Lift off the veil of the antenna, peel off the coat of the antenna

01

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Lift the veil of the antenna

As we all know, antennas are used by base stations and mobile phones to transmit signals.

The English word for antenna is Antenna, which originally means tentacles. The tentacles are the two long filaments on the top of the insect's head. Don't underestimate such inconspicuous things. Insects use the various chemical signals sent by these tentacles to transmit various social information.

Similarly, in the human world, wireless communication also transmits information through antennas, except that it transmits electromagnetic waves that carry useful information. The figure below is an example of mutual communication between a mobile phone and a base station.

So what do the actual antennas look like? Due to different uses, there are too many forms of antennas, ranging from pots (parabolic antennas) for receiving TV signals to small antennas hidden in mobile phones. They have different shapes due to different functions.

Speaking of antennas, the most common thing most people see is the antenna of the wireless router at home.

It is this stick-like antenna that allows us to enjoy the same internet speed as flying.

A long time ago, listening to the radio was a very fashionable thing. The radio had a long antenna that stretched section by section. This antenna is the same as the router antenna. It is called a whip antenna, also called a telescopic antenna or a rod. antenna.

In prehistoric times, the tallest building in every city must be the TV tower. TVs also receive signals from the TV tower through antennas. The two whip antennas with the same antennae on top form a lot of people-to-antennas. Initial impression. Both the shape and the function are completely similar to the tentacles of insects.

In addition, there are a variety of different types of antennas, which can be given different types according to different classification methods.

1. According to the nature of work, it can be divided into transmitting antenna and receiving antenna.

2. According to the purpose, it can be divided into communication antennas, broadcast antennas, TV antennas, radar antennas, etc.

3. According to the directivity, it can be divided into omnidirectional antenna and directional antenna.

4. According to the working wavelength, it can be divided into ultra long wave antenna, long wave antenna, medium wave antenna, short wave antenna, ultra short wave antenna, microwave antenna, etc.

5. According to the structure and working principle, it can be divided into line antenna and surface antenna.

6. According to the number of dimensions, it can be divided into two types: one-dimensional antenna and two-dimensional antenna.

7. Antennas can be divided into three categories: handheld station antennas, vehicle antennas, and base antennas.

Just like a blind person touching an elephant, each classification method can only describe one side or one type of feature of the antenna. Only by combining all the features targeted by these classification methods can you see the full picture of the antenna.

In order to reduce the complexity, we start with the most intuitive classification method as the starting point. It is better to hear it than to see, let everyone see what real antennas for different purposes look like.

Many people should have seen this kind of antenna in the picture below. It used to be mainly installed on the roof to receive TV signals (the whip antenna that comes with the TV is really limited). This kind of fishbone antenna is called a Yagi antenna.

It is not necessary to count whether there are 8 rods on the antenna. The name Yagi antenna is because its inventor is a Japanese named Yagi Hideji. Yagi antennas are mainly used for the reception of TV signals, and there are not many scenarios for wireless communication.

The picture below is a parabolic antenna used for radar, just like huge pots, which are spectacular. When the radar is launched, the energy must be concentrated and radiated to the direction that needs to be irradiated. This shape is very suitable.

The following "pots" will be smaller. These are the microwave antennas used to send and receive microwave signals to convey information. The wavelength of electromagnetic waves such as microwaves is very short, and they mainly propagate in a straight line. The transmitting and receiving antennas must be aligned with each other to work. They are mainly used for transmission in wireless communication.

If you look at the above pictures carefully, you will find that there are some plate-shaped things at the top of the tower. This is the protagonist of this article: communication antenna (the subdivision type is directional antenna: the signal is sent and received in a certain direction). This is the one who often flirts with the phone directly.

Since there are directional antennas, there must be omnidirectional antennas. As the name implies, the omnidirectional antenna can transmit and receive signals at 360° without dead angles, outdoor omnidirectional antennas, and ceiling antennas for indoor coverage.

Back to the protagonist of this article: directional antenna. To unravel the mystery of this cargo, you must take it apart to see what's inside.

The interior is empty, the structure is not complicated, it is composed of a vibrator, a reflector, a feed network and a radome. What do these internal structures do, and how to realize the function of directional transmitting and receiving signals?

It all starts with electromagnetic waves.

02

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Strip off the coat of the antenna

The reason why the antenna can transmit information at a high speed is because it can emit electromagnetic waves carrying information into the air, propagate at the speed of light, and finally reach the receiving antenna.

It's like using high-speed trains to transport passengers. If you compare information to passengers, then the means of transporting passengers: high-speed trains are electromagnetic waves, and antennas are equivalent to stations, responsible for managing and dispatching the transmission of electromagnetic waves.

So, what are electromagnetic waves?

Scientists have studied the two mysterious forces of electricity and magnetism for hundreds of years. In the end, Maxwell of the United Kingdom proposed that: electric current can generate electric field around it, changing electric field generates magnetic field, and changing magnetic field generates electric field. In the end, this theory was confirmed by Hertz's experiment.

In such a periodic transformation of the electromagnetic field, electromagnetic waves radiate out and propagate into space.

As shown in the figure above, the red line represents the electric field, and the blue line represents the magnetic field. The propagation direction of the electromagnetic wave is perpendicular to the direction of the electric field and the magnetic field at the same time.

So, how does the antenna send out these electromagnetic waves? After reading the picture below, you will understand.

The above two wires that generate electromagnetic waves are called "oscillators". In general, the size of the vibrator works best when it is half the wavelength, so it is often called "half-wave vibrator".

With the vibrator, electromagnetic waves can be emitted continuously. As shown below.

The real vibrator looks like the picture below.

The half-wave vibrator continuously propagates electromagnetic waves to space, but the signal strength is not uniformly distributed in space, like a tire-like ring.

But in fact, the coverage of our base station needs to be farther in the horizontal direction. After all, the people who need to call are on the ground; the vertical direction is high in the sky, and there are no people who need to fly and brush the vibrato (route Coverage is another topic, I will talk about it next time), therefore, in the emission of electromagnetic wave energy, it is necessary to strengthen the horizontal direction and weaken the vertical direction.

According to the principle of energy conservation, energy will neither increase nor decrease. If you want to increase the emitted energy in the horizontal direction, you must weaken the energy in the vertical direction. Therefore, the only way to flatten the energy radiation pattern of the standard half-wave array is as shown in the figure below.

So how do you slap it? The answer is to increase the number of half-wave oscillators. The emission of multiple oscillators converges in the center, and the energy at the edge is weakened, which achieves the purpose of flattening the radiation direction and concentrating the energy in the horizontal direction.

In general macro base station systems, the use of directional antennas is the most common. In general, a base station is divided into 3 sectors, covered by 3 antennas, and each antenna covers a range of 120 degrees.

The above figure is a base station coverage plan of a patch area. We can clearly see that each base station is composed of three sectors, and each sector is represented by a different color, which requires three directional antennas.

So, how does the antenna realize the directional emission of electromagnetic waves?

This is certainly not difficult for smart designers. Isn't it enough to add a reflector to the vibrator to reflect the signal that should be radiated to the other side?

In this way, adding a vibrator to allow electromagnetic waves to travel farther in the horizontal direction, and then adding a reflector to control the direction. After these two tossings, the prototype of the directional antenna was born, and the electromagnetic wave emission direction became as shown in the figure below.

The horizontal main lobe is far away from the launch site, but the upper and lower side lobes are produced in the vertical direction. At the same time, due to incomplete reflection, there is a tail behind it, called the back lobe.

At this point, the most important indicator of the antenna: "gain" is explained.

As the name implies, gain means that the antenna can enhance the signal. It stands to reason that the antenna does not need a power source, but only emits the electromagnetic waves transmitted to it. How can there be "gain"?

In fact, whether there is "gain" depends on who and how.

As shown in the figure below, compared to an ideal point radiation source and half-wave oscillator, the antenna can concentrate energy in the direction of the main lobe, and can send electromagnetic waves farther, which is equivalent to increasing in the direction of the main lobe. In other words, the so-called gain is relative to a point radiation source or a half-wave oscillator in a certain direction.

So, how do you measure the coverage and gain of the antenna's main lobe? This requires the introduction of a "beam width" concept. We call the beam width the range when the electromagnetic wave intensity on both sides of the center line on the main lobe attenuates to half.

Because the intensity is attenuated by half, which is 3dB, the beam width is also called "half power angle", or "3dB power angle".

Common antenna half-power angles are mostly 60°, and there are also narrower 33° antennas. The narrower the half power angle, the farther the signal propagates in the direction of the main lobe, and the higher the gain.

Next, we combine the horizontal pattern of the antenna with the vertical pattern to get the three-dimensional radiation pattern, which looks much more intuitive.

Obviously, the existence of the back lobe destroys the directivity of the directional antenna, and it is necessary to minimize it. The energy ratio between the front and rear lobes is called the "front-to-back ratio". The larger the value, the better, and it is an important indicator of the antenna.

The precious power of the upper side lobe is radiated to the sky in vain, which is not a small waste, so when designing a directional antenna, the upper side lobe should be minimized as much as possible.

In addition, there are some holes between the main lobe and the lower side lobes, which are also called lower nulls, which result in poor signals near the antenna. When designing the antenna, these holes should be minimized, which is called "zero point filling".

03

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Meet the antenna candidly

Let's talk about another important concept of the antenna: polarization.

As mentioned earlier, the propagation of electromagnetic waves is essentially the propagation of electromagnetic fields, and electric fields are directional.

If the direction of the electric field is perpendicular to the ground, we call it a vertically polarized wave. Similarly, parallel to the ground is a horizontally polarized wave.

If the direction of the electric field and the ground form an angle of 45°, we call it ±45° polarization.

Due to the characteristics of electromagnetic waves, the horizontally polarized signal will generate a polarized current on the surface of the ground when it is close to the ground, so that the electric field signal decays quickly, while the vertical polarization method is not easy to generate a polarized current, thus avoiding energy loss. Significant attenuation ensures the effective propagation of the signal.

As a compromise optimization solution, the current mainstream antennas are superimposed on the two polarization modes of ±45°, and two orthogonal polarization waves are formed in one unit by two vibrators, which is called dual polarization. While ensuring performance, this implementation also greatly improves the integration level of the antenna.

This is the reason why I like to draw a number of forks in the antenna diagram. These forks visually indicate the polarization direction and the number of vibrators.

With a high-gain directional antenna, can it be hung directly on the tower?

Obviously, the ground is low and the building is blocked too much, which is not good; when the ground is high, no one is in the air, the signal is wasted, and if the signal is transmitted too far, the base station can barely accept it, but the transmission power of the mobile phone is too small. The base station can't receive it.

Therefore, the antenna has to be directed at the human ground to transmit signals, and the coverage area has to be controlled. This requires the antenna to be tilted down to an angle, like a street lamp, each antenna is responsible for the coverage of its own area.

This introduces the concept of antenna downtilt.

All antennas are equipped with a knob with an angle scale on the mounting bracket. By twisting the knob to control the mechanical movement of the bracket, the downtilt angle can be adjusted. Therefore, adjusting the downward tilt angle in this way is also called mechanical downward tilt.

But this approach has two obvious drawbacks.

The first is trouble. In order to adjust the angle of network optimization, it is necessary for engineers to climb the tower. The actual effect is hard to say. It is really inconvenient and costly.

The second is that the mechanical downtilt adjustment method is too simple and rough, and the amplitude of the vertical component and the horizontal component of the antenna is unchanged, which will cause distortion of the coverage pattern.

After so much effort, the coverage before and after the adjustment was completely changed, and it was difficult to achieve the expected effect. Moreover, the interference to other base stations was increased due to the upturn of the back flap, so the mechanical downtilt angle could only be adjusted slightly.

So, is there a better way?

There is a real way, which is to use electronic downtilt. The principle of electronic downward tilt is to change the phase of the collinear array antenna element, change the amplitude of the vertical component and the horizontal component, and change the field strength of the composite component, so that the vertical pattern of the antenna is downward tilted.

In other words, the electronic downward tilt does not need to actually tilt the antenna, only the engineer is in front of the computer, clicks the mouse, and adjusts it with software. Moreover, the downward tilt of electrons will not cause distortion of the radiation pattern.

The simplicity and convenience of electronic downward tilting does not come out of thin air, but is achieved through the joint efforts of the industry.

In 2001, several antenna manufacturers came together to form an organization called AISG (Antenna Interface Standards Group), which wanted to standardize the interface of electronically adjustable antennas.

Up to now, there have been two versions of the agreement: AISG 1.0 and AISG 2.0.

With these two protocols, even if the antenna and the base station are produced by different manufacturers, as long as they all comply with the same AISG protocol, they can communicate the control information of the antenna downtilt to each other and realize the remote adjustment of the downtilt angle.

With the backward evolution of the AISG protocol, not only the vertical downtilt angle can be adjusted remotely, but also the horizontal azimuth angle, as well as the width and gain of the main lobe can be adjusted remotely.

Moreover, due to the increasing number of wireless frequency bands of various operators, and the sharp increase in the number of antenna ports required by technologies such as 4G MIMO, antennas are gradually evolving from single-frequency dual-port to multi-frequency multi-port.

The principle of the antenna seems simple, but there is no end to the pursuit of performance excellence. So far, this article has only qualitatively described the basic knowledge of base stations. As for the deeper mystery, how to better support the evolution to 5G, waves of communication people are still searching up and down.