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Understand antenna design and matching network

  • Categories:News Center
  • Time of issue:2021-02-02 14:42
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(Summary description)This article is not intended to explain a solid theory of how antennas and matching networks work. We have a very broad understanding of a theory that explains how point charges radiate (Maxwell's equation), a theory that explains the necessity of matching (microwave theory), and another theory that explains how dipole antennas map their radiation patterns on paper. However, when it comes to actual antennas in the real world, much of the knowledge is empirical. First, for most antennas, there is no closed radiation equation. Secondly, even if the equations are solved for some antennas, the mathematical operations are quite complicated and difficult to understand. Experience and practice in the field of antenna design are much faster than the development of theoretical knowledge. This is understandable when considering the complexity of these energy converters. Therefore, it is difficult to claim that one person can formulate all the basic laws of antenna working principles. Even if this is successfully achieved, these laws are almost useless in solving practical problems in antenna design.

Understand antenna design and matching network

(Summary description)This article is not intended to explain a solid theory of how antennas and matching networks work. We have a very broad understanding of a theory that explains how point charges radiate (Maxwell's equation), a theory that explains the necessity of matching (microwave theory), and another theory that explains how dipole antennas map their radiation patterns on paper. However, when it comes to actual antennas in the real world, much of the knowledge is empirical. First, for most antennas, there is no closed radiation equation. Secondly, even if the equations are solved for some antennas, the mathematical operations are quite complicated and difficult to understand. Experience and practice in the field of antenna design are much faster than the development of theoretical knowledge. This is understandable when considering the complexity of these energy converters. Therefore, it is difficult to claim that one person can formulate all the basic laws of antenna working principles. Even if this is successfully achieved, these laws are almost useless in solving practical problems in antenna design.

  • Categories:News Center
  • Author:
  • Origin:Asem Elshimi, Silicon Labs IoT wireless solutions for design engineers
  • Time of issue:2021-02-02 14:42
  • Views:
Information

This article is not intended to explain a solid theory of how antennas and matching networks work. We have a very broad understanding of a theory that explains how point charges radiate (Maxwell's equation), a theory that explains the necessity of matching (microwave theory), and another theory that explains how dipole antennas map their radiation patterns on paper. However, when it comes to actual antennas in the real world, much of the knowledge is empirical. First, for most antennas, there is no closed radiation equation. Secondly, even if the equations are solved for some antennas, the mathematical operations are quite complicated and difficult to understand. Experience and practice in the field of antenna design are much faster than the development of theoretical knowledge. This is understandable when considering the complexity of these energy converters. Therefore, it is difficult to claim that one person can formulate all the basic laws of antenna working principles. Even if this is successfully achieved, these laws are almost useless in solving practical problems in antenna design.

First, I will share my intuition about how wireless electronics work on the physical layer. There are many physical (hardware) and non-physical (software) layers in wireless electronic devices. Engineers usually tend to understand some of them, especially when they only deal with specific tasks, such as designing matching networks or phased array antennas. And I tend to connect points—from the radiant charge that oscillates at a non-relative speed, to the Bluetooth communication channel that transmits water meter readings to the gateway. For novices in RF engineering, I hope this article will help shape a broad understanding of antenna design and matching networks; for wireless experts, I hope to emphasize best practices and hard-won wisdom.

Figure 1 shows some common antenna designs. The one we are most familiar with is the monopole antenna, because it used to be the mainstream antenna for TV broadcast reception, first-generation mobile phones and even toys. Some engineers who have been engaged in analog and wireless for a long time will recognize the Yagi-Uda antenna, which has been used as the TV receiver antenna on our roof until the late 1990s. Due to economic and mechanical reasons, the most common antenna in wireless electronic products is a microstrip patch antenna. In my opinion, the easiest antenna to explain is the horn antenna. Having said that, the concepts I will explain on the horn antenna are also applicable to other types of antennas. It only takes a little more imagination and an understanding of electromagnetics. Look at them from the same perspective.

Flexible application of optocouplers in green energy and energy storage systems

Figure 1 Antenna design example.

The antenna is a kind of energy converter. It absorbs electromagnetic waves from one side and radiates spherical waves in free space from the other. Each wire does this to a certain extent. Each wire basically emits part of the electromagnetic energy passing through it, which is one of the reasons for using electrical insulation. However, when talking about antennas that radiate electromagnetic energy, it actually refers to a very special type of radiation-useful electromagnetic radiation. In 2020, useful electromagnetic radiation is simply an electromagnetic wave that oscillates at a frequency allowed by the standard (FCC, ETSI, etc.), and has enough power to traverse the target range of the application. For example, a Bluetooth antenna must be able to send/radiate tens of milliwatts of electromagnetic waves, which can travel through a space of several meters. We will return to this example shortly. For now, let’s focus on the antenna as an energy converter for a specific frequency and output power.

In order to clarify the ambiguity of energy converters, let’s look at a familiar example: a power converter that absorbs electrical energy in one form and transmits electrical energy in a slightly different form. It converts voltage to the current ratio of an electrical signal. In other words, it changes the wave impedance of the electrical signal (according to Ohm's law, voltage/current = impedance). A common example of a converter is the two-winding transformer learned in high school, which is still used in the power grid today. Power plants generate relatively high current and low voltage electrical signals. In order to convert this signal across hundreds of kilometers with minimal loss, a converter is used to increase the wave impedance. In other words, to increase the voltage and reduce the current, a smaller current can flow through a longer wire with less loss.


Figure 2 (Left) Electronic transformer; (Right) Energy conversion diagram of the entire transformer.

In a purely electrical sense, the antenna is like a converter, and the role of a horn antenna is very similar to that of a converter. Observing a rectangular waveguide with a horn antenna at its end, you can see how it prepares electromagnetic waves so that they are emitted toward free space (Figure 3). This gradual expansion of the horn antenna is basically an energy converter, which receives a guided wave with an impedance of 50 ohms from the coaxial cable and converts it into a free space wave with an impedance of 377 ohms. Without using any mathematical formulas, only some relevant and obvious explanations for antennas are given: they are matching elements that match guided waves with free-space waves. Why is this matching important? Because like a transformer, guided waves also need this kind of energy conversion to be able to travel through free space with minimal loss (if the wave impedance of electromagnetic waves is different from the impedance of free space, then it will not propagate in free space at all.)


Figure 3 Schematic diagram of electromagnetic energy conversion in a horn antenna.

What is wave impedance? It is the ratio of electrical energy to magnetic energy in electromagnetic waves. What does it mean that the wave impedance of free space is 377 ohms? This means that for a wave to pass through free space, its wave impedance must be 377 ohms. How do you know this number? The Maxwell equation can be solved in free space and the wave impedance is found to be 377 ohms. Alternatively, experiments can be performed to measure the ratio of electrical energy to magnetic energy in a free space wave and get the same value with incredible accuracy. So far, this is one of the most impressive scientific verifications in human history. What about 50 ohms? Why is the wave impedance inside the waveguide 50 ohms? This is a very good question. 50 ohms is the standard value for microwave circuits. Even if some are 75 ohms or higher, in contemporary microwave technology, that is, chip microwave circuits, no one cares about this 50 ohm number. Where does this standard come from? Obviously, this is a compromise found by coaxial cable designers in the past between maximum power carrying and cable loss. This value is 50 ohms, which has become a quality factor used by every wireless engineer.


Figure 4 50 ohms is a compromise between maximum power carrying and coaxial cable loss.

Now, this article attempts to build an SoC to detect and process water meter data sent wirelessly to the gateway. The data stored in the SoC memory is represented by 1 and 0. Some memory switches are OFF and some are ON; ON switch represents digit 1, OFF represents digit 0. You can read the memory one by one, and then get ready to send all the data. We also have an energy converter called an antenna. We know that it can absorb electromagnetic energy from wires and change the impedance, and then send it to free space. Do we only apply these 1 and 0 directly to the antenna? Does this still work?

In the early radio transmissions, developers were able to successfully do this by using the "on/off" key on one end of the antenna to control the signal, and then another receiver in another location would read the signal. In contemporary radio frequency engineering, this cannot be done for many reasons. First, these 1s and 0s are based on the operating frequency of the microcontroller (MCU), which is usually several tens of MHz. The length of the antenna is about 15 meters to effectively convert the 50 ohm guided wave of 10MHz into 377 ohm. This size is huge for any current electronic product-I want to look at a smartphone with a built-in 15-meter antenna. So, why does the antenna have to be so long? This is to make the antenna as efficient as possible, it needs to resonate near the frequency of the transmitted wave. Resonance keeps the electromagnetic energy oscillating between the two ends of the antenna structure. Therefore, keeping as much energy as possible in the structure (rather than reflecting it back to the source) can achieve higher radiated power. Resonance requires that the antenna size be equal to half the wavelength of the propagating wave. Essentially, a useful antenna should be on the order of the wavelength of the propagating wave. Speed ​​of light = wavelength × frequency, this is the relationship between the speed of light, the frequency of the propagating wave, and the wavelength. This article uses this relationship to calculate the antenna size as 15 meters.

To use a smaller antenna, all you need is a higher signal frequency, which is what you need to do when applying modulation to the signal. Modulation is to encode the low-frequency signal in the information of the high-frequency transmission signal. A simple method (but not the only method) is to multiply the low-frequency signal by the high-frequency carrier, and the result is an amplitude modulation (AM) signal. Yes, it's like AM in old car radios. When using Bluetooth, the frequency of the carrier is 2.4GHz, which reduces the size of the antenna to about 2 cm. This is one of the reasons why we no longer see the antenna. Because they are small enough, they can be hidden in electronic devices. Okay, now this whole modulation technique will give us another huge advantage: coexistence.


Figure 5 Modulation is to encode the low-frequency signal in the information of the high-frequency transmission signal.

When I was a child in the 1990s, I was puzzled that my father and my sister could talk on the mobile phone at the same time. How can you not hear each other? My father seems to be sending sound into this electromagnetic ether, but it is not clear why the sound data is not coupled to my sister's phone. Facts have proved that cellular phones and wireless electronic devices use modulation to avoid this problem. The electromagnetic ether or spectrum can be divided into smaller bandwidths, called channels in radio frequency engineering. Whenever two Bluetooth nodes or any other communication standard, but this article continues to use Bluetooth as an example to try to create a connection, they will choose a channel for communication. Then, they modulate all bits (1 and 0) on the carrier associated with this channel. Now, even if another Bluetooth connection occurs nearby, the first connection will not be significantly affected, because the two are orthogonal in the spectrum space, and each connection is located on a different carrier frequency. Therefore, it is only necessary to demodulate the specific carrier frequency used for the channel to decode the information on the intended connection.


Figure 6 Frequency division in the 2.4GHz band.

Let's study a part of the wireless communication problem, and then we can look at the big picture and summarize it. Now, with a 2.4GHz modulated carrier and trying to transmit information through the Bluetooth channel, we have a 5mm miniature antenna that can receive 50 ohm waves and convert them into 377 ohm free space waves. A closer look at what you have will realize that more work still needs to be done. We have prepared a 2.4GHz signal on the chip, which means it is a low-power signal. Now it is time to convert this low-power signal to high power (high and low are relative terms). Low power here means a few microwatts, and high power refers to a few milliwatts. Compared with kilowatt signals, power electronics engineers believe that both are noise, and this can be done by using a power amplifier.

Now that the theoretical dynamics of how antennas radiate power have been discussed, here are some practical considerations that can make antenna design more effective:

The need for perfect antenna size stems from the need for good antenna gain and coverage, which may vary depending on the target solution. For example, if a Bluetooth mouse operates within 50 cm and 5kbps (low data rate), this means that the size range will be affected. For example, the antenna required for a wireless mouse may be much smaller than the wavelength/2. The smaller size means that the antenna is no longer a perfect matching element-but if the application only needs to radiate a small portion of electromagnetic energy into space, who cares about it;
Although wavelength/2 is the theoretical size of the antenna, it is always possible to reduce it to wavelength/4 to achieve a smaller size. Only need to integrate a ground plane under the wavelength/4 antenna. Based on the mirror image theory and some electromagnetic theory practice, the performance of the wavelength/4 antenna with a ground plane is similar to that of the wavelength/2 antenna;
The ground plane design must be wide enough and usually continuous;
The plastic shell of the final product must also be checked carefully. The dielectric constant (different wave impedance) of plastic is higher than that of air. Therefore, if the antenna is designed to radiate perfectly into the air, its performance may be reduced once it is packaged by plastic. Depending on the tightness of the enclosure, it may even affect the near-field dynamics of the antenna and have more adverse effects on performance;
Pay close attention to the antenna feed. This structure is solely responsible for receiving the signal from the device and feeding it to the antenna harmoniously. The feeding will directly affect the bandwidth and the reliability of the overall design.
This article has discussed the surrounding environment of the antenna, and then take a closer look at the matching network. This may be confusing, why do you need to match again? Matching is essentially energy conversion. When we use a power amplifier to create high-energy waves, it has a certain wave impedance, but the wave impedance of a standard antenna (and all connectors and traces from the chip to the antenna) is 50 ohms. Therefore, in order to effectively spread energy, it is necessary to ensure that the wave leaving the power amplifier is converted to 50 ohms, and we do this by using a matching network.

Here is a metaphor of fluid mechanics for matching networks: Have you ever played with water pipes as a child? When you squeeze the opening of the water pipe, the water pressure will increase and the water will spray farther. According to fluid mechanics, there is a certain relationship between mass, cross-sectional area and flow rate. Changing the cross-sectional area of ​​water flow can increase the flow rate. The cross-sectional area of ​​hydrodynamics is similar to the wave impedance of electrodynamics, and the function of the matching network is very similar to that of squeezing the opening of a water pipe.


Figure 7 Squeezing the opening of the water pipe will make the water spray farther.

That's it. This is a pile of electrons swinging between different storage units on the chip, which can tell us which storage units store 1 and which store 0. Then, these swinging electrons are obtained and other electrons swinging at a higher rate are modulated, that is, 2.4 GHz swing/sec. Expose the input of the power amplifier to 2.4GHz swinging electrons, and the power amplifier uses strong electromagnetic waves to infuse the antenna. Finally, the electrons on the surface of the antenna are oscillated at the precise frequency of the channel, and a free space electromagnetic wave of tens of milliwatts is generated, which spans a space filled with many other electromagnetic waves (other radio waves and light waves, and many other waves). Then, the wave causes the electrons on the surface of the receiver's antenna to swing at the same frequency, and then the entire swinging electronic world jumps on the receiver chain to decode the information of the originally encoded ones and zeros.

This is a short story about contemporary wireless communication, how Marconi crossed the Atlantic to send his radio frequency waves that changed the world, and how honeycomb technology sends sounds to base stations, and then transmits voice data across the globe to contact my parents on the other side of the world. In order to realize this evolved radio frequency technology, a large number of designed and organized oscillations must be generated on the chip.

 

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