ETS-Lindgren 7604 Shielded Coil Antenna - Manual (Page 18)

18 | Theory of Operation
It is important to understand that the Model 7604, like any loop antenna,
responds to the average density of the magnetic flux enclosed within the coil.
The importance of this is that two loop antennas of different size will produce
different responses to the same source at the same position. This is because the
measurements are made in the near field of the source, and the flux density in
the near field is a non-linear function of the distance from the source. The varying
flux density causes the integral of the flux in two differently-sized areas centered
at the same point to be different, resulting in a different average flux density and
thus a different response from each loop.
E QUATION (3)
E QUATION (4)
The transition region between the near and far fields around a typical
magnetic field source is centered at a distance equal to the
wavelength divided by 2
π
. The near field thus extends out to a
distance of about 100 m at 500 kHz, 1 km at 50 kHz, and 2500 km at
20 Hz.
Equation (4) is derived in Appendix B on page 21. For the Model 7604,
(4) becomes (5).
E QUATION (5)