EM waves and traditional method of transmission

In summary, the conversation discusses the use of EM waves to transfer power in AC systems and the challenges that arise with long wavelengths. The participants also touch on the role of transmission lines in this process and the impact of impedance mismatch on the current waveform. They mention the use of traditional methods in circuit analysis and the need for a different approach at high frequencies. Additionally, they mention the study of switching surges and their different transmission line models compared to power frequency.
  • #1
Jackson Lee
77
1
Hey, guys. We all know power of AC currents is transferred via EM waves, but we seldom use that to calculate power in AC system. The reason for this is wavelength is very long. I want to know if there is really existed a transmission line which is 500km, just the length of EM wavelength, then could we still use the traditional method to calculate AC system? For example, could we still regard the current in the line to be same in every spot?
 
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  • #2
Jackson Lee said:
I want to know if there is really existed a transmission line which is 500km

I would imagine there's quite a few around the world
there are a number of them in Australia alone

Jackson Lee said:
For example, could we still regard the current in the line to be same in every spot?

why wouldn't it be ?
 
  • #3
davenn said:
why wouldn't it be ?

Wouldn't it depend upon the phase of the EM wave at that point?
 
  • #4
Drakkith said:
Wouldn't it depend upon the phase of the EM wave at that point?

hmmm, good point ... that's possible, would need some one better informed than me to confirm that
 
  • #5
Jackson Lee said:
I want to know if there is really existed a transmission line which is 500km, just the length of EM wavelength, then could we still use the traditional method to calculate AC system?
The length of the line is not important if it is impedance matched at both ends. The wave propagates along the line at close to the speed of light, it takes time to do so. The wave progressively reaches different points along the line, so at any instant the V and I waves will be different everywhere.

If there is an impedance mismatch at the receiving end, a wave will be reflected back to the generator. On a transmission line the forward and reflected waves are quite independent of each other and so pass without any influence on each other. But if you measure the current on the line without a directional coupler or hybrid transformer, it will be the sum of both that you are measuring. That is where standing waves come from.

At 60Hz, the wavelength on a bare metal transmission line is not 500 km but is closer to 5000 km.
 
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  • #6
Baluncore said:
The length of the line is not important if it is impedance matched at both ends. The wave propagates along the line at close to the speed of light, it takes time to do so. The wave progressively reaches different points along the line, so at any instant the V and I waves will be different everywhere.

If there is an impedance mismatch at the receiving end, a wave will be reflected back to the generator. On a transmission line the forward and reflected waves are quite independent of each other and so pass without any influence on each other. But if you measure the current on the line without a directional coupler or hybrid transformer, it will be the sum of both that you are measuring. That is where standing waves come from.

At 60Hz, the wavelength on a bare metal transmission line is not 500 km but is closer to 5000 km.
Oh, yes. You are right. It is close to 5000km. Besides, what puzzled me most is in traditional method of electric circuit analysis, both AC and DC, we always assume current is same at every point in one transmission line, I mean if there is no branch, but if it is not the case as what we talked above, then how to analyze? Use wave formula you have mentioned?
 
  • #7
Jackson Lee said:
Besides, what puzzled me most is in traditional method of electric circuit analysis, both AC and DC, we always assume current is same at every point in one transmission line

Unless you are dealing with high-frequency AC transmission, perhaps in the form of microwave communication or radar, the electrical frequency is so low that for all intents and purposes the current is the same at every point and there is no need to complicate things by including it.
 
  • #8
Jackson Lee said:
Besides, what puzzled me most is in traditional method of electric circuit analysis, both AC and DC, we always assume current is same at every point in one transmission line,

that works for a cct on your bench where wire lengths are minimal
 
  • #9
Drakkith said:
Unless you are dealing with high-frequency AC transmission, perhaps in the form of microwave communication or radar, the electrical frequency is so low that for all intents and purposes the current is the same at every point and there is no need to complicate things by including it.
davenn said:
that works for a cct on your bench where wire lengths are minimal
So, you mean if frequency is very high, then we have to use another novel way to deal with circuit analysis.Right?
 
  • #10
Jackson Lee said:
So, you mean if frequency is very high, then we have to use another novel way to deal with circuit analysis.Right?

As far as I know, yes.
 
  • #11
If you have an impedance mismatch, the resulting waveform will often show ringing:
nexlogic%20pcb%20design%20figure%203%20350.jpg
 
  • #12
Svein said:
If you have an impedance mismatch, the resulting waveform will often show ringing:
nexlogic%20pcb%20design%20figure%203%20350.jpg
Oh， it seems interesting. Maybe I will learn it laterly in wave formula. Thx
 
  • #13
Drakkith said:
As far as I know, yes.
Thx a lot. You are really helpful, guy
 
  • #14
Jackson Lee said:
Oh, yes. You are right. It is close to 5000km. Besides, what puzzled me most is in traditional method of electric circuit analysis, both AC and DC, we always assume current is same at every point in one transmission line, I mean if there is no branch, but if it is not the case as what we talked above, then how to analyze? Use wave formula you have mentioned?

In power system analysis, for the 60 hertz power, we assume current is the same everywhere (neglecting distributed shunt capacitance),
But we also study what we call "switching surges", where the EM wave front moves at near light speed, but the surges disappear in a few milliseconds. Lightning strikes are like switching surges. Surges and strikes have string high frequency components. The transmission line models for surges and the models for power frequency are very different. For example, impedance matching and reflections are not a factor at 60 hertz.

I resist calling a 60 hertz signal an EM wave; 60 hertz is closer to a time varying DC signal than it is to a surge EM wave. Those semantic differences may be confusing the OP.
 

1. What are EM waves?

EM waves, or electromagnetic waves, are a type of energy that can travel through space. They are created by the movement of electrically charged particles and can range in frequency and wavelength, from radio waves to gamma rays.

2. How are EM waves traditionally transmitted?

EM waves are traditionally transmitted through the use of antennas. The antenna converts the electrical signals into EM waves and broadcasts them into space. These waves can then be received by another antenna, which converts them back into electrical signals. This method is commonly used for radio and television broadcasting.

3. What are the advantages of using EM waves for transmission?

EM waves have several advantages for transmission, including the ability to travel long distances without the need for a physical medium, such as wires. They also have the ability to penetrate through certain materials, making them useful for communication and imaging technologies. Additionally, they can be easily manipulated and controlled for various applications.

4. What are the limitations of using EM waves for transmission?

While EM waves have many advantages, they also have limitations. One limitation is that they can be affected by various factors, such as interference and obstacles, which can weaken or disrupt the signal. Additionally, different types of EM waves have different properties and limitations, making certain frequencies more suitable for specific applications.

5. How are scientists studying and researching EM waves?

Scientists use a variety of methods to study and research EM waves, including mathematical models, laboratory experiments, and observations in natural environments. They also use advanced technologies, such as antennas, detectors, and spectrometers, to collect and analyze data on EM waves. This research is crucial for understanding the properties and behaviors of EM waves and developing new technologies that utilize them.

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