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Antenna Reciprocity

  1. Apr 28, 2008 #1
    Could someone give me a quick explanation of antenna reciprocity and if/how antenna gain can increase RX sensitivity? Thanks.
     
  2. jcsd
  3. Apr 28, 2008 #2

    berkeman

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    Staff: Mentor

    Welcome to the PF Dave. Reciprocity just means that the gain pattern of an antenna will be the same whether it is receiving or transmitting. So if you transmit a test signal and map out the gain pattern in the far field of the antenna, then this tells you the receive gain pattern as well. Here's an intro to antennas:

    http://en.wikipedia.org/wiki/Antenna_(radio)

    So yes, if you have an antenna with TX gain, it will have the same gain in the RX direction. A multi-element Yagi antenna has gain for both TX and RX, for example.
     
  4. Apr 28, 2008 #3
    Thanks.

    I guess what I'm really asking is how does gain increase the distance at which you can receive, loss being what it is in free space? As an example: -80dB is -80dB at the antenna whether you're receiving on an omni-directional antenna or a high-gain Yagi. If there is enough signal to reach your antenna what difference does RX gain really make? Maybe I'm misunderstanding gain?
     
  5. Apr 28, 2008 #4

    berkeman

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    Gain gives you a bigger RX signal for a given signal power. So if you have two antennas, where the 2nd antenna has a 3dBi gain over the first, then the RX signal will be 3dB higher for the 2nd antenna. This lets you receive signal sources that are farther away.
     
  6. Apr 28, 2008 #5
    Okay, so what you're saying is that I would now be able to receive a signal which is -83dB where before I could only receive a signal of -80db because the antenna is "creating" 3dB of signal?

    I don't mean to sound ignorant. Thanks for your patience.
     
  7. Apr 28, 2008 #6

    rbj

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    that is true, berk, except for the word "just", i think. i think that if you have a pair of antennae, one for receiving and one for transmitting, that the power "gain" (it's really a big loss) using antenna A as the transmitter and B as the receiver, from terminals to terminals, is the same as if you switched them around and used antenna B as the transmitter and A as the receiver.
     
  8. Apr 28, 2008 #7

    berkeman

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    Good point, your are correct. Glad that you pointed that out, rbj.

    Correct. Remember, an antenna that has gain over 0dBi in some direction, will have gain less than 0dBi in other directions. There is no free energy here -- maybe that is the part that has you a little uneasy. If you look down on a horizontal dipole antenna, for example, you have gain in the direction perpendicular to the dipole elements, and you have practically no gain (negative many dBi) in the direction parallel to the elements. What you gain has to come from somewhere. Was that part of your confusion?
     
  9. Apr 28, 2008 #8
    No, it's always made sense to me that an antenna "creates" TX gain by conserving energy. It's never made sense to me how an antenna "creates" RX gain.

    Okay, so the same physical properties of the antenna that conserve energy (directors, reflectors, etc.), thereby creating TX gain, also increase RX gain by conserving energy in the opposite direction? Is this correct?
     
  10. Apr 28, 2008 #9

    berkeman

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    Yes. The reflector/director elements are a good way to think about this. Just picture a parabolic dish, for example, where you can make a dish that is several wavelengths across or more. There it's a lot more obvious how the act of reflection is providing gain.
     
  11. Apr 28, 2008 #10
    Thanks. It makes perfect sense now. I have a problem thinking backwards sometimes.
     
  12. Apr 28, 2008 #11
    Okay, I'm going to add to this...

    Let's say I have an antenna that the manufacturer states has a vertical beamwidth of 30 degrees. Isn't the vertical beamwidth going to change depending on the amount of power applied to the antenna? Or am I misunderstanding antenna propagation patterns? Maybe propagation is limited by the physical characteristics of the antenna and the half-power points on the main lobe will always be the same regardless of applied power?

    I guess I'm confused as to what a propagation pattern actually shows me.
     
  13. Apr 29, 2008 #12

    rbj

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    i think what is correct is that the RX antenna has increased gain (in the direction it is pointing) because with the other parasitic elements, it has an increased "aperture". it becomes a bigger net to scoop up radiant energy.
     
  14. Apr 29, 2008 #13

    berkeman

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    That is correct. The antenna pattern is normalized, and is independent of TX power.
     
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