UCLA BPPL - Radiation from Magnetic Loop Antennas
Magnetic antennas are used to excite and receive electromagnetic waves. In a magnetized plasma, we have excited whistlers with pulsed loop antennas and measured the wave magnetic field B(r, t) from the near zone into the far zone in 3-D space and time.
 "Click image to see larger version (10 kB)")
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Experimental setup and basic parameters. |
4-D Fourier
transformation yields B(k,
) and an experimental
verification of the dispersion of plane oblique whistler waves.
The electric field is calculated in Fourier space via Maxwell's equations
and Ohm's law. Inverse transformation yields E(r, t),
uniquely separated into inductive and space charge fields which are
equally important in oblique whistlers but cannot be distinguished in
dipole probe measurements. Electric and magnetic fields yield the
Poynting's vector and radiation resistance.
</i>]
show the radiation pattern of a loop
antenna with dipole axis parallel to <b>B</b><sub>0</sub>.',
600,
238) "Click image to see larger version (57 kB)")
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Contours of
r2
Re[Sz(x=0,
y,
z,
|
)]
show the radiation pattern of a loop
antenna with dipole axis parallel to B0.
Damping is by collisions rather than Landau damping. In EMHD whistler wave packets, the vector potential, magnetic field, and current density are nearly parallel. The helicity densities A·B, B·J have different signs for waves propagating in opposite directions to the dc magnetic field, hence no net helicity is injected for symmetric wave excitation. The measured antenna properties are compared with computer simulations. Antenna configurations have been designed which optimize the radiation into the Gendrin mode (oblique whistlers with parallel energy flow).
| = const superimposed on the theoretical dispersion surface
in <b>n</b> = <i>c</i><b>k</b>/<img src=\'research/Symbols/omega-F.gif\'
width=\'13px\'
height=\'15px\'
class=\'in-line\'
alt=\'<i>omega</i>\' />-space.
A quadrant has been removed to show the close agreement of the data
with theory. The spectral density maximizes for oblique
<i>c</i><b>k</b>/<img src=\'research/Symbols/omega-F.gif\'
width=\'13px\'
height=\'15px\'
class=\'in-line\'
alt=\'<i>omega</i>\' />
showing that the antenna couples to oblique whistler modes.',
600,
386) "Click image to see larger version (89 kB)")
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Surface of
|B(ck/ |
References
- Pulsed currents carried by whistlers. V. Detailed new results of magnetic antenna excitation (1.9 MB), C. L. Rousculp, R. L. Stenzel, and J. M. Urrutia, Phys. Plasmas 2, 4083-4093 (1995). [Link to original publication.]
- Inductive and space-charge electric Fields in a whistler wave packet (1.7 MB), C. L. Rousculp, R. L. Stenzel and J. M. Urrutia, Phys. Rev. Lett. 72, 1658-1661 (1994). [Link to original publication.]