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Power and gain in antennae

  1. Jan 17, 2005 #1
    I'm having difficulty understanding how the gain of an antenna works. I appreciate that the gain effectively concentrates the available RF energy into a narrower beam but i can't fathom how this seems to mean that if i have a transmitter transmitting at say 100dBm that i can use antenna gain to produce a power higher than this at a receiver. How is this possible to seemingly 'create' power?
    If i supply an isotropic antenna with 40W of power then this theoretically will be dissipated equally in all direction so at any point away fom this i will receive a fraction of this. If i conentrate the energy by beam forming then surely the most i can expect to get is no more than the original 40W.

    Also, when we say that the power at a certain point from the transmitter is XdBm what does this mean? i.e. surely it depends on the size of the receiver at that point so you can't make a general statement that at that point the power is XdBm????

    Thanks in advance

  2. jcsd
  3. Jan 17, 2005 #2
    I don't know what you mean exactly, when I want to boost gain on my antenna, I need an operational amplifier circuit. The ability to boost the power comes from a 12V or 9V supply.

    This formula gives the fraction of transmitted power when a EM wave is normally incident on a recieving medium.

    [tex]T=1 - (\frac {\eta_2 - \eta_1} {\eta_2 + \eta_1})^2[/tex]
    where [tex]\eta[/tex] is impedance of medium 1 or 2.
    So the power transmitted will always be less, unless you have an effective impedance set up relevantly.

    Concentrating Electric flux...well you will get more power per unit area, but the same power overall.
  4. Jan 17, 2005 #3
    I guess the thing that confuses me is the following equation:

    If if have a system wherein i transmit a power of 4dBm to an antenna, there is a loss of 6db in the transmission line and a gain of 18dBi then the following applies:

    EIRP = Pout - Ct + Gt = 16 dBm

    This to me suggests that an initial supply of 4dBm somehow magically becomes 16dBm.
  5. Jan 17, 2005 #4
  6. Jan 17, 2005 #5


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    Think of it this way. You have an omni directional antenna transmitting a signal. You place a receiving antenna at a short distance and note the received signals strength. Now you replace the transmitting antenna with antenna that has a gain of 10 db. The signal strength in the receiver now is 10 db higher than the previously noted reading. You sacraficed some signal in one direction and added signal in another. Now if you positioned the transmitting antenna wrong you may end up with the receiving antenna showing a lower reading than previously. You can do the same with a receiving antenna.
  7. Jan 17, 2005 #6
    Hi Beckett,

    I hope this analogy helps: Let's assume we can see the radiated RF power from the omnidirectional antenna as patterns of light--sort of like a flourescent light bulb. At a certain distance we can still see the light source but not too strong. Let's say without changing our location, there's a way to increase the intensity of the light source by using mirrors and lenses. If we add mirrors and/or lenses at the light source and situate them in away it increases the visibility or intensity (i.e. gain) at a certain direction (hopefully, to us!), the light source will appear brighter--and, if done correctly, much brighter to us!

    (Of course, that is if our "receiver" eyes can see correctly)

    Another simpler way of thinking about antennas and their patterns are similar to the type of light sources that are available, such as spot lights, flood lights, etc.

    Hope this helps you visualize what's going on. :)
  8. Jan 18, 2005 #7
    Folks, thanks for your help, i think i've got it now, these forums are so useful.
  9. Mar 5, 2005 #8
    Its all about superposition of waves....if you take a single simple antenna element it will have a uniform dispersal. Multiple elements will add or subtract to the waves from each other creating areas where the signal is stronger or weaker. The total power is still the same but there are areas where it is multiplied. Reflective surfaces can also cause the same effects.
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