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Antennas/Radiation

Learn why moving charges radiate and understand the physics behind the observed flattening of field lines. Explore concepts such as charge acceleration, radiation fields, and the impact of velocity on electromagnetic waves propagation.

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Antennas/Radiation

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  1. Antennas/Radiation Six 22 m antennas comprising the Australia Telescope Compact Array (ATCA)

  2. Why do moving charges radiate? http://www.cco.caltech.edu/~phys1/java/phys1/MovingCharge/MovingCharge.html http://webphysics.davidson.edu/applets/retard/Retard_FEL.html

  3. Uniformly moving charge Low speed: Fields track Charge with Spherical symmetry High speed: Fields track Charge, but Squash field Lines along Transport direction

  4. Why do the lines flatten? Observed size of train (a bit longer !!) Light from the back left earlier to reach on time. So it shows the back of the train a bit earlier, and thus a bit further Real size of train At time of taking picture, light from front and back reached the camera Simultaneously.

  5. Why do the lines flatten? T=0, back emits light T=Dt, back light reaches front and then the front emits light

  6. Why do the lines flatten? Dt = L’/c L’ L Dt = (L’-L)/v L’ = L/[1-v/c]

  7. Why do the lines flatten? L’ = L/[1-vcosq/c] dV’ = dV/[1-vcosq/c] (Apparent volume larger) So q’ = rdW’ = q/[1-vcosq/c]

  8. Why do the lines flatten? V = q/4pe0rret[1-vcosq/c] A = qvm0/4prret[1-vcosq/c] whererret=|r – r(t’)| B =  x A E = -V – ∂A/∂t

  9. Accelerate charge • Accelerate charge, and then let it move at higher velocity thereafter • A pulse propagates spherically outwards • Outside pulse, fields point towards where charge was supposed to be • Inside pulse, fields point to where charge is

  10. Accelerate charge • The transverse field components (kinks) are needed to connect the outside and inside world – these are radiation fields • The radiation field is largest perpendicular to the motion where the disconnect is largest • The r field drops as 1/r2, but the disconnect grows with r, so Eq ~ 1/r

  11. Why do moving charges radiate? • Each jiggling atom creates a spherical looking wave • The spherical wave actually is not fully symmetric(it moves preferably perpendicular to the source) • More importantly, as we saw earlier, its strength decays as 1/R. • So a field due to a static point source goes as 1/R2 Its flux propagates, but power doesn’t • A field due to an oscillating point source goes as 1/R Its power also propagates

  12. Hertzian Dipole p = I0l/w Eq= m0 d2p/dt2 xsinqq/4pr Static charge Uniform velocity Sudden acceleration Uniform acceleration = (qm0/4p)a(t)sinqq/r • Need accelerating charges a(t) • Transverse field components are the ones to notice Eq • 1/r dependence (radiation fields propagate far away) • Angle dependence (radiate perp. to oscillation)  sinq (Remember Brewster angle. Also watch kinks)

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