the test site, which is located on the border of two regions. In this case, the phone reliably received signals from a base station from a neighboring region.

The factory Radant 911 antenna was used as a directional antenna.

Frequency range 890 – 960 MHz, input impedance 50 Ohm, beamwidth 45 degrees, vertical polarization, gain 11 dBi. It was suspended under the roof of the gazebo, at a height of three meters from the ground. A two-meter 50 Ohm cable was connected to a patch antenna on the table. In the patch antenna, instead of the director, there was a mobile phone on a plastic pedestal.

In my experience with the repeater, I deliberately provide for additional losses by installing the antenna under the roof of the gazebo (1 cm of wood, plus soft roofing) to ensure that the passive repeater is working.

The simplest way to increase the signal level of your cell phone in a few minutes. You will need absolutely nothing other than a piece of wire of a certain length. This method will definitely help in difficult life situations.
Before modifying a cell phone, I show the initial signal level.

Now it is equal to: minus 99 dB - 7 units.
We disassemble the smartphone, namely, remove the back cover. SIM cards, batteries, etc. are hidden under it. We are interested in antenna connectors.

Usually, there is one connector for connecting an external GSM antenna, but sometimes there can be several of them: wi-fi, 4G, 3G. See their captions. We are interested in the GSM socket.

Making a simple antenna for a cell phone

Now you need to take a piece of insulated wire, with a central core of such a diameter that will be inserted into the central socket of this connector. The wiring must be exposed on one side for connection.

But it's not so simple: this wiring must be of a certain length in order for it to work as efficiently as possible as an external antenna.
Namely, its length should be 1/4 of the wavelength. The wavelength can be calculated from the frequency at which your cell phone operates.
Most phones operate in a range where the average frequency will be 1.9 GHz. But there are exceptions, as in my case - 0.8 GHz.
Now you need to calculate the wavelength, for this you can use numerous online services online.
So:

For a frequency of 0.8 GHz - 38 cm, now divide by 4 and get the length of the antenna wiring - 9.5 cm.
And for a frequency of 1.9 GHz - 16 cm, divide by 4 and get - 4 cm.

We cut the required length of wire and insert the bare end into the antenna connector.