Lossless transmission line and a lossless antenna

In summary: And this is why you sometimes see higher power outputs in lossless systems when the mismatch doesn't seem too great.
  • #1
Wannabeagenius
91
0
Hi All,

I understand that if you have a lossless transmission line and a lossless antenna and there is a mismatch at the antenna/transmission line junction, no energy is lost. If the transmitter is delivering 100 watts into the line, 100 watts will be radiated from the antenna.

I'm wrestling with this concept and getting nowhere because it seems impossible. How is the energy in the rearward moving wave rereflected 100%

I've been told that the impedance of the transmitter is matched to that of the transmission line by the transmitter having an impedance that is the complex conjugate of that of the transmission line. But, as I see it, this only means that power from the transmitter to the transmission line is smoothly transferred with no reflections. This doesn't address the wave reflected back from the antenna.

I'm thinking that the rearward wave gets completely absorbed by the transmitter and the transmitter increases its power in order to compensate! I'm thinking complete absorption because the impedance is perfectly matched from the transmission line into the transmitter.

As you can see, I'm guessing like wild! Please explain how this is done.

Thank you,

Bob
 
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  • #2


Wannabeagenius said:
Hi All,

I understand that if you have a lossless transmission line and a lossless antenna and there is a mismatch at the antenna/transmission line junction, no energy is lost. If the transmitter is delivering 100 watts into the line, 100 watts will be radiated from the antenna.

I'm wrestling with this concept and getting nowhere because it seems impossible. How is the energy in the rearward moving wave rereflected 100%

I've been told that the impedance of the transmitter is matched to that of the transmission line by the transmitter having an impedance that is the complex conjugate of that of the transmission line. But, as I see it, this only means that power from the transmitter to the transmission line is smoothly transferred with no reflections. This doesn't address the wave reflected back from the antenna.

I'm thinking that the rearward wave gets completely absorbed by the transmitter and the transmitter increases its power in order to compensate! I'm thinking complete absorption because the impedance is perfectly matched from the transmission line into the transmitter.

As you can see, I'm guessing like wild! Please explain how this is done.

Thank you,

Bob

You are thinking along the correct lines, but some of the initial premise is incorrect.

If the TL and antenna are mismatched, some of the TX energy will be reflected, so less than 100% makes it to the antenna to be radiated. The amount of reflected power varies with the extent of the mismatch.

And yes, if the TX amplifier is matched to the TL, then all of the reflected energy will be absorbed by the TX amp.
 
  • #3


berkeman said:
And yes, if the TX amplifier is matched to the TL, then all of the reflected energy will be absorbed by the TX amp.

OK. This means that the transmitter than has to put its normal output plus this addional absorbed power back out in order to get 100% of the intitial energy radiated.

Does the transmitter somehow redirect this absorbed energy back to the transmission line? If so, how? What kind of circuitry are we talking about?

Thanks again,
Bob
 
  • #4


Hello Bob,

The reflected energy gets back to your transmitter and presents the transmitter with what it perceives to be a mismatched load. Depending on the antenna's load and the length of the transmission line (in wavelengths), the transmitter can see a mismatch that is a combination of being off resistively, but also with either capacitance or inductance elements.

Some transmitters have a tuned stage on their output that can be tuned to match the seen impedance. Thus, the transmitter can tune to the mismatch. In other cases, a separate tuning device, the antenna matcher, can transform the impedance seen at the transmitter to the conjugate of the one at the transmission line.

Of course, if the mismatch between the antenna and transmission line is too great, a good deal of energy can be wasted in the transmission line. Thus, some matching devices are placed at the junction of the antenna and transmission line and operated remotely.

- Mike
 
  • #5


The transmitter sees the reflected power as a mismatch in the line.

If it sees a low impedance, this becomes the load of the transmit devices. So, they can overheat when they draw too much current.

If it sees a high impedance, this becomes the load for the transmit devices and the result can be excessive voltages being developed which may destroy transistors etc.

So, to avoid these effects, commercial transceivers have a built in Standing Wave bridge which reduces the drive to the output transistors of the transmitter if it detects poor matching on the antenna.

So, you not only lose the power that is reflected, you get less power out of the transmitter to start with.
 

Related to Lossless transmission line and a lossless antenna

1. What is a lossless transmission line and how does it work?

A lossless transmission line is a type of cable or wire used to transmit signals without any energy loss. This is achieved by using conductors with very low resistance and dielectric materials with low loss. Lossless transmission lines work by using electromagnetic fields to transfer energy from one point to another, without dissipating any of that energy along the way.

2. What are the advantages of using a lossless transmission line?

The main advantage of using a lossless transmission line is that it allows for efficient transmission of signals without any loss of energy. This means that the signals can travel longer distances without being weakened, and can also carry higher frequencies. Lossless transmission lines are also less susceptible to interference, resulting in clearer and more reliable signal transmission.

3. What is a lossless antenna and how is it different from a regular antenna?

A lossless antenna is an antenna that is designed and constructed using materials and components that minimize energy loss. This allows for the antenna to efficiently radiate and receive electromagnetic waves without losing any energy. Regular antennas, on the other hand, may have higher resistance and lose some of the energy in the form of heat.

4. What factors affect the efficiency of a lossless transmission line and antenna?

The efficiency of a lossless transmission line and antenna can be affected by various factors such as the quality of the materials used, the design and construction of the components, and the frequency of the signals being transmitted. Additionally, external factors such as environmental conditions and interference can also impact the efficiency of these systems.

5. Are there any limitations to using lossless transmission lines and antennas?

While lossless transmission lines and antennas offer many advantages, there are some limitations to their use. One limitation is that they can be more expensive to manufacture and maintain compared to regular transmission lines and antennas. Additionally, lossless transmission lines and antennas may have limited bandwidth capabilities, which can limit their use for certain applications.

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