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Radar antenna: measuring H or E?

  1. May 18, 2015 #1
    Hi there,
    Maybe this is a stupid question. Maybe not.

    I want to make a statement in my thesis that says that all radar antennas on satellites derive the EM field by: receiving the time-varying H field, and from the induced current derive the EM field. Am I correct in saying that? Or are there any different types of antennas?

    The reason I ask, is because I want to make a statement for noise induced in certain scenarios where the H field lags the E field.
     
  2. jcsd
  3. May 19, 2015 #2
    I can't make sense of "derive the EM field".
     
  4. May 19, 2015 #3
    I mean 'and from the measured induced current derive the electromagnetic E field. Or is it something else I might define incorrectly?

    Another way to ask this question is: can a radar antenna measure a phase-shift between the H and E field (where the H field lags the E field)?
     
  5. May 19, 2015 #4
    The E and H field of an EM wave in free space are in-phase, even for noise. If there is a phase difference, it is a local effect caused by the antenna or some object.
     
  6. May 19, 2015 #5

    Baluncore

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    There is a simple assumption used when modelling conductive antennas. The current induced in the conductive antenna material is due to the magnetic component of the EM field.

    The voltage at some place on the antenna will be determined by the product of current and impedance where that voltage is measured relative to some reference.
    The impedance of free space is real = resistive, not complex = reactive.
    If E and M are not in phase then it must be propagating through a material with complex impedance. The EM wave will then be in a dielectric or a magnetic material.
     
  7. May 19, 2015 #6
    You're right, but I am processing data that have partly travelled trough some conductive media before received back to the satellite as backscatter.

    I guess definition of the question is actually harder than I thought :-).
     
  8. May 19, 2015 #7
    Thanks! You mean 'The EM wave will then be in a conductive or a magnetic material'?
     
  9. May 19, 2015 #8

    davenn

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    no, a dielectric is non conductive
     
  10. May 19, 2015 #9
    Yep, but the statement was: "If E and M are not in phase then it must be propagating through a material with complex impedance. The EM wave will then be in a dielectric or a magnetic material." So I think Baluncore meant conductive.
     
  11. May 19, 2015 #10

    Baluncore

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    No, I mean reactive.
    Electric effects are due to permittivity, magnetic effects are due to permeability, resistance represents a loss. Any change of impedance in the propagating medium will result in an impedance mismatch with partial reflection. A GHz signal cannot pass through a good conductor because almost all the energy will spread across the surface and/or be reflected and scattered.

    If you receive a plane polarised EM wave that has passed through space then the E and M must be in phase as they propagate through space. Any EM phase difference generated by the glass, paint or oxide layer on the satellite will result in an energy mismatch and scatter, but the returning wave through free space must have E&M in phase.

    Maybe you are being confused by a reflection or by circular polarisation in some way.
    How might you measure the phase difference of the E and M vectors?
     
  12. May 20, 2015 #11
    I'm confused by the term "in phase". While the phase relationship is fixed (for far field effects) whether it's in phase or out of phase would seem to depend on the choice of reference. Perhaps the two of you are using different reference choices and this is adding to the confusion?
     
  13. May 20, 2015 #12

    nsaspook

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    Typically non-cw advanced radar (and some communications signal methods like spread spectrum ) are non-sinusoidal so phase changes are very easy to detect with the reference being the source waveform.
    http://www.ntia.doc.gov/legacy/osmhome/uwbtestplan/barret_history_(piersw-figs).pdf
     
  14. May 20, 2015 #13
  15. May 21, 2015 #14

    Baluncore

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    There is confusion, but I don't know where it lies. If we shake things up it may be revealed.

    One convention is that a horizontal wire dipole transmits a horizontal signal. The horizontal therefore refers to the Electric field direction. The M far-field could then be described as vertical.
    If a radar transmits a plane polarised signal from a dipole, then receives the reflected wave with crossed dipoles capable of resolving the Horizontal and Vertical components, those components might possibly be referred to as the E and M plane orientation relative to the transmitter dipole. There may be a phase difference in the two received signals due to rotation or circular polarisation. That distinction between EM and HV orientation arises because the radar antenna is not usually fixed on the face of the Earth, aimed at the horizon.

    Yet another convention is to compute the directionally induced current in the antenna from the M component of the wave incident on the dipole. That assumes the dipole is in free space, with mutually perpendicular E, M and Poynting vectors. The ratio of E/M is the impedance of free space, Zfs = c * 4π*10–7 = 376.7303 ohms. The propagating E and M fields are then in time phase because Zfs is real.
     
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