An ongoing challenge in electrical engineering is the design of antennas whose size is small
compared to the broadcast wavelength λ. One difficulty is that the radiation resistance of
a small antenna is small compared to that of the typical transmission lines that feed the
antenna,1 so that much of the power in the feed line is reflected off the antenna rather than
radiated.
The radiation resistance of an antenna that emits dipole radiation is proportional to
the square of the peak (electric or magnetic) dipole moment of the antenna. This dipole
moment is roughly the product of the peak charge times the length of the antenna in the
case of a linear (electric) antenna, and is the product of the peak current times the area
of the antenna in the case of a loop (magnetic) antenna. Hence, it is hard to increase the
radiation resistance of small linear or loop antennas by altering their shapes.2
One suggestion for a small antenna is the so-called 'crossed-field' antenna [2]. Its proponents
are not very explicit as to the design of this antenna, so this problem is based on a
conjecture as to its motivation.
It is well known that in the far zone of a dipole antenna the electric and magnetic fields
have equal magnitudes (in Gaussian units), and their directions are at right angles to each
other and to the direction of propagation of the radiation. Furthermore, the far zone electric
and magnetic fields are in phase. The argument is, I believe, that it is desirable if these
conditions could also be met in the near zone of the antenna.
The proponents appear to argue that in the near zone the magnetic field B is in phase
with the current in a simple, small antenna, while the electric field E is in phase with the
charge, but the charge and current have a 90◦ phase difference. Hence, they imply, the
electric and magnetic fields are 90◦ out of phase in the near zone, so that the radiation
(which is proportional to E ×
is weak.The concept of the 'crossed-field' antenna seems to be based on the use of two small
antennas driven 90◦ out of phase. The expectation is that the electric field of one of the
antennas will combine with the magnetic field of the other to produce radiation that is much
more powerful than that from either of the two antennas separately.
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http://puhep1.princeton.edu/~mcdonald/exam...rossedfield.pdf
