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Antenna pattern and wavelength

  1. Aug 3, 2011 #1
    Dear all,
    Let's assume that I have just bought the perfect omnidirectional antenna and I try to understand how the wavelengths are transmitted. If I got it right, there are wavelengths getting 'out' towards the 360 degrees directions.

    Now let's assume that I have a perfect receiver antenna that it will only captures wavelengths that come from the 180 degrees exactly. So in theory only one wavelength will be captured.

    In that ideal world of receiver and transmitter antenna, how I can find
    a) How much energy this one wavelength will carry (assuming that I know how much energy was transmitted from the transmitter antenna)?
    b) What are the 'dimensions' of this signal? I know the wavelength but how I can also find the height of it? In other words if I know wavelength and height I can define the area that this signal will occupy.

    I would like to thank you in advance for your help.
    (I need this kind of information to get some basic understanding what I am capturing)

    Best Regards
    Alex
     
  2. jcsd
  3. Aug 3, 2011 #2

    sophiecentaur

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    You need to be defining your geometry better. If you are looking in a horizontal plane at the pattern of a simple vertical dipole then the pattern is omnidirectional. If you lay the dipole down then the pattern is more like a figure of 8 with nulls on the axis of the wire.

    I can't think what your receiving antenna layout is, from your description but it would not have a single frequency response. Why should it?

    A radio signal doesn't have a "height". It has a wavelength and a field strength. The size of an individual antenna element is related more to what goes on in and on the antenna system than the wavelength of the em radiation involved.

    Remember - most of the energy transmitted from an antenna never gets received at all- it mostly keeps going off into free space. A receiving antenna will only intercept a small proportion. (There is an exception for very directive antennae with very close spacing)
    I will wait for your reply when you can, perhaps, refine your query a bit. I think there may be some fundamental misconceptions here.
     
  4. Aug 3, 2011 #3
    Dear sophiecentaur I would like to thank you for your answer.

    I did the following assumption so to simplify things:
    I have a perfect omnidirectional antenna at H and E plane, for the transmitter. All the energy is equally distributed to everywhere

    For the reception antenna, let's assume that is exactly the same . A perfect omnidirectional to H and E plane.

    I connect the transmitter antenna to a signal generator and I create a signal of -50dbm (no modulation) strength.... then I put the receiver antenna perfectly aligned to the transmitter antenna and only one meter apart.

    This receiver antenna is connected to a Spectrum analyzer which shows that the peak is -80dbm. There is a 30dbm loss. I know that some energy is dissipated at free space, some energy is lost due to antenna imperfections, cable losses, connectors, spectrum analyzer imperfections. and so on.
    I also know that the transmission is done inside so there are many reflections.. but still 30dbm loss is a lot and I think this is because of the -50dbm energy radiated only a small portion of it is send to the "right direction"

    So I am trying to understand how
    a) the -50dbm energy is radiated from antenna and
    b) what will be the energy of one of the radiated wavelengths towards one direction?

    I hope this clarifies a bit what is in my brain.


    Best Regards
    Alex
     
  5. Aug 3, 2011 #4

    sophiecentaur

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    Perhaps it would help if you imagine the whole of the output power being spread out over a sphere. Then, rather than asking for the power in "one direction", think of the the power over a small solid angle in that general direction. Then the proportion will be that solid angle over 4π (the solid angle of the whole sphere. The receiving antenna will have an effective collecting area and will pick up that proportion of the power. Even a microscopically thin piece of wire as a dipole, will still have a very finite collecting area.

    Of course, you can't get a perfect omnidirectional radiator so the above needs to be modified in practice.
     
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