Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Lossless transmission line and a lossless antenna

  1. Dec 26, 2009 #1
    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
     
  2. jcsd
  3. Dec 26, 2009 #2

    berkeman

    User Avatar

    Staff: Mentor

    Re: Rereflections

    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.
     
  4. Dec 26, 2009 #3
    Re: Rereflections

    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
     
  5. Dec 26, 2009 #4
    Re: Rereflections

    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
     
  6. Dec 26, 2009 #5

    vk6kro

    User Avatar
    Science Advisor

    Re: Rereflections

    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.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook