Jimmie wrote:
Just because it is canceld in the far field does not mean the coil did not
radiate.

It is generally accepted that if RF energy doesn't escape to the far-
field, that energy is not "radiated". Energy can appear in the near-
field around a wire without being "radiated".

I would not even say it is canceled in the far field although this
is a convenent way of looking at radiation. All you can really say for sure
is that the out of phase voltages in the receiving antenna combine
destructively.

If the fields combine destructively in the near-field, that field energy
is returned to the system and never reaches the receiving antenna in the
far field. That's how transmission lines are supposed to work and why
the spacing between the two conductors needs to be negligible compared
to a wavelength.

The near-balanced currents on each side of a coil, traveling in
opposite directions, cause destructive interference in the near field,
thus limiting the amount of energy radiated by the coil. An electrical 1/4WL
helical antenna radiates slightly less RF than does a 1/4WL straight wire.
That implies the amount of radiation from an air-core coil is slightly less
than from a wire the same overall length as the coil.
--
73, Cecil http://www.qsl.net/w5dxp


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